diff --git a/config/vufind/facets.ini b/config/vufind/facets.ini
index d99dd52eeb2..7ced6130fb4 100644
--- a/config/vufind/facets.ini
+++ b/config/vufind/facets.ini
@@ -148,9 +148,6 @@ facet_limit = 30
 ; Limit facets based on a prefix on a per-field basis:
 ;facet_prefix_by_field[building] = 22
 
-; Filter facet values to those matching a regular expression on a per-field basis:
-;facet_matches_by_field[era_facet] = ".+0"
-
 ; By default, the side facets will only show 6 facets and then the "show more"
 ; button. This can be configured with the showMore settings. A positive value is
 ; required for "normal" facets, but for hierarchical facets you can use 0 to disable
diff --git a/config/vufind/searches.ini b/config/vufind/searches.ini
index 963739c633b..5775a27bd6d 100644
--- a/config/vufind/searches.ini
+++ b/config/vufind/searches.ini
@@ -138,9 +138,10 @@ always_display_reset_filters = false
 ; caveats described at
 ; https://opensourceconnections.com/blog/2018/02/20/edismax-and-multiterm-synonyms-oddities/
 ;
-; Parameters are entered as query parameters and they can be repeated like in Solr's
-; query string syntax:
-; default_parameters[search] = "fq=field%3A%2Fmatch%2F&fq=another%3A2"
+; Parameters are entered as a JSON object according to Solr's JSON Request API.
+; See https://solr.apache.org/guide/solr/latest/query-guide/json-request-api.html
+; Parameters with multiple values can be entered using an array like:
+; default_parameters[search] = '{ "filter": ["format:DVD", "author:Smith"] }'
 ;
 ; Parameters can be defined for the following search contexts:
 ; *                 All contexts not otherwise defined here
@@ -150,8 +151,8 @@ always_display_reset_filters = false
 ; terms             Return terms
 ; alphabeticBrowse  Return information from the browse index
 ; workExpressions   Return work expressions (record versions)
-;default_parameters[*] = "echoParams=none&debugQuery=false"
-;default_parameters[search] = "sow=false"
+;default_parameters[*] = '{ "params": { "echoParams": "none", "debugQuery": true } }'
+;default_parameters[search] = '{ "params": { "sow": false } }'
 
 ; If you set this setting, VuFind will use a secondary Solr field to try to find
 ; previous record IDs when primary ID lookups fail. If you leave the setting
diff --git a/module/VuFind/src/VuFind/ChannelProvider/SimilarItems.php b/module/VuFind/src/VuFind/ChannelProvider/SimilarItems.php
index 2d7e7b6eb5f..8e35de24c78 100644
--- a/module/VuFind/src/VuFind/ChannelProvider/SimilarItems.php
+++ b/module/VuFind/src/VuFind/ChannelProvider/SimilarItems.php
@@ -212,7 +212,7 @@ protected function buildChannelFromRecord(
         }
 
         $retVal['limit'] = $this->batchSize;
-        $params = new \VuFindSearch\ParamBag(['rows' => $this->batchSize]);
+        $params = new \VuFindSearch\ParamBag(['limit' => $this->batchSize]);
         $command = new SimilarCommand(
             $driver->getSourceIdentifier(),
             $driver->getUniqueID(),
diff --git a/module/VuFind/src/VuFind/Hierarchy/TreeDataSource/Solr.php b/module/VuFind/src/VuFind/Hierarchy/TreeDataSource/Solr.php
index ac952e829d4..f20dcb35b43 100644
--- a/module/VuFind/src/VuFind/Hierarchy/TreeDataSource/Solr.php
+++ b/module/VuFind/src/VuFind/Hierarchy/TreeDataSource/Solr.php
@@ -31,7 +31,7 @@
 
 use VuFind\Hierarchy\TreeDataFormatter\PluginManager as FormatterManager;
 use VuFindSearch\Backend\Solr\Command\RawJsonSearchCommand;
-use VuFindSearch\ParamBag;
+use VuFindSearch\NestingParamBag;
 use VuFindSearch\Query\Query;
 use VuFindSearch\Service;
 
@@ -152,12 +152,14 @@ public function getXML($id, $options = [])
     protected function getDefaultSearchParams(): array
     {
         return [
-            'fq' => $this->filters,
-            'hl' => ['false'],
-            'fl' => ['title,id,hierarchy_parent_id,hierarchy_top_id,'
+            'filter' => $this->filters,
+            'fields' => ['title,id,hierarchy_parent_id,hierarchy_top_id,'
                 . 'is_hierarchy_id,hierarchy_sequence,title_in_hierarchy'],
-            'wt' => ['json'],
-            'json.nl' => ['arrarr'],
+            'params' => [
+                'hl' => ['false'],
+                'wt' => ['json'],
+                'json.nl' => ['arrarr'],
+            ],
         ];
     }
 
@@ -172,7 +174,7 @@ protected function getDefaultSearchParams(): array
      */
     protected function searchSolrLegacy(Query $query, $rows): array
     {
-        $params = new ParamBag($this->getDefaultSearchParams());
+        $params = NestingParamBag::fromArray($this->getDefaultSearchParams());
         $command = new RawJsonSearchCommand(
             $this->backendId,
             $query,
@@ -198,16 +200,15 @@ protected function searchSolrCursor(Query $query, $rows): array
         $cursorMark = '*';
         $records = [];
         while ($cursorMark !== $prevCursorMark) {
-            $params = new ParamBag(
-                $this->getDefaultSearchParams() + [
-                    // Sort is required
-                    'sort' => ['id asc'],
-                    // Override any default timeAllowed since it cannot be used with
-                    // cursorMark
-                    'timeAllowed' => -1,
-                    'cursorMark' => $cursorMark,
-                ]
-            );
+            $params = NestingParamBag::fromArray($this->getDefaultSearchParams());
+            // Sort is required
+            $params->add('sort', 'id asc');
+            $params->addMultiNested('params', [
+                // Override any default timeAllowed since it cannot be used with
+                // cursorMark
+                'timeAllowed' => -1,
+                'cursorMark' => $cursorMark,
+            ]);
             $command = new RawJsonSearchCommand(
                 $this->backendId,
                 $query,
diff --git a/module/VuFind/src/VuFind/RecordDriver/SolrDefault.php b/module/VuFind/src/VuFind/RecordDriver/SolrDefault.php
index 645f680397a..2bc61268e98 100644
--- a/module/VuFind/src/VuFind/RecordDriver/SolrDefault.php
+++ b/module/VuFind/src/VuFind/RecordDriver/SolrDefault.php
@@ -33,6 +33,8 @@
 namespace VuFind\RecordDriver;
 
 use VuFindSearch\Command\SearchCommand;
+use VuFindSearch\NestingParamBag;
+use VuFindSearch\ParamBag;
 
 use function count;
 use function in_array;
@@ -317,7 +319,7 @@ public function getChildRecordCount()
             'hierarchy_parent_id:"' . $safeId . '"'
         );
         // Disable highlighting for efficiency; not needed here:
-        $params = new \VuFindSearch\ParamBag(['hl' => ['false']]);
+        $params = new NestingParamBag(['params' => new ParamBag(['hl' => ['false']])]);
         $command = new SearchCommand($this->sourceIdentifier, $query, 0, 0, $params);
         return $this->searchService
             ->invoke($command)->getResult()->getTotal();
diff --git a/module/VuFind/src/VuFind/Search/Blender/Params.php b/module/VuFind/src/VuFind/Search/Blender/Params.php
index 31442387a7b..2c8222c857b 100644
--- a/module/VuFind/src/VuFind/Search/Blender/Params.php
+++ b/module/VuFind/src/VuFind/Search/Blender/Params.php
@@ -446,7 +446,7 @@ public function getBackendParameters(): ParamBag
         $result = parent::getBackendParameters();
         foreach ($this->unsupportedFilters as $backendId => $filters) {
             if ($filters) {
-                $result->add('fq', "-blender_backend:\"$backendId\"");
+                $result->add('filter', "-blender_backend:\"$backendId\"");
             }
         }
         foreach ($this->searchParams as $params) {
diff --git a/module/VuFind/src/VuFind/Search/Factory/AbstractSolrBackendFactory.php b/module/VuFind/src/VuFind/Search/Factory/AbstractSolrBackendFactory.php
index 3b819062e26..c9c2fa84201 100644
--- a/module/VuFind/src/VuFind/Search/Factory/AbstractSolrBackendFactory.php
+++ b/module/VuFind/src/VuFind/Search/Factory/AbstractSolrBackendFactory.php
@@ -494,8 +494,8 @@ protected function createConnector()
         $handlers = [
             'select' => [
                 'fallback' => true,
-                'defaults' => ['fl' => $defaultFields],
-                'appends'  => ['fq' => []],
+                'defaults' => ['fields' => $defaultFields],
+                'appends'  => ['filter' => []],
             ],
             'terms' => [
                 'functions' => ['terms'],
@@ -503,7 +503,7 @@ protected function createConnector()
         ];
 
         foreach ($this->getHiddenFilters() as $filter) {
-            array_push($handlers['select']['appends']['fq'], $filter);
+            array_push($handlers['select']['appends']['filter'], $filter);
         }
 
         $connector = new $this->connectorClass(
diff --git a/module/VuFind/src/VuFind/Search/Params/FacetLimitTrait.php b/module/VuFind/src/VuFind/Search/Params/FacetLimitTrait.php
index 8618e24b3ea..a664b137684 100644
--- a/module/VuFind/src/VuFind/Search/Params/FacetLimitTrait.php
+++ b/module/VuFind/src/VuFind/Search/Params/FacetLimitTrait.php
@@ -77,10 +77,10 @@ trait FacetLimitTrait
     protected function initFacetLimitsFromConfig(?Config $config = null)
     {
         if (is_numeric($config->facet_limit ?? null)) {
-            $this->setFacetLimit($config->facet_limit);
+            $this->setFacetLimit((int)($config->facet_limit));
         }
         foreach ($config->facet_limit_by_field ?? [] as $k => $v) {
-            $this->facetLimitByField[$k] = $v;
+            $this->facetLimitByField[$k] = (int)$v;
         }
     }
 
@@ -91,7 +91,7 @@ protected function initFacetLimitsFromConfig(?Config $config = null)
      *
      * @return void
      */
-    public function setFacetLimit($l)
+    public function setFacetLimit(int $l): void
     {
         $this->facetLimit = $l;
     }
@@ -125,7 +125,7 @@ public function getHierarchicalFacetLimit()
      *
      * @return void
      */
-    public function setHierarchicalFacetLimit($limit)
+    public function setHierarchicalFacetLimit(int $limit)
     {
         $this->hierarchicalFacetLimit = $limit;
     }
diff --git a/module/VuFind/src/VuFind/Search/Params/FacetRestrictionsTrait.php b/module/VuFind/src/VuFind/Search/Params/FacetRestrictionsTrait.php
index a310b1c691b..03208b3e0fe 100644
--- a/module/VuFind/src/VuFind/Search/Params/FacetRestrictionsTrait.php
+++ b/module/VuFind/src/VuFind/Search/Params/FacetRestrictionsTrait.php
@@ -50,13 +50,6 @@ trait FacetRestrictionsTrait
      */
     protected $facetPrefixByField = [];
 
-    /**
-     * Per-field facet matches
-     *
-     * @var array
-     */
-    protected $facetMatchesByField = [];
-
     /**
      * Initialize facet prefix and matches from a Config object.
      *
@@ -69,9 +62,6 @@ protected function initFacetRestrictionsFromConfig(?Config $config = null)
         foreach ($config->facet_prefix_by_field ?? [] as $k => $v) {
             $this->facetPrefixByField[$k] = $v;
         }
-        foreach ($config->facet_matches_by_field ?? [] as $k => $v) {
-            $this->facetMatchesByField[$k] = $v;
-        }
     }
 
     /**
@@ -86,18 +76,6 @@ public function setFacetPrefixByField(array $new)
         $this->facetPrefixByField = $new;
     }
 
-    /**
-     * Set Facet Matches by Field
-     *
-     * @param array $new Associative array of $field name => $limit
-     *
-     * @return void
-     */
-    public function setFacetMatchesByField(array $new)
-    {
-        $this->facetMatchesByField = $new;
-    }
-
     /**
      * Get the facet prefix for the specified field.
      *
@@ -110,17 +88,4 @@ protected function getFacetPrefixForField($field)
         $prefix = $this->facetPrefixByField[$field] ?? '';
         return $prefix;
     }
-
-    /**
-     * Get the facet matches for the specified field.
-     *
-     * @param string $field Field to look up
-     *
-     * @return string
-     */
-    protected function getFacetMatchesForField($field)
-    {
-        $matches = $this->facetMatchesByField[$field] ?? '';
-        return $matches;
-    }
 }
diff --git a/module/VuFind/src/VuFind/Search/Solr/CustomFilterListener.php b/module/VuFind/src/VuFind/Search/Solr/CustomFilterListener.php
index 4c5b03702cf..bfe486a16a8 100644
--- a/module/VuFind/src/VuFind/Search/Solr/CustomFilterListener.php
+++ b/module/VuFind/src/VuFind/Search/Solr/CustomFilterListener.php
@@ -74,7 +74,7 @@ class CustomFilterListener
      *
      * @var string
      */
-    protected $filterParam = 'fq';
+    protected $filterParam = 'filter';
 
     /**
      * Constructor.
diff --git a/module/VuFind/src/VuFind/Search/Solr/DeduplicationListener.php b/module/VuFind/src/VuFind/Search/Solr/DeduplicationListener.php
index ed653963e50..1936e8ec4df 100644
--- a/module/VuFind/src/VuFind/Search/Solr/DeduplicationListener.php
+++ b/module/VuFind/src/VuFind/Search/Solr/DeduplicationListener.php
@@ -154,15 +154,15 @@ public function onSearchPre(EventInterface $event)
                     $this->enabled && 'getids' !== $context
                     && !$this->hasChildFilter($params)
                 ) {
-                    $fq = '-merged_child_boolean:true';
+                    $filter = '-merged_child_boolean:true';
                     if ($context == 'similar' && $id = $event->getParam('id')) {
-                        $fq .= ' AND -local_ids_str_mv:"'
+                        $filter .= ' AND -local_ids_str_mv:"'
                             . addcslashes($id, '"') . '"';
                     }
                 } else {
-                    $fq = '-merged_boolean:true';
+                    $filter = '-merged_boolean:true';
                 }
-                $params->add('fq', $fq);
+                $params->add('filter', $filter);
             }
         }
         return $event;
@@ -177,7 +177,7 @@ public function onSearchPre(EventInterface $event)
      */
     public function hasChildFilter($params)
     {
-        $filters = $params->get('fq');
+        $filters = $params->get('filter');
         return $filters != null && in_array('merged_child_boolean:true', $filters);
     }
 
diff --git a/module/VuFind/src/VuFind/Search/Solr/DefaultParametersListener.php b/module/VuFind/src/VuFind/Search/Solr/DefaultParametersListener.php
index 89ce70a75c2..e588300f0c8 100644
--- a/module/VuFind/src/VuFind/Search/Solr/DefaultParametersListener.php
+++ b/module/VuFind/src/VuFind/Search/Solr/DefaultParametersListener.php
@@ -29,9 +29,11 @@
 
 namespace VuFind\Search\Solr;
 
+use JsonException;
 use Laminas\EventManager\EventInterface;
 use Laminas\EventManager\SharedEventManagerInterface;
 use VuFindSearch\Backend\Solr\Backend;
+use VuFindSearch\NestingParamBag;
 use VuFindSearch\Service;
 
 /**
@@ -119,16 +121,21 @@ public function onSearchPre(EventInterface $event): EventInterface
                 $context = null;
             }
             $context = $this->contextMap[$context] ?? $context;
-            $defaultParams = $this->defaultParams[$context]
+            $defaultParamsText = $this->defaultParams[$context]
                 ?? $this->defaultParams['*']
                 ?? '';
-            if ($defaultParams && $params = $command->getSearchParameters()) {
-                foreach (explode('&', $defaultParams) as $keyVal) {
-                    $parts = explode('=', $keyVal, 2);
-                    if (!isset($parts[1])) {
-                        continue;
-                    }
-                    $params->add(urldecode($parts[0]), urldecode($parts[1]));
+            if ($defaultParamsText && $params = NestingParamBag::from($command->getSearchParameters())) {
+                $command->setSearchParameters($params);
+                try {
+                    $defaultParams = json_decode($defaultParamsText, true, flags: JSON_THROW_ON_ERROR);
+                } catch (JsonException $e) {
+                    throw new \Exception(
+                        'Default parameters must be expressed in JSON, using Solr\'s JSON Request API '
+                        . '(starting in VuFind 12)'
+                    );
+                }
+                foreach ($defaultParams as $name => $param) {
+                    $params->addMultiNested($name, $param);
                 }
             }
         }
diff --git a/module/VuFind/src/VuFind/Search/Solr/Explanation.php b/module/VuFind/src/VuFind/Search/Solr/Explanation.php
index a639d167e1f..8b857542259 100644
--- a/module/VuFind/src/VuFind/Search/Solr/Explanation.php
+++ b/module/VuFind/src/VuFind/Search/Solr/Explanation.php
@@ -31,6 +31,7 @@
 namespace VuFind\Search\Solr;
 
 use VuFindSearch\Backend\Solr\Command\RawJsonSearchCommand;
+use VuFindSearch\NestingParamBag;
 use VuFindSearch\ParamBag;
 
 use function count;
@@ -249,13 +250,16 @@ public function performRequest($recordId)
 
         // prepare search params
         $params = $this->getParams()->getBackendParameters();
-        $params->set('spellcheck', 'false');
-        $explainParams = new ParamBag([
-            'fl' => 'id,score',
-            'debug' => 'true',
-            'indent' => 'true',
-            'echoParams' => 'all',
-            'explainOther' => 'id:"' . addcslashes($recordId, '"') . '"',
+        $params = NestingParamBag::from($params);
+        $params->setNested('params', 'spellcheck', 'false');
+        $explainParams = new NestingParamBag([
+            'fields' => 'id,score',
+            'params' => new ParamBag([
+                'debug' => 'true',
+                'indent' => 'true',
+                'echoParams' => 'all',
+                'explainOther' => 'id:"' . addcslashes($recordId, '"') . '"',
+            ]),
         ]);
         $params->mergeWith($explainParams);
 
diff --git a/module/VuFind/src/VuFind/Search/Solr/FilterFieldConversionListener.php b/module/VuFind/src/VuFind/Search/Solr/FilterFieldConversionListener.php
index febfa19f488..d60259e8f86 100644
--- a/module/VuFind/src/VuFind/Search/Solr/FilterFieldConversionListener.php
+++ b/module/VuFind/src/VuFind/Search/Solr/FilterFieldConversionListener.php
@@ -91,12 +91,12 @@ public function attach(SharedEventManagerInterface $manager)
     public function onSearchPre(EventInterface $event)
     {
         $params = $event->getParam('command')->getSearchParameters();
-        $fq = $params->get('fq');
-        if (is_array($fq) && !empty($fq)) {
+        $filters = $params->get('filter');
+        if (is_array($filters) && !empty($filters)) {
             // regex lookahead to ignore strings inside quotes:
             $lookahead = '(?=(?:[^\"]*+\"[^\"]*+\")*+[^\"]*+$)';
-            $new_fq = [];
-            foreach ($fq as $currentFilter) {
+            $new_filters = [];
+            foreach ($filters as $currentFilter) {
                 foreach ($this->map as $oldField => $newField) {
                     $currentFilter = preg_replace(
                         "/\b$oldField:$lookahead/",
@@ -104,9 +104,9 @@ public function onSearchPre(EventInterface $event)
                         $currentFilter
                     );
                 }
-                $new_fq[] = $currentFilter;
+                $new_filters[] = $currentFilter;
             }
-            $params->set('fq', $new_fq);
+            $params->set('filter', $new_filters);
         }
 
         return $event;
diff --git a/module/VuFind/src/VuFind/Search/Solr/InjectConditionalFilterListener.php b/module/VuFind/src/VuFind/Search/Solr/InjectConditionalFilterListener.php
index 47470d9938b..3f31ef4b35a 100644
--- a/module/VuFind/src/VuFind/Search/Solr/InjectConditionalFilterListener.php
+++ b/module/VuFind/src/VuFind/Search/Solr/InjectConditionalFilterListener.php
@@ -147,12 +147,12 @@ public function onSearchPre(EventInterface $event)
         }
 
         $params = $command->getSearchParameters();
-        $fq = $params->get('fq');
-        if (!is_array($fq)) {
-            $fq = [];
+        $filters = $params->get('filter');
+        if (!is_array($filters)) {
+            $filters = [];
         }
-        $new_fq = array_merge($fq, $this->filterList);
-        $params->set('fq', $new_fq);
+        $new_filters = array_merge($filters, $this->filterList);
+        $params->set('filter', $new_filters);
 
         return $event;
     }
diff --git a/module/VuFind/src/VuFind/Search/Solr/InjectHighlightingListener.php b/module/VuFind/src/VuFind/Search/Solr/InjectHighlightingListener.php
index 0e286f545c2..aafa3b016e1 100644
--- a/module/VuFind/src/VuFind/Search/Solr/InjectHighlightingListener.php
+++ b/module/VuFind/src/VuFind/Search/Solr/InjectHighlightingListener.php
@@ -32,6 +32,7 @@
 use Laminas\EventManager\EventInterface;
 use Laminas\EventManager\SharedEventManagerInterface;
 use VuFindSearch\Backend\BackendInterface;
+use VuFindSearch\NestingParamBag;
 use VuFindSearch\Service;
 
 /**
@@ -120,18 +121,19 @@ public function onSearchPre(EventInterface $event)
         }
         if ($command->getTargetIdentifier() === $this->backend->getIdentifier()) {
             if ($params = $command->getSearchParameters()) {
+                $params = NestingParamBag::from($params);
                 // Set highlighting parameters unless explicitly disabled:
-                $hl = $params->get('hl');
+                $hl = $params->getNested('params', 'hl');
                 if (($hl[0] ?? 'true') != 'false') {
                     $this->active = true;
                     // Set extra parameters first so they don't override necessary
                     // core parameters:
                     foreach ($this->extraHighlightingParameters as $key => $val) {
-                        $params->set($key, $val);
+                        $params->addMultiNested($key, $val);
                     }
-                    $params->set('hl', 'true');
-                    $params->set('hl.simple.pre', '{{{{START_HILITE}}}}');
-                    $params->set('hl.simple.post', '{{{{END_HILITE}}}}');
+                    $params->setNested('params', 'hl', 'true');
+                    $params->setNested('params', 'hl.simple.pre', '{{{{START_HILITE}}}}');
+                    $params->setNested('params', 'hl.simple.post', '{{{{END_HILITE}}}}');
 
                     // Turn on hl.q generation in query builder:
                     $this->backend->getQueryBuilder()
diff --git a/module/VuFind/src/VuFind/Search/Solr/InjectSpellingListener.php b/module/VuFind/src/VuFind/Search/Solr/InjectSpellingListener.php
index bee6156eeeb..d7cc8a74a7f 100644
--- a/module/VuFind/src/VuFind/Search/Solr/InjectSpellingListener.php
+++ b/module/VuFind/src/VuFind/Search/Solr/InjectSpellingListener.php
@@ -35,7 +35,7 @@
 use VuFind\Log\LoggerAwareTrait;
 use VuFindSearch\Backend\BackendInterface;
 use VuFindSearch\Backend\Solr\Response\Json\Spellcheck;
-use VuFindSearch\ParamBag;
+use VuFindSearch\NestingParamBag;
 use VuFindSearch\Query\Query;
 use VuFindSearch\Service;
 
@@ -128,8 +128,10 @@ public function onSearchPre(EventInterface $event)
         }
         if ($command->getTargetIdentifier() === $this->backend->getIdentifier()) {
             if ($params = $command->getSearchParameters()) {
+                $params = NestingParamBag::from($params);
+
                 // Set spelling parameters when enabled:
-                $sc = $params->get('spellcheck');
+                $sc = $params->getNested('params', 'spellcheck');
                 if (isset($sc[0]) && $sc[0] != 'false') {
                     $this->active = true;
                     if (empty($this->dictionaries)) {
@@ -140,8 +142,9 @@ public function onSearchPre(EventInterface $event)
 
                     // Set relevant Solr parameters:
                     reset($this->dictionaries);
-                    $params->set('spellcheck', 'true');
-                    $params->set(
+                    $params->setNested('params', 'spellcheck', 'true');
+                    $params->setNested(
+                        'params',
                         'spellcheck.dictionary',
                         current($this->dictionaries)
                     );
@@ -175,7 +178,8 @@ public function onSearchPost(EventInterface $event)
         if ($command->getTargetIdentifier() === $this->backend->getIdentifier()) {
             $result = $command->getResult();
             $params = $command->getSearchParameters();
-            $spellcheckQuery = $params->get('spellcheck.q');
+            $params = NestingParamBag::from($params);
+            $spellcheckQuery = $params->getNested('params', 'spellcheck.q');
             if (!empty($spellcheckQuery)) {
                 $this->aggregateSpellcheck(
                     $result->getSpellcheck(),
@@ -197,9 +201,9 @@ public function onSearchPost(EventInterface $event)
     protected function aggregateSpellcheck(Spellcheck $spellcheck, $query)
     {
         while (next($this->dictionaries) !== false) {
-            $params = new ParamBag();
-            $params->set('spellcheck', 'true');
-            $params->set('spellcheck.dictionary', current($this->dictionaries));
+            $params = new NestingParamBag();
+            $params->setNested('params', 'spellcheck', 'true');
+            $params->setNested('params', 'spellcheck.dictionary', current($this->dictionaries));
             $queryObj = new Query($query, 'AllFields');
             try {
                 $collection = $this->backend->search($queryObj, 0, 0, $params);
diff --git a/module/VuFind/src/VuFind/Search/Solr/MultiIndexListener.php b/module/VuFind/src/VuFind/Search/Solr/MultiIndexListener.php
index cee2cbb8f27..745caf73c60 100644
--- a/module/VuFind/src/VuFind/Search/Solr/MultiIndexListener.php
+++ b/module/VuFind/src/VuFind/Search/Solr/MultiIndexListener.php
@@ -32,6 +32,8 @@
 use Laminas\EventManager\EventInterface;
 use Laminas\EventManager\SharedEventManagerInterface;
 use VuFindSearch\Backend\BackendInterface;
+use VuFindSearch\NestingParamBag;
+use VuFindSearch\ParamBag;
 use VuFindSearch\Service;
 
 use function in_array;
@@ -126,11 +128,12 @@ public function onSearchPre(EventInterface $event)
         $command = $event->getParam('command');
         if ($command->getTargetIdentifier() === $this->backend->getIdentifier()) {
             $params = $command->getSearchParameters();
+            $params = NestingParamBag::from($params);
             $allShardsContexts = ['retrieve', 'retrieveBatch'];
             if (in_array($command->getContext(), $allShardsContexts)) {
                 // If we're retrieving by id(s), we should pull all shards to be
                 // sure we find the right record(s).
-                $params->set('shards', implode(',', $this->shards));
+                $params->setNested('params', 'shards', implode(',', $this->shards));
             } else {
                 // In any other context, we should make sure our field values are
                 // all legal.
@@ -138,7 +141,7 @@ public function onSearchPre(EventInterface $event)
                 // Normalize array of strings containing comma-separated values to
                 // simple array of values; check if $params->get('shards') returns
                 // an array to prevent invalid argument warnings.
-                $shards = $params->get('shards');
+                $shards = $params->getNested('params', 'shards');
                 $shards = explode(
                     ',',
                     implode(',', (is_array($shards) ? $shards : []))
@@ -146,8 +149,16 @@ public function onSearchPre(EventInterface $event)
                 $fields = $this->getFields($shards);
                 $specs  = $this->getSearchSpecs($fields);
                 $this->backend->getQueryBuilder()->setSpecs($specs);
-                $facets = $params->get('facet.field') ?: [];
-                $params->set('facet.field', array_diff($facets, $fields));
+                if ($params->get('facet')) {
+                    $facets = $params->get('facet')[0]->getArrayCopy();
+                    $facets = array_filter(
+                        $facets,
+                        fn ($facet) =>
+                            (!($facet[0] instanceof ParamBag))
+                            || !in_array($facet[0]->getArrayCopy()['field'][0], $fields)
+                    );
+                    $params->set('facet', new ParamBag($facets));
+                }
             }
         }
         return $event;
diff --git a/module/VuFind/src/VuFind/Search/Solr/Params.php b/module/VuFind/src/VuFind/Search/Solr/Params.php
index f4136e8942d..3f1e0bcf95c 100644
--- a/module/VuFind/src/VuFind/Search/Solr/Params.php
+++ b/module/VuFind/src/VuFind/Search/Solr/Params.php
@@ -32,7 +32,7 @@
 
 use VuFind\Config\Config;
 use VuFind\Config\ConfigManagerInterface;
-use VuFindSearch\ParamBag;
+use VuFindSearch\NestingParamBag;
 
 use function count;
 use function in_array;
@@ -58,17 +58,17 @@ class Params extends \VuFind\Search\Base\Params
      * Search with facet.contains
      * cf. https://lucene.apache.org/solr/guide/7_3/faceting.html
      *
-     * @var string
+     * @var ?string
      */
-    protected $facetContains = null;
+    protected ?string $facetContains = null;
 
     /**
      * Ignore Case when using facet.contains
      * cf. https://lucene.apache.org/solr/guide/7_3/faceting.html
      *
-     * @var bool
+     * @var ?bool
      */
-    protected $facetContainsIgnoreCase = null;
+    protected ?bool $facetContainsIgnoreCase = null;
 
     /**
      * Offset for facet results
@@ -237,20 +237,8 @@ public function getFacetSettings()
 
         if (!empty($this->facetConfig)) {
             $dateRangeTypes = $this->getOptions()->getDateRangeFieldTypes();
-            $facetSet['limit'] = $this->facetLimit;
             foreach (array_keys($this->facetConfig) as $facetField) {
-                $fieldLimit = $this->getFacetLimitForField($facetField);
-                if ($fieldLimit != $this->facetLimit) {
-                    $facetSet["f.{$facetField}.facet.limit"] = $fieldLimit;
-                }
-                $fieldPrefix = $this->getFacetPrefixForField($facetField);
-                if (!empty($fieldPrefix)) {
-                    $facetSet["f.{$facetField}.facet.prefix"] = $fieldPrefix;
-                }
-                $fieldMatches = $this->getFacetMatchesForField($facetField);
-                if (!empty($fieldMatches)) {
-                    $facetSet["f.{$facetField}.facet.matches"] = $fieldMatches;
-                }
+                // TODO Figure out date range field
                 if ('DateRangeField' === ($dateRangeTypes[$facetField] ?? null)) {
                     $startYear = $this->getOptions()->getDateRangeSliderMinValue($facetField)
                         ?? VUFIND_DEFAULT_EARLIEST_YEAR;
@@ -263,29 +251,36 @@ public function getFacetSettings()
                     $facetSet["f.{$facetField}.facet.range.gap"] = '+1YEAR';
                     $facetSet['range'][] = $facetField;
                 } else {
+                    $facetFieldName = $facetField;
+                    $fieldLimit = $this->getFacetLimitForField($facetField);
+
+                    $facet = [
+                        'type' => 'terms',
+                        'field' => $facetField,
+                        'limit' => $fieldLimit,
+                    ];
+
+                    $facet['sort'] = in_array($facetField, $this->indexSortedFacets ?? [])
+                        ? 'index'
+                        : ($this->facetSort ?: 'count');
+
+                    $fieldPrefix = $this->getFacetPrefixForField($facetField);
+                    if (!empty($fieldPrefix)) {
+                        $facet['prefix'] = $fieldPrefix;
+                    } elseif ($this->facetPrefix != null) {
+                        $facet['prefix'] = $this->facetPrefix;
+                    }
+
+                    if ($this->facetOffset != null) {
+                        $facet['offset'] = $this->facetOffset;
+                    }
+
                     if ($this->getFacetOperator($facetField) == 'OR') {
-                        $facetField = '{!ex=' . $facetField . '_filter}' . $facetField;
+                        $facet['domain'] ??= [];
+                        $facet['domain']['excludeTags'] = $facetField . '_filter';
                     }
-                    $facetSet['field'][] = $facetField;
-                }
-            }
-            if ($this->facetContains != null) {
-                $facetSet['contains'] = $this->facetContains;
-            }
-            if ($this->facetContainsIgnoreCase != null) {
-                $facetSet['contains.ignoreCase']
-                    = $this->facetContainsIgnoreCase ? 'true' : 'false';
-            }
-            if ($this->facetOffset != null) {
-                $facetSet['offset'] = $this->facetOffset;
-            }
-            if ($this->facetPrefix != null) {
-                $facetSet['prefix'] = $this->facetPrefix;
-            }
-            $facetSet['sort'] = $this->facetSort ?: 'count';
-            if ($this->indexSortedFacets != null) {
-                foreach ($this->indexSortedFacets as $field) {
-                    $facetSet["f.{$field}.facet.sort"] = 'index';
+
+                    $facetSet[$facetFieldName] = $facet;
                 }
             }
         }
@@ -337,6 +332,16 @@ public function setFacetContains($p)
         $this->facetContains = $p;
     }
 
+    /**
+     * Get Facet Contains
+     *
+     * @return ?string The contains value
+     */
+    public function getFacetContains()
+    {
+        return $this->facetContains;
+    }
+
     /**
      * Set Facet Contains Ignore Case
      *
@@ -349,6 +354,16 @@ public function setFacetContainsIgnoreCase($val)
         $this->facetContainsIgnoreCase = $val;
     }
 
+    /**
+     * Get Facet Contains Ignore Case
+     *
+     * @return ?bool The boolean value
+     */
+    public function getFacetContainsIgnoreCase()
+    {
+        return $this->facetContainsIgnoreCase;
+    }
+
     /**
      * Set Facet Offset
      *
@@ -567,14 +582,15 @@ protected function normalizeSort($sort)
     /**
      * Create search backend parameters for advanced features.
      *
-     * @return ParamBag
+     * @return NestingParamBag
      */
     public function getBackendParameters()
     {
-        $backendParams = new ParamBag();
+        $backendParams = new NestingParamBag();
 
         // Spellcheck
-        $backendParams->set(
+        $backendParams->setNested(
+            'params',
             'spellcheck',
             $this->getOptions()->spellcheckEnabled() ? 'true' : 'false'
         );
@@ -582,20 +598,13 @@ public function getBackendParameters()
         // Facets
         $facets = $this->getFacetSettings();
         if (!empty($facets)) {
-            $backendParams->add('facet', 'true');
-
-            foreach ($facets as $key => $value) {
-                // prefix keys with "facet" unless they already have a "f." prefix:
-                $fullKey = str_starts_with($key, 'f.') ? $key : "facet.$key";
-                $backendParams->add($fullKey, $value);
-            }
-            $backendParams->add('facet.mincount', 1);
+            $backendParams->addMultiNested('facet', $facets);
         }
 
         // Filters
         $filters = $this->getFilterSettings();
         foreach ($filters as $filter) {
-            $backendParams->add('fq', $filter);
+            $backendParams->add('filter', $filter);
         }
 
         // Shards
@@ -611,7 +620,7 @@ public function getBackendParameters()
             foreach ($shards as $current) {
                 $selectedShards[$current] = $allShards[$current];
             }
-            $backendParams->add('shards', implode(',', $selectedShards));
+            $backendParams->addNested('params', 'shards', implode(',', $selectedShards));
         }
 
         // Sort
@@ -633,14 +642,14 @@ public function getBackendParameters()
 
         // Highlighting -- on by default, but we should disable if necessary:
         if (!$this->getOptions()->highlightEnabled()) {
-            $backendParams->add('hl', 'false');
+            $backendParams->addNested('params', 'hl', 'false');
         }
 
         // Pivot facets for visual results
 
         if ($pf = $this->getPivotFacets()) {
-            $backendParams->add('facet.pivot', $pf);
-            $backendParams->set('facet', 'true');
+            $backendParams->addNested('params', 'facet.pivot', $pf);
+            $backendParams->setNested('params', 'facet', 'true');
         }
 
         return $backendParams;
diff --git a/module/VuFind/src/VuFind/Search/Solr/Results.php b/module/VuFind/src/VuFind/Search/Solr/Results.php
index f0041be3cc5..c5612ea727c 100644
--- a/module/VuFind/src/VuFind/Search/Solr/Results.php
+++ b/module/VuFind/src/VuFind/Search/Solr/Results.php
@@ -30,6 +30,7 @@
 namespace VuFind\Search\Solr;
 
 use VuFindSearch\Command\SearchCommand;
+use VuFindSearch\NestingParamBag;
 use VuFindSearch\Query\AbstractQuery;
 use VuFindSearch\Query\QueryGroup;
 
@@ -194,13 +195,14 @@ protected function performSearch()
         $limit  = $this->getParams()->getLimit();
         $offset = $this->getStartRecord() - 1;
         $params = $this->getParams()->getBackendParameters();
+        $params = NestingParamBag::from($params);
         $searchService = $this->getSearchService();
         $cursorMark = $this->getCursorMark();
         if (null !== $cursorMark) {
-            $params->set('cursorMark', '' === $cursorMark ? '*' : $cursorMark);
+            $params->setNested('params', 'cursorMark', '' === $cursorMark ? '*' : $cursorMark);
             // Override any default timeAllowed since it cannot be used with
             // cursorMark
-            $params->set('timeAllowed', -1);
+            $params->setNested('params', 'timeAllowed', -1);
         }
 
         try {
@@ -432,9 +434,22 @@ public function getPartialFieldFacets(
 
         // Do search
         $result = $clone->getFacetList();
-        $filteredCounts = $clone->getFilteredFacetCounts();
+
+        // Apply "facet contains"
+        if ($facetContains = $params->getFacetContains()) {
+            $ignoreCase = $params->getFacetContainsIgnoreCase();
+            foreach ($result as &$values) {
+                $values['list'] = array_filter(
+                    $values['list'],
+                    fn ($value) => $ignoreCase
+                        ? stripos($value['value'], $facetContains) !== false
+                        : str_contains($value['value'], $facetContains)
+                );
+            }
+        }
 
         // Reformat into a hash:
+        $filteredCounts = $clone->getFilteredFacetCounts();
         foreach ($result as $key => $value) {
             // Detect next page and crop results if necessary
             $more = false;
diff --git a/module/VuFind/src/VuFindTest/Feature/MockSearchCommandTrait.php b/module/VuFind/src/VuFindTest/Feature/MockSearchCommandTrait.php
index 584cfaa1b5e..98bab166025 100644
--- a/module/VuFind/src/VuFindTest/Feature/MockSearchCommandTrait.php
+++ b/module/VuFind/src/VuFindTest/Feature/MockSearchCommandTrait.php
@@ -59,12 +59,20 @@ protected function getMockSearchCommand(
         string $backendId = 'foo',
         $result = null
     ): Command {
+        $storedParams = $params;
         $command = $this->getMockBuilder(Command::class)
             ->disableOriginalConstructor()
             ->getMock();
-        if ($params) {
-            $command->expects($this->any())->method('getSearchParameters')->willReturn($params);
-        }
+        $command->expects($this->any())->method('getSearchParameters')->willReturnCallback(
+            function () use (&$storedParams) {
+                return $storedParams;
+            }
+        );
+        $command->expects($this->any())->method('setSearchParameters')->willReturnCallback(
+            function ($params) use (&$storedParams): void {
+                $storedParams = $params;
+            }
+        );
         if ($context) {
             $command->expects($this->any())->method('getContext')->willReturn($context);
         }
diff --git a/module/VuFind/tests/unit-tests/src/VuFindTest/ChannelProvider/SimilarItemsTest.php b/module/VuFind/tests/unit-tests/src/VuFindTest/ChannelProvider/SimilarItemsTest.php
index 5fd1547809a..18bad6d9c9c 100644
--- a/module/VuFind/tests/unit-tests/src/VuFindTest/ChannelProvider/SimilarItemsTest.php
+++ b/module/VuFind/tests/unit-tests/src/VuFindTest/ChannelProvider/SimilarItemsTest.php
@@ -53,7 +53,7 @@ class SimilarItemsTest extends \PHPUnit\Framework\TestCase
      */
     public function testGetFromRecord(): void
     {
-        [$similar, $expectedResult] = $this->configureTestTargetAndExpectations(['rows' => 24]);
+        [$similar, $expectedResult] = $this->configureTestTargetAndExpectations(['limit' => 24]);
         $recordDriver = $this->getDriver();
         $this->assertSame($expectedResult, $similar->getFromRecord($recordDriver));
     }
@@ -82,7 +82,7 @@ public function testGetFromSearch(): void
         $recordDriver = $this->getDriver();
         $results->expects($this->once())->method('getResults')
             ->willReturn([$recordDriver]);
-        [$similar, $expectedResult] = $this->configureTestTargetAndExpectations(['rows' => 24]);
+        [$similar, $expectedResult] = $this->configureTestTargetAndExpectations(['limit' => 24]);
         $this->assertSame($expectedResult, $similar->getFromSearch($results));
     }
 
@@ -122,7 +122,7 @@ public function testGetFromSearchWhenChannelsIsEmpty(): void
         $results->expects($this->once())->method('getResults')
             ->willReturn([$recordDriver]);
         [$similar, $expectedResult]  = $this->configureTestTargetAndExpectations(
-            ['maxRecordsToExamine' => 0, 'rows' => 24],
+            ['maxRecordsToExamine' => 0, 'limit' => 24],
             true
         );
         $this->assertSame(
@@ -145,14 +145,14 @@ public function configureTestTargetAndExpectations(
         $options = [],
         $fetchFromSearchService = false
     ) {
-        $options = array_merge(['maxRecordsToExamine' => 1, 'rows' => 20], $options);
+        $options = array_merge(['maxRecordsToExamine' => 1, 'limit' => 20], $options);
         $mockObjects = $this->getSimilarItems($options);
         $similar = $mockObjects['similar'];
         $search = $mockObjects['search'];
         $url = $mockObjects['url'];
         $router = $mockObjects['router'];
         $similar->setProviderId('foo_ProviderId');
-        $params = new ParamBag(['rows' => $options['rows']]);
+        $params = new ParamBag(['limit' => $options['limit']]);
         $retrieveParams = new ParamBag();
         $commandObj = $this->createMock(\VuFindSearch\Command\AbstractBase::class);
         $collection = $this->createMock(\VuFindSearch\Response\RecordCollectionInterface::class);
diff --git a/module/VuFind/tests/unit-tests/src/VuFindTest/Recommend/RandomRecommendTest.php b/module/VuFind/tests/unit-tests/src/VuFindTest/Recommend/RandomRecommendTest.php
index f49b09556c3..e512b139bb6 100644
--- a/module/VuFind/tests/unit-tests/src/VuFindTest/Recommend/RandomRecommendTest.php
+++ b/module/VuFind/tests/unit-tests/src/VuFindTest/Recommend/RandomRecommendTest.php
@@ -164,10 +164,13 @@ public function testCanInitialise()
                 && $command->getTargetIdentifier() === 'Solr'
                 && $command->getArguments()[0]->getAllTerms() === 'john smith'
                 && $command->getArguments()[1] === 10
-                && $command->getArguments()[2]->getArrayCopy() ===
-                    ['spellcheck' => ['true'],
-                    'fq' => ['facet1:"value1"', 'facet2:"value2"'],
-                    'hl' => ['false']];
+                && $command->getArguments()[2]->jsonSerialize() == [
+                        'params' => [
+                            'spellcheck' => 'true',
+                            'hl' => 'false',
+                        ],
+                        'filter' => ['facet1:"value1"', 'facet2:"value2"'],
+                    ];
         };
         $service->expects($this->once())->method('invoke')
             ->with($this->callback($checkCommand))
@@ -208,10 +211,14 @@ public function testCanInitialiseInDisregardMode()
                 && $command->getTargetIdentifier() === 'Solr'
                 && $command->getArguments()[0]->getAllTerms() === 'john smith'
                 && $command->getArguments()[1] === 10
-                && $command->getArguments()[2]->getArrayCopy() ===
-                    ['spellcheck' => ['true'],
-                    'fq' => ['facet1:"value1"',
-                    'facet2:"value2"'], 'hl' => ['false']];
+                && $command->getArguments()[2]->jsonSerialize() ==
+                    [
+                        'params' => [
+                            'spellcheck' => 'true',
+                            'hl' => 'false',
+                        ],
+                        'filter' => ['facet1:"value1"', 'facet2:"value2"'],
+                    ];
         };
         $service->expects($this->once())->method('invoke')
             ->with($this->callback($checkCommand))
@@ -255,10 +262,14 @@ protected function getConfiguredModule($recConfig): Random
                 && $command->getTargetIdentifier() === 'Solr'
                 && $command->getArguments()[0]->getAllTerms() === 'john smith'
                 && $command->getArguments()[1] === 10
-                && $command->getArguments()[2]->getArrayCopy() ===
-                    ['spellcheck' => ['true'],
-                    'fq' => ['facet1:"value1"',
-                    'facet2:"value2"'], 'hl' => ['false']];
+                && $command->getArguments()[2]->jsonSerialize() ==
+                    [
+                        'params' => [
+                            'spellcheck' => 'true',
+                            'hl' => 'false',
+                        ],
+                        'filter' => ['facet1:"value1"', 'facet2:"value2"'],
+                    ];
         };
         $service->expects($this->once())->method('invoke')
             ->with($this->callback($checkCommand))
diff --git a/module/VuFind/tests/unit-tests/src/VuFindTest/Search/Blender/ParamsTest.php b/module/VuFind/tests/unit-tests/src/VuFindTest/Search/Blender/ParamsTest.php
index d4be227f282..1ce560221cf 100644
--- a/module/VuFind/tests/unit-tests/src/VuFindTest/Search/Blender/ParamsTest.php
+++ b/module/VuFind/tests/unit-tests/src/VuFindTest/Search/Blender/ParamsTest.php
@@ -307,7 +307,7 @@ public function testFacetsAndFilters(): void
                 'format:"bar"',
                 '-blender_backend:"EDS"',
             ],
-            $backendParams->get('fq')
+            $backendParams->get('filter')
         );
 
         $solrParams = $backendParams->get('params_Solr')[0];
@@ -317,7 +317,7 @@ public function testFacetsAndFilters(): void
 
         $this->assertEquals(
             ['formatSolr:"bar"'],
-            $solrParams->get('fq')
+            $solrParams->get('filter')
         );
         $this->assertEquals(
             [
@@ -338,7 +338,7 @@ public function testFacetsAndFilters(): void
         // Remove a filter and check that EDS is enabled again:
         $params->removeFilter('format:bar');
         $backendParams = $params->getBackendParameters();
-        $this->assertNull($backendParams->get('fq'));
+        $this->assertNull($backendParams->get('filter'));
 
         // Add multiple filters:
         $params->addFilter('~format:bar');
@@ -357,7 +357,7 @@ public function testFacetsAndFilters(): void
                 '{!tag=formatSolr_filter}formatSolr:(formatSolr:"bar"'
                     . ' OR formatSolr:"baz")',
             ],
-            $solrParams->get('fq')
+            $solrParams->get('filter')
         );
         $this->assertEquals(
             [
@@ -383,12 +383,12 @@ public function testFacetsAndFilters(): void
         $params->removeFilter('~format:bar');
         $params->removeFilter('~format:baz');
         $backendParams = $params->getBackendParameters();
-        $this->assertEquals(['fulltext:"1"'], $backendParams->get('fq'));
+        $this->assertEquals(['fulltext:"1"'], $backendParams->get('filter'));
 
         $solrParams = $backendParams->get('params_Solr')[0];
         $primoParams = $backendParams->get('params_Primo')[0];
         $edsParams = $backendParams->get('params_EDS')[0];
-        $this->assertEquals(['fulltext_boolean:"1"'], $solrParams->get('fq'));
+        $this->assertEquals(['fulltext_boolean:"1"'], $solrParams->get('filter'));
         $this->assertEquals(
             [
                 [
@@ -404,12 +404,12 @@ public function testFacetsAndFilters(): void
         $params->removeAllFilters('building');
         $params->removeFilter('fulltext:1');
         $backendParams = $params->getBackendParameters();
-        $this->assertNull($backendParams->get('fq'));
+        $this->assertNull($backendParams->get('filter'));
 
         $solrParams = $backendParams->get('params_Solr')[0];
         $primoParams = $backendParams->get('params_Primo')[0];
         $edsParams = $backendParams->get('params_EDS')[0];
-        $this->assertNull($solrParams->get('fq'));
+        $this->assertNull($solrParams->get('filter'));
         $this->assertEquals(
             [
                 [
@@ -426,12 +426,12 @@ public function testFacetsAndFilters(): void
         $params = $this->getParams();
         $params->addFilter('fulltext:1');
         $backendParams = $params->getBackendParameters();
-        $this->assertEquals(['fulltext:"1"'], $backendParams->get('fq'));
+        $this->assertEquals(['fulltext:"1"'], $backendParams->get('filter'));
 
         $solrParams = $backendParams->get('params_Solr')[0];
         $primoParams = $backendParams->get('params_Primo')[0];
         $edsParams = $backendParams->get('params_EDS')[0];
-        $this->assertEquals(['fulltext_boolean:"1"'], $solrParams->get('fq'));
+        $this->assertEquals(['fulltext_boolean:"1"'], $solrParams->get('filter'));
         $this->assertEquals(
             [
                 [
@@ -455,7 +455,7 @@ public function testFacetsAndFilters(): void
                 '-blender_backend:"Primo"',
                 '-blender_backend:"EDS"',
             ],
-            $backendParams->get('fq')
+            $backendParams->get('filter')
         );
 
         // Add a filter that maps to two values:
@@ -485,7 +485,7 @@ public function testFacetsAndFilters(): void
         $params->addFilter('-format:double');
         $backendParams = $params->getBackendParameters();
         $solrParams = $backendParams->get('params_Solr')[0];
-        $this->assertEquals(['-formatSolr:"double"'], $solrParams->get('fq'));
+        $this->assertEquals(['-formatSolr:"double"'], $solrParams->get('filter'));
         $primoParams = $backendParams->get('params_Primo')[0];
         $this->assertEquals(
             [
@@ -510,7 +510,7 @@ public function testFacetsAndFilters(): void
         $params->removeFilter('-format:double');
         $backendParams = $params->getBackendParameters();
         $solrParams = $backendParams->get('params_Solr')[0];
-        $this->assertEquals(null, $solrParams->get('fq'));
+        $this->assertEquals(null, $solrParams->get('filter'));
         $primoParams = $backendParams->get('params_Primo')[0];
         $this->assertEquals(
             [
@@ -528,12 +528,12 @@ public function testFacetsAndFilters(): void
         $params->addFilter('format:double');
         $params->removeAllFilters();
         $backendParams = $params->getBackendParameters();
-        $this->assertNull($backendParams->get('fq'));
+        $this->assertNull($backendParams->get('filter'));
 
         $solrParams = $backendParams->get('params_Solr')[0];
         $primoParams = $backendParams->get('params_Primo')[0];
         $edsParams = $backendParams->get('params_EDS')[0];
-        $this->assertNull($solrParams->get('fq'));
+        $this->assertNull($solrParams->get('filter'));
         $this->assertEquals(
             [
                 [
@@ -555,7 +555,7 @@ public function testFacetsAndFilters(): void
             [
                 'fulltext:"1"',
             ],
-            $backendParams->get('fq')
+            $backendParams->get('filter')
         );
 
         // Test a daterange filter:
@@ -564,13 +564,13 @@ public function testFacetsAndFilters(): void
         $backendParams = $params->getBackendParameters();
         $this->assertEquals(
             ['publish_date:[2020 TO 2022]'],
-            $backendParams->get('fq')
+            $backendParams->get('filter')
         );
 
         $solrParams = $backendParams->get('params_Solr')[0];
         $primoParams = $backendParams->get('params_Primo')[0];
         $edsParams = $backendParams->get('params_EDS')[0];
-        $this->assertEquals(['publishDate:[2020 TO 2022]'], $solrParams->get('fq'));
+        $this->assertEquals(['publishDate:[2020 TO 2022]'], $solrParams->get('filter'));
         $this->assertEquals(
             [
                 [
@@ -609,7 +609,7 @@ public function testHiddenFilters(): void
                 'format:"bar"',
                 '-blender_backend:"EDS"',
             ],
-            $backendParams->get('fq')
+            $backendParams->get('filter')
         );
 
         $solrParams = $backendParams->get('params_Solr')[0];
@@ -619,7 +619,7 @@ public function testHiddenFilters(): void
 
         $this->assertEquals(
             ['formatSolr:"bar"'],
-            $solrParams->get('fq')
+            $solrParams->get('filter')
         );
         $this->assertEquals(
             [
@@ -650,15 +650,15 @@ public function testBlenderFilters(): void
         $params->addFilter('blender_backend:Primo');
         $backendParams = $params->getBackendParameters();
 
-        $this->assertEquals(['blender_backend:"Primo"'], $backendParams->get('fq'));
+        $this->assertEquals(['blender_backend:"Primo"'], $backendParams->get('filter'));
         $solrParams = $backendParams->get('params_Solr')[0];
         $this->assertInstanceOf(ParamBag::class, $solrParams);
-        $this->assertNull($solrParams->get('fq'));
+        $this->assertNull($solrParams->get('filter'));
 
         // Remove the filter and check:
         $params->removeFilter('blender_backend:Primo');
         $backendParams = $params->getBackendParameters();
-        $this->assertNull($backendParams->get('fq'));
+        $this->assertNull($backendParams->get('filter'));
 
         // Add as a hidden filter:
         $params = $this->getParams();
@@ -671,11 +671,11 @@ public function testBlenderFilters(): void
                 '{!tag=blender_backend_filter}blender_backend:('
                 . 'blender_backend:"Primo" OR blender_backend:"EDS")',
             ],
-            $backendParams->get('fq')
+            $backendParams->get('filter')
         );
         $solrParams = $backendParams->get('params_Solr')[0];
         $this->assertInstanceOf(ParamBag::class, $solrParams);
-        $this->assertNull($solrParams->get('fq'));
+        $this->assertNull($solrParams->get('filter'));
     }
 
     /**
@@ -702,7 +702,7 @@ public function testBlenderFacets(): void
         $backendParams = $params->getBackendParameters();
 
         // Make sure no backend was disabled:
-        $this->assertNull($backendParams->get('fq'));
+        $this->assertNull($backendParams->get('filter'));
         $primoParams = $backendParams->get('params_Primo')[0];
         $this->assertInstanceOf(ParamBag::class, $primoParams);
         $this->assertEquals(
@@ -733,7 +733,7 @@ public function testIgnoredFilters(): void
                 'format:"bar"',
                 '-blender_backend:"EDS"',
             ],
-            $backendParams->get('fq')
+            $backendParams->get('filter')
         );
 
         // Test ignoring all values:
@@ -749,7 +749,7 @@ public function testIgnoredFilters(): void
             [
                 'format:"bar"',
             ],
-            $backendParams->get('fq')
+            $backendParams->get('filter')
         );
         $edsParams = $backendParams->get('params_EDS')[0];
         $this->assertInstanceOf(ParamBag::class, $edsParams);
@@ -770,7 +770,7 @@ public function testIgnoredFilters(): void
             [
                 'format:"bar"',
             ],
-            $backendParams->get('fq')
+            $backendParams->get('filter')
         );
         $edsParams = $backendParams->get('params_EDS')[0];
         $this->assertInstanceOf(ParamBag::class, $edsParams);
@@ -786,7 +786,7 @@ public function testIgnoredFilters(): void
                 'format:"baz"',
                 '-blender_backend:"EDS"',
             ],
-            $backendParams->get('fq')
+            $backendParams->get('filter')
         );
     }
 
@@ -837,7 +837,7 @@ public function testFacetMappings(): void
                 'building:"0/Main/"',
                 '-blender_backend:"Primo"',
             ],
-            $backendParams->get('fq')
+            $backendParams->get('filter')
         );
 
         $solrParams = $backendParams->get('params_Solr')[0];
@@ -845,32 +845,24 @@ public function testFacetMappings(): void
 
         $this->assertEquals(
             [
-                'spellcheck' => [
-                    'true',
-                ],
                 'facet' => [
-                    'true',
-                ],
-                'facet.limit' => [
-                    30,
-                ],
-                'facet.field' => [
-                    '{!ex=building_filter}building',
-                ],
-                'facet.sort' => [
-                    'count',
-                ],
-                'facet.mincount' => [
-                    1,
-                ],
-                'fq' => [
-                    'building:"0/Main/"',
+                    'building' => [
+                        'type' => 'terms',
+                        'field' => 'building',
+                        'limit' => 30,
+                        'sort' => 'count',
+                        'domain' => [
+                            'excludeTags' => 'building_filter',
+                        ],
+                    ],
                 ],
-                'hl' => [
-                    'false',
+                'filter' => 'building:"0/Main/"',
+                'params' => [
+                    'spellcheck' => 'true',
+                    'hl' => 'false',
                 ],
             ],
-            $solrParams->getArrayCopy()
+            $solrParams->jsonSerialize()
         );
 
         $this->assertEquals(
@@ -899,7 +891,7 @@ public function testFacetMappings(): void
                 'building:"0/Sub/"',
                 '-blender_backend:"Primo"',
             ],
-            $backendParams->get('fq')
+            $backendParams->get('filter')
         );
 
         $edsParams = $backendParams->get('params_EDS')[0];
@@ -993,7 +985,7 @@ public function testInitFromRequest(): void
         $this->assertInstanceOf(ParamBag::class, $solrParams);
         $this->assertInstanceOf(ParamBag::class, $primoParams);
         $this->assertInstanceOf(ParamBag::class, $edsParams);
-        $this->assertNull($solrParams->get('fq'));
+        $this->assertNull($solrParams->get('filter'));
 
         $solrQuery = $backendParams->get('query_Solr')[0];
         $primoQuery = $backendParams->get('query_Primo')[0];
diff --git a/module/VuFind/tests/unit-tests/src/VuFindTest/Search/Solr/ConditionalFilterListenerTest.php b/module/VuFind/tests/unit-tests/src/VuFindTest/Search/Solr/ConditionalFilterListenerTest.php
index ed0ff32be26..a87ed873296 100644
--- a/module/VuFind/tests/unit-tests/src/VuFindTest/Search/Solr/ConditionalFilterListenerTest.php
+++ b/module/VuFind/tests/unit-tests/src/VuFindTest/Search/Solr/ConditionalFilterListenerTest.php
@@ -144,8 +144,8 @@ public function testConditionalFilterWithoutAuthorizationService()
         $event = $this->getMockPreEvent($params);
         $listener->onSearchPre($event);
 
-        $fq = $params->get('fq');
-        $this->assertEquals([], $fq);
+        $filter = $params->get('filter');
+        $this->assertEquals([], $filter);
     }
 
     /**
@@ -159,7 +159,7 @@ public function testConditionalFilterWithoutAuthorizationServiceWithParams()
     {
         $params = new ParamBag(
             [
-                'fq' => ['fulltext:VuFind', 'field2:novalue'],
+                'filter' => ['fulltext:VuFind', 'field2:novalue'],
             ]
         );
         $listener = new InjectConditionalFilterListener($this->backend, self::$searchConfig);
@@ -167,11 +167,11 @@ public function testConditionalFilterWithoutAuthorizationServiceWithParams()
         $event = $this->getMockPreEvent($params);
         $listener->onSearchPre($event);
 
-        $fq = $params->get('fq');
+        $filter = $params->get('filter');
         $this->assertEquals(
             [0 => 'fulltext:VuFind',
             1 => 'field2:novalue'],
-            $fq
+            $filter
         );
     }
 
@@ -190,8 +190,8 @@ public function testConditionalFilterEmptyConfig()
         $event = $this->getMockPreEvent($params);
         $listener->onSearchPre($event);
 
-        $fq = $params->get('fq');
-        $this->assertEquals([], $fq);
+        $filter = $params->get('filter');
+        $this->assertEquals([], $filter);
     }
 
     /**
@@ -204,7 +204,7 @@ public function testConditionalFilterEmptyConfigWithFQ()
     {
         $params = new ParamBag(
             [
-                'fq' => ['fulltext:VuFind', 'field2:novalue'],
+                'filter' => ['fulltext:VuFind', 'field2:novalue'],
             ]
         );
         $listener = new InjectConditionalFilterListener($this->backend, self::$emptySearchConfig);
@@ -214,11 +214,11 @@ public function testConditionalFilterEmptyConfigWithFQ()
         $event = $this->getMockPreEvent($params);
         $listener->onSearchPre($event);
 
-        $fq = $params->get('fq');
+        $filter = $params->get('filter');
         $this->assertEquals(
             [0 => 'fulltext:VuFind',
             1 => 'field2:novalue'],
-            $fq
+            $filter
         );
     }
 
@@ -241,17 +241,17 @@ public function testConditionalFilter()
         $event = $this->getMockPreEvent($params);
         $listener->onSearchPre($event);
 
-        $fq = $params->get('fq');
+        $filter = $params->get('filter');
         $this->assertEquals(
             [0 => 'institution:"MyInst"'],
-            $fq
+            $filter
         );
 
         // Check that a filter is not added for wrong backend:
         $params = new ParamBag([]);
         $event = $this->getMockPreEvent($params, 'Other');
         $listener->onSearchPre($event);
-        $this->assertEmpty($params->get('fq'));
+        $this->assertEmpty($params->get('filter'));
     }
 
     /**
@@ -273,8 +273,8 @@ public function testNegativeConditionalFilter()
         $event = $this->getMockPreEvent($params);
         $listener->onSearchPre($event);
 
-        $fq = $params->get('fq');
-        $this->assertEquals([0 => '(NOT institution:"MyInst")'], $fq);
+        $filter = $params->get('filter');
+        $this->assertEquals([0 => '(NOT institution:"MyInst")'], $filter);
     }
 
     /**
@@ -287,7 +287,7 @@ public function testNegativeConditionalFilterWithFQ()
     {
         $params = new ParamBag(
             [
-                'fq' => ['fulltext:VuFind', 'field2:novalue'],
+                'filter' => ['fulltext:VuFind', 'field2:novalue'],
             ]
         );
 
@@ -300,13 +300,13 @@ public function testNegativeConditionalFilterWithFQ()
         $event = $this->getMockPreEvent($params);
         $listener->onSearchPre($event);
 
-        $fq = $params->get('fq');
+        $filter = $params->get('filter');
         $this->assertEquals(
             [0 => 'fulltext:VuFind',
             1 => 'field2:novalue',
             2 => '(NOT institution:"MyInst")',
             ],
-            $fq
+            $filter
         );
     }
 
@@ -320,7 +320,7 @@ public function testConditionalFilterWithFQ()
     {
         $params = new ParamBag(
             [
-                'fq' => ['fulltext:VuFind', 'field2:novalue'],
+                'filter' => ['fulltext:VuFind', 'field2:novalue'],
             ]
         );
 
@@ -333,13 +333,13 @@ public function testConditionalFilterWithFQ()
         $event = $this->getMockPreEvent($params);
         $listener->onSearchPre($event);
 
-        $fq = $params->get('fq');
+        $filter = $params->get('filter');
         $this->assertEquals(
             [0 => 'fulltext:VuFind',
             1 => 'field2:novalue',
             2 => 'institution:"MyInst"',
             ],
-            $fq
+            $filter
         );
     }
 }
diff --git a/module/VuFind/tests/unit-tests/src/VuFindTest/Search/Solr/CustomFilterListenerTest.php b/module/VuFind/tests/unit-tests/src/VuFindTest/Search/Solr/CustomFilterListenerTest.php
index bcb8495949c..c62f6649dca 100644
--- a/module/VuFind/tests/unit-tests/src/VuFindTest/Search/Solr/CustomFilterListenerTest.php
+++ b/module/VuFind/tests/unit-tests/src/VuFindTest/Search/Solr/CustomFilterListenerTest.php
@@ -104,13 +104,13 @@ public function testRemapping(): void
             'vufind:"normal"' => 'field1:normal OR field2:alsoNormal',
         ];
         $listener = $this->getListener($normal);
-        $params = new ParamBag(['fq' => ['foo:"bar"', 'vufind:"normal"']]);
+        $params = new ParamBag(['filter' => ['foo:"bar"', 'vufind:"normal"']]);
         $command = $this->getMockSearchCommand($params, 'search', 'Solr');
         $event = new Event(null, null, compact('command'));
         $listener->onSearchPre($event);
         $this->assertEquals(
             ['foo:"bar"', 'field1:normal OR field2:alsoNormal'],
-            $params->get('fq')
+            $params->get('filter')
         );
     }
 
@@ -125,13 +125,13 @@ public function testMismatchedBackendIsIgnored(): void
             'vufind:"normal"' => 'field1:normal OR field2:alsoNormal',
         ];
         $listener = $this->getListener($normal);
-        $params = new ParamBag(['fq' => ['foo:"bar"', 'vufind:"normal"']]);
+        $params = new ParamBag(['filter' => ['foo:"bar"', 'vufind:"normal"']]);
         $command = $this->getMockSearchCommand($params, 'search', 'Search2');
         $event = new Event(null, null, compact('command'));
         $listener->onSearchPre($event);
         $this->assertEquals(
             ['foo:"bar"', 'vufind:"normal"'],
-            $params->get('fq')
+            $params->get('filter')
         );
     }
 
@@ -146,13 +146,13 @@ public function testWrongContextIsIgnored(): void
             'vufind:"normal"' => 'field1:normal OR field2:alsoNormal',
         ];
         $listener = $this->getListener($normal);
-        $params = new ParamBag(['fq' => ['foo:"bar"', 'vufind:"normal"']]);
+        $params = new ParamBag(['filter' => ['foo:"bar"', 'vufind:"normal"']]);
         $command = $this->getMockSearchCommand($params, 'weird', 'Solr');
         $event = new Event(null, null, compact('command'));
         $listener->onSearchPre($event);
         $this->assertEquals(
             ['foo:"bar"', 'vufind:"normal"'],
-            $params->get('fq')
+            $params->get('filter')
         );
     }
 
@@ -168,13 +168,13 @@ public function testMissingInvertedFilterAddsContent(): void
             'vufind:"inverted"' => 'field3:invertedFilter',
         ];
         $listener = $this->getListener([], $inverted);
-        $params = new ParamBag(['fq' => ['foo:"bar"']]);
+        $params = new ParamBag(['filter' => ['foo:"bar"']]);
         $command = $this->getMockSearchCommand($params, 'search', 'Solr');
         $event = new Event(null, null, compact('command'));
         $listener->onSearchPre($event);
         $this->assertEquals(
             ['foo:"bar"', 'field3:invertedFilter'],
-            $params->get('fq')
+            $params->get('filter')
         );
     }
 
@@ -190,10 +190,10 @@ public function testInvertedFilterPreventsAdditionOfContent(): void
             'vufind:"inverted"' => 'field3:invertedFilter',
         ];
         $listener = $this->getListener([], $inverted);
-        $params = new ParamBag(['fq' => ['foo:"bar"', 'vufind:"inverted"']]);
+        $params = new ParamBag(['filter' => ['foo:"bar"', 'vufind:"inverted"']]);
         $command = $this->getMockSearchCommand($params, 'search', 'Solr');
         $event = new Event(null, null, compact('command'));
         $listener->onSearchPre($event);
-        $this->assertEquals(['foo:"bar"'], $params->get('fq'));
+        $this->assertEquals(['foo:"bar"'], $params->get('filter'));
     }
 }
diff --git a/module/VuFind/tests/unit-tests/src/VuFindTest/Search/Solr/DefaultParametersListenerTest.php b/module/VuFind/tests/unit-tests/src/VuFindTest/Search/Solr/DefaultParametersListenerTest.php
index 3d500d9e6f9..15429b76131 100644
--- a/module/VuFind/tests/unit-tests/src/VuFindTest/Search/Solr/DefaultParametersListenerTest.php
+++ b/module/VuFind/tests/unit-tests/src/VuFindTest/Search/Solr/DefaultParametersListenerTest.php
@@ -141,10 +141,10 @@ public function testSearch(
 
         // Set up listener
         $listenerConfig = [
-            'search' => 'foo=1&foo=2',
+            'search' => '{"foo": ["1", "2"]}',
         ];
         if ($catchAllConfig) {
-            $listenerConfig['*'] = 'bar=3&bar';
+            $listenerConfig['*'] = '{"bar": ["3"]}';
         }
         $listener = new DefaultParametersListener($this->backends['primary'], $listenerConfig);
 
@@ -164,6 +164,7 @@ public function testSearch(
         );
         $listener->onSearchPre($event);
 
+        $params = $command->getSearchParameters();
         $this->assertEquals($expectFoo, $params->get('foo'));
         $this->assertEquals($expectBar, $params->get('bar'));
     }
diff --git a/module/VuFind/tests/unit-tests/src/VuFindTest/Search/Solr/FilterFieldConversionListenerTest.php b/module/VuFind/tests/unit-tests/src/VuFindTest/Search/Solr/FilterFieldConversionListenerTest.php
index 7c34f211350..3879181548c 100644
--- a/module/VuFind/tests/unit-tests/src/VuFindTest/Search/Solr/FilterFieldConversionListenerTest.php
+++ b/module/VuFind/tests/unit-tests/src/VuFindTest/Search/Solr/FilterFieldConversionListenerTest.php
@@ -74,7 +74,7 @@ public function testFilterTranslation()
     {
         $params   = new ParamBag(
             [
-                'fq' => [
+                'filter' => [
                     'foo:value',
                     'baz:"foo:value"',
                     'foofoo:value',
@@ -97,7 +97,7 @@ public function testFilterTranslation()
         );
         $listener->onSearchPre($event);
 
-        $fq   = $params->get('fq');
+        $fq   = $params->get('filter');
         $expected = [
             'bar:value',
             'boo:"foo:value"',
diff --git a/module/VuFind/tests/unit-tests/src/VuFindTest/Search/Solr/InjectHighlightingListenerTest.php b/module/VuFind/tests/unit-tests/src/VuFindTest/Search/Solr/InjectHighlightingListenerTest.php
index dbf2615be6a..0b1e0c9be20 100644
--- a/module/VuFind/tests/unit-tests/src/VuFindTest/Search/Solr/InjectHighlightingListenerTest.php
+++ b/module/VuFind/tests/unit-tests/src/VuFindTest/Search/Solr/InjectHighlightingListenerTest.php
@@ -32,7 +32,7 @@
 use Laminas\EventManager\Event;
 use VuFind\Search\Solr\InjectHighlightingListener;
 use VuFindSearch\Backend\Solr\QueryBuilder;
-use VuFindSearch\ParamBag;
+use VuFindSearch\NestingParamBag;
 use VuFindSearch\Service;
 
 /**
@@ -72,7 +72,7 @@ protected function setUp(): void
     {
         $this->backend = $this->createMock(\VuFindSearch\Backend\Solr\Backend::class);
         $this->backend->method('getIdentifier')->willReturn('foo');
-        $this->listener = new InjectHighlightingListener($this->backend, 'bar,baz', ['xyzzy' => 'true']);
+        $this->listener = new InjectHighlightingListener($this->backend, 'bar,baz', ['params' => ['xyzzy' => 'true']]);
     }
 
     /**
@@ -96,9 +96,11 @@ public function testAttach()
      */
     public function testParameters()
     {
-        $params = new ParamBag(
-            [
-                'hl' => 'true',
+        $params = NestingParamBag::fromArray(
+            ['params' =>
+                [
+                  'hl' => 'true',
+                ],
             ]
         );
         $command = $this->getMockSearchCommand(
@@ -119,12 +121,14 @@ public function testParameters()
         $this->listener->onSearchPre($event);
         $this->assertEquals(
             [
-                'hl' => ['true'],
-                'xyzzy' => ['true'],
-                'hl.simple.pre' => ['{{{{START_HILITE}}}}'],
-                'hl.simple.post' => ['{{{{END_HILITE}}}}'],
+                'params' => [
+                    'hl' => 'true',
+                    'xyzzy' => 'true',
+                    'hl.simple.pre' => '{{{{START_HILITE}}}}',
+                    'hl.simple.post' => '{{{{END_HILITE}}}}',
+                ],
             ],
-            $params->getArrayCopy()
+            $params->jsonSerialize()
         );
     }
 }
diff --git a/module/VuFind/tests/unit-tests/src/VuFindTest/Search/Solr/MultiIndexListenerTest.php b/module/VuFind/tests/unit-tests/src/VuFindTest/Search/Solr/MultiIndexListenerTest.php
index f2de8ed703c..26f5e185811 100644
--- a/module/VuFind/tests/unit-tests/src/VuFindTest/Search/Solr/MultiIndexListenerTest.php
+++ b/module/VuFind/tests/unit-tests/src/VuFindTest/Search/Solr/MultiIndexListenerTest.php
@@ -34,7 +34,7 @@
 use VuFindSearch\Backend\Solr\Backend;
 use VuFindSearch\Backend\Solr\Connector;
 use VuFindSearch\Backend\Solr\HandlerMap;
-use VuFindSearch\ParamBag;
+use VuFindSearch\NestingParamBag;
 use VuFindSearch\Service;
 
 /**
@@ -146,10 +146,16 @@ function () {
      */
     public function testStripFacetFields()
     {
-        $params = new ParamBag(
+        $params = NestingParamBag::fromArray(
             [
-                'facet.field' => ['field_1', 'field_2', 'field_3'],
-                'shards' => [self::$shards['b'], self::$shards['c']],
+                'facet' => [
+                    'field_1' => ['field' => 'field_1'],
+                    'field_2' => ['field' => 'field_2'],
+                    'field_3' => ['field' => 'field_3'],
+                ],
+                'params' => [
+                    'shards' => [self::$shards['b'], self::$shards['c']],
+                ],
             ]
         );
         $command = $this->getMockSearchCommand($params);
@@ -160,9 +166,10 @@ public function testStripFacetFields()
         );
         $this->listener->onSearchPre($event);
 
-        $facets   = $params->get('facet.field');
-        sort($facets);
-        $this->assertEquals(['field_1', 'field_2'], $facets);
+        $facets   = $params->jsonSerialize()['facet'];
+        $facetFields = array_map(fn ($facet) => $facet['field'], $facets);
+        sort($facetFields);
+        $this->assertSame(['field_1', 'field_2'], $facetFields);
     }
 
     /**
@@ -172,10 +179,12 @@ public function testStripFacetFields()
      */
     public function testAllShardsUsedForRecordRetrieval()
     {
-        $params   = new ParamBag(
-            [
-                'shards' => [self::$shards['b'], self::$shards['c']],
-            ]
+        $params   = NestingParamBag::fromArray(
+            ['params' =>
+                [
+                    'shards' => [self::$shards['b'], self::$shards['c']],
+                ],
+            ],
         );
         $command  = $this->getMockSearchCommand($params, 'retrieve');
         $event    = new Event(
@@ -185,8 +194,8 @@ public function testAllShardsUsedForRecordRetrieval()
         );
         $this->listener->onSearchPre($event);
 
-        $shards = $params->get('shards');
-        $this->assertEquals(
+        $shards = $params->getNested('params', 'shards');
+        $this->assertSame(
             [implode(',', [self::$shards['a'], self::$shards['b'], self::$shards['c']])],
             $shards
         );
diff --git a/module/VuFind/tests/unit-tests/src/VuFindTest/Search/Solr/ParamsTest.php b/module/VuFind/tests/unit-tests/src/VuFindTest/Search/Solr/ParamsTest.php
index 50f35d120a9..5823242abe3 100644
--- a/module/VuFind/tests/unit-tests/src/VuFindTest/Search/Solr/ParamsTest.php
+++ b/module/VuFind/tests/unit-tests/src/VuFindTest/Search/Solr/ParamsTest.php
@@ -59,7 +59,7 @@ public function testFilters(): void
         $params->addFacet('building', 'building_label');
 
         // No filters:
-        $this->assertEquals(null, $params->getBackendParameters()->get('fq'));
+        $this->assertEquals(null, $params->getBackendParameters()->get('filter'));
 
         // Add multiple filters:
         $params->addFilter('~format:bar');
@@ -70,7 +70,7 @@ public function testFilters(): void
                 'building:"main"',
                 '{!tag=format_filter}format:(format:"bar" OR format:"baz")',
             ],
-            $params->getBackendParameters()->get('fq')
+            $params->getBackendParameters()->get('filter')
         );
 
         // Add a hidden filter:
@@ -81,7 +81,7 @@ public function testFilters(): void
                 'building:"main"',
                 '{!tag=format_filter}format:(format:"bar" OR format:"baz")',
             ],
-            $params->getBackendParameters()->get('fq')
+            $params->getBackendParameters()->get('filter')
         );
 
         // Remove format filters:
@@ -91,7 +91,7 @@ public function testFilters(): void
                 'building:"sub"',
                 'building:"main"',
             ],
-            $params->getBackendParameters()->get('fq')
+            $params->getBackendParameters()->get('filter')
         );
 
         // Remove building filter:
@@ -100,7 +100,7 @@ public function testFilters(): void
             [
                 'building:"sub"',
             ],
-            $params->getBackendParameters()->get('fq')
+            $params->getBackendParameters()->get('filter')
         );
     }
 
diff --git a/module/VuFind/tests/unit-tests/src/VuFindTest/Search/Solr/ResultsTest.php b/module/VuFind/tests/unit-tests/src/VuFindTest/Search/Solr/ResultsTest.php
index 601234713f6..c019956ea17 100644
--- a/module/VuFind/tests/unit-tests/src/VuFindTest/Search/Solr/ResultsTest.php
+++ b/module/VuFind/tests/unit-tests/src/VuFindTest/Search/Solr/ResultsTest.php
@@ -86,16 +86,18 @@ class ResultsTest extends \PHPUnit\Framework\TestCase
         'response' => [
             'numFound' => 5,
         ],
-        'facet_counts' => [
-            'facet_fields' => [
-                'topic_facet' => [
-                    ['Research', 16],
-                    ['Psychotherapy', 8],
+        'facets' => [
+            'topic_facet' => [
+                'buckets' => [
+                    ['val' => 'Research', 'count' => 16],
+                    ['val' => 'Psychotherapy', 'count' => 8],
                 ],
-                'building' => [
-                    ['0/Main/', 11],
-                    ['1/Main/Fiction/', 5],
-                    ['0/Sub/', 2],
+            ],
+            'building' => [
+                'buckets' => [
+                    ['val' => '0/Main/', 'count' => 11],
+                    ['val' => '1/Main/Fiction/', 'count' => 5],
+                    ['val' => '0/Sub/', 'count' => 2],
                 ],
             ],
         ],
@@ -143,22 +145,27 @@ public function testFacetTranslations(): void
         $searchService = $this->getSearchServiceWithMockSearchMethod(
             [
                 'response' => ['numFound' => 5],
-                'facet_counts' => [
-                    'facet_fields' => [
-                        'dewey-raw' => [
-                            ['000', 100],
+                'facets' => [
+                    'dewey-raw' => [
+                        'buckets' => [
+                            ['val' => '000', 'count' => 100],
                         ],
                     ],
                 ],
             ],
             [
-                'spellcheck' => ['true'],
-                'hl' => ['false'],
-                'facet' => ['true'],
-                'facet.limit' => [30],
-                'facet.field' => ['dewey-raw'],
-                'facet.sort' => ['count'],
-                'facet.mincount' => [1],
+                'facet' => [
+                    'dewey-raw' => [
+                        'type' => 'terms',
+                        'field' => 'dewey-raw',
+                        'limit' => 30,
+                        'sort' => 'count',
+                    ],
+                ],
+                'params' => [
+                    'spellcheck' => 'true',
+                    'hl' => 'false',
+                ],
             ]
         );
         $results = $this->getResults($params, $searchService);
@@ -198,10 +205,10 @@ public function testGetResultTotal(): void
     {
         $searchService = $this->getSearchServiceWithMockSearchMethod(
             ['response' => ['numFound' => 5]],
-            [
-                'spellcheck' => ['true'],
-                'hl' => ['false'],
-            ]
+            [ 'params' => [
+                'spellcheck' => 'true',
+                'hl' => 'false',
+            ]]
         );
         $results = $this->getResults(null, $searchService);
         $this->assertEquals(5, $results->getResultTotal());
@@ -232,7 +239,7 @@ protected function getSearchServiceWithMockSearchMethod(
                 && get_class($command->getArguments()[0]) === \VuFindSearch\Query\Query::class
                 && $command->getArguments()[1] === 0
                 && $command->getArguments()[2] === 20
-                && $command->getArguments()[3]->getArrayCopy() == $expectedParams;
+                && $command->getArguments()[3]->jsonSerialize() == $expectedParams;
         };
         $searchService->expects($this->once())->method('invoke')
             ->with($this->callback($checkCommand))
diff --git a/module/VuFindSearch/src/VuFindSearch/Backend/Blender/Response/Json/RecordCollection.php b/module/VuFindSearch/src/VuFindSearch/Backend/Blender/Response/Json/RecordCollection.php
index f21219b53de..86572070172 100644
--- a/module/VuFindSearch/src/VuFindSearch/Backend/Blender/Response/Json/RecordCollection.php
+++ b/module/VuFindSearch/src/VuFindSearch/Backend/Blender/Response/Json/RecordCollection.php
@@ -142,7 +142,7 @@ public function initBlended(
             }
         }
 
-        $this->response['facet_counts']['facet_fields']
+        $this->response['facets']
             = $this->getMergedFacets($collections);
 
         return $backendRecords;
@@ -341,14 +341,17 @@ function ($a, $b) {
 
         $mergedFacets['blender_backend'] = $this->getBlenderFacetStats($collections);
 
-        // Convert the array back to Solr-style array with two elements
+        // Convert the array back to Solr-style array from JSON Facet API
         $facetFields = [];
         foreach ($mergedFacets as $facet => $values) {
-            $list = [];
-            foreach ($values as $key => $value) {
-                $list[] = [$key, $value];
+            $buckets = [];
+            foreach ($values as $key => $count) {
+                $buckets[] = [
+                    'val' => $key,
+                    'count' => $count,
+                ];
             }
-            $facetFields[$facet] = $list;
+            $facetFields[$facet] = ['buckets' => $buckets];
         }
 
         return $facetFields;
diff --git a/module/VuFindSearch/src/VuFindSearch/Backend/Solr/Backend.php b/module/VuFindSearch/src/VuFindSearch/Backend/Solr/Backend.php
index 6658f99708a..ea92d496c7c 100644
--- a/module/VuFindSearch/src/VuFindSearch/Backend/Solr/Backend.php
+++ b/module/VuFindSearch/src/VuFindSearch/Backend/Solr/Backend.php
@@ -40,6 +40,7 @@
 use VuFindSearch\Feature\RandomInterface;
 use VuFindSearch\Feature\RetrieveBatchInterface;
 use VuFindSearch\Feature\SimilarInterface;
+use VuFindSearch\NestingParamBag;
 use VuFindSearch\ParamBag;
 use VuFindSearch\Query\AbstractQuery;
 use VuFindSearch\Query\WorkKeysQuery;
@@ -135,6 +136,7 @@ public function search(
         $limit,
         ?ParamBag $params = null
     ) {
+        $params = NestingParamBag::from($params, false);
         if ($query instanceof WorkKeysQuery) {
             return $this->workKeysSearch($query, $offset, $limit, $params);
         }
@@ -161,11 +163,11 @@ public function rawJsonSearch(
         $limit,
         ?ParamBag $params = null
     ) {
-        $params = $params ?: new ParamBag();
+        $params = NestingParamBag::from($params);
         $this->injectResponseWriter($params);
 
-        $params->set('rows', $limit);
-        $params->set('start', $offset);
+        $params->set('limit', $limit);
+        $params->set('offset', $offset);
         $params->mergeWith($this->getQueryBuilder()->build($query, $params));
         return $this->connector->search($params);
     }
@@ -208,17 +210,17 @@ public function getIds(
         $limit,
         ?ParamBag $params = null
     ) {
-        $params = $params ?: new ParamBag();
+        $params = NestingParamBag::from($params);
         $this->injectResponseWriter($params);
 
-        $params->set('rows', $limit);
-        $params->set('start', $offset);
-        $flParts = [$this->getConnector()->getUniqueKey()];
-        if ($fl = $params->get('fl')) {
+        $params->set('limit', $limit);
+        $params->set('offset', $offset);
+        $fieldParts = [$this->getConnector()->getUniqueKey()];
+        if ($fields = $params->get('fields')) {
             // Merge multiple values if necessary, then split on delimiter:
-            $flParts = array_unique(array_merge($flParts, explode(',', implode(',', $fl))));
+            $fieldParts = array_unique(array_merge($fieldParts, explode(',', implode(',', $fields))));
         }
-        $params->set('fl', implode(',', $flParts));
+        $params->set('fields', implode(',', $fieldParts));
         $params->mergeWith($this->getQueryBuilder()->build($query));
         $response   = $this->connector->search($params);
         $collection = $this->createRecordCollection($response);
@@ -241,7 +243,7 @@ public function random(
         $limit,
         ?ParamBag $params = null
     ) {
-        $params = $params ?: new ParamBag();
+        $params = NestingParamBag::from($params);
         $this->injectResponseWriter($params);
 
         $random = rand(0, 1000000);
@@ -261,7 +263,7 @@ public function random(
      */
     public function retrieve($id, ?ParamBag $params = null)
     {
-        $params = $params ?: new ParamBag();
+        $params = NestingParamBag::from($params);
         $this->injectResponseWriter($params);
 
         $response   = $this->connector->retrieve($id, $params);
@@ -280,7 +282,7 @@ public function retrieve($id, ?ParamBag $params = null)
      */
     public function retrieveBatch($ids, ?ParamBag $params = null)
     {
-        $params = $params ?: new ParamBag();
+        $params = NestingParamBag::from($params);
 
         // Callback function for formatting IDs:
         $formatIds = function ($i) {
@@ -292,9 +294,9 @@ public function retrieveBatch($ids, ?ParamBag $params = null)
         while (count($ids) > 0) {
             $currentPage = array_splice($ids, 0, $this->pageSize, []);
             $currentPage = array_map($formatIds, $currentPage);
-            $params->set('q', 'id:(' . implode(' OR ', $currentPage) . ')');
-            $params->set('start', 0);
-            $params->set('rows', $this->pageSize);
+            $params->set('query', 'id:(' . implode(' OR ', $currentPage) . ')');
+            $params->set('offset', 0);
+            $params->set('limit', $this->pageSize);
             $this->injectResponseWriter($params);
             $next = $this->createRecordCollection(
                 $this->connector->search($params)
@@ -321,10 +323,10 @@ public function retrieveBatch($ids, ?ParamBag $params = null)
      */
     public function similar($id, ?ParamBag $params = null)
     {
-        $params = $params ?: new ParamBag();
+        $params = NestingParamBag::from($params);
         $this->injectResponseWriter($params);
 
-        $params->mergeWith($this->getSimilarBuilder()->build($id));
+        $params = $this->getSimilarBuilder()->build($id, $params);
         $response   = $this->connector->similar($id, $params);
         $collection = $this->createRecordCollection($response);
         $this->injectSourceIdentifier($collection);
@@ -354,29 +356,29 @@ public function terms(
         }
 
         // Create empty ParamBag if none provided:
-        $params = $params ?: new ParamBag();
+        $params = NestingParamBag::from($params);
         $this->injectResponseWriter($params);
 
         // Always enable terms:
-        $params->set('terms', 'true');
+        $params->setNested('params', 'terms', 'true');
 
         // Use parameters if provided:
         if (null !== $field) {
-            $params->set('terms.fl', $field);
+            $params->setNested('params', 'terms.fl', $field);
         }
         if (null !== $start) {
-            $params->set('terms.lower', $start);
+            $params->setNested('params', 'terms.lower', $start);
         }
         if (null !== $limit) {
-            $params->set('terms.limit', $limit);
+            $params->setNested('params', 'terms.limit', $limit);
         }
 
         // Set defaults unless overridden:
-        if (!$params->hasParam('terms.lower.incl')) {
-            $params->set('terms.lower.incl', 'false');
+        if (!$params->hasNestedParam('params', 'terms.lower.incl')) {
+            $params->setNested('params', 'terms.lower.incl', 'false');
         }
-        if (!$params->hasParam('terms.sort')) {
-            $params->set('terms.sort', 'index');
+        if (!$params->hasNestedParam('params', 'terms.sort')) {
+            $params->setNested('params', 'terms.sort', 'index');
         }
 
         $response = $this->connector->terms($params);
@@ -405,7 +407,8 @@ public function alphabeticBrowse(
         $params = null,
         $offsetDelta = 0
     ) {
-        $params = $params ?: new ParamBag();
+        // TODO Does alphabrowse also need to be converted?  Custom request handler...
+        $params = NestingParamBag::from($params);
         $this->injectResponseWriter($params);
 
         $params->set('from', $from);
@@ -607,26 +610,26 @@ protected function refineBrowseException(RemoteErrorException $e)
      * @throws InvalidArgumentException Response writer and named list
      * implementation already set to an incompatible type.
      */
-    protected function injectResponseWriter(ParamBag $params)
+    protected function injectResponseWriter(NestingParamBag $params)
     {
-        if (array_diff($params->get('wt') ?: [], ['json'])) {
+        if (array_diff($params->getNested('params', 'wt') ?: [], ['json'])) {
             throw new InvalidArgumentException(
                 sprintf(
                     'Invalid response writer type: %s',
-                    implode(', ', $params->get('wt'))
+                    implode(', ', $params->getNested('params', 'wt'))
                 )
             );
         }
-        if (array_diff($params->get('json.nl') ?: [], ['arrarr'])) {
+        if (array_diff($params->getNested('params', 'json.nl') ?: [], ['arrarr'])) {
             throw new InvalidArgumentException(
                 sprintf(
                     'Invalid named list implementation type: %s',
-                    implode(', ', $params->get('json.nl'))
+                    implode(', ', $params->getNested('params', 'json.nl'))
                 )
             );
         }
-        $params->set('wt', ['json']);
-        $params->set('json.nl', ['arrarr']);
+        $params->setNested('params', 'wt', 'json');
+        $params->setNested('params', 'json.nl', 'arrarr');
     }
 
     /**
@@ -660,12 +663,12 @@ protected function workKeysSearch(
 
         $params = $defaultParams ? clone $defaultParams : new \VuFindSearch\ParamBag();
         $this->injectResponseWriter($params);
-        $params->set('q', "{!terms f=work_keys_str_mv separator=\"\u{001f}\"}" . implode("\u{001f}", $workKeys));
+        $params->set('query', "{!terms f=work_keys_str_mv separator=\"\u{001f}\"}" . implode("\u{001f}", $workKeys));
         if (!$query->getIncludeSelf()) {
-            $params->add('fq', sprintf('-id:"%s"', addcslashes($id, '"')));
+            $params->add('filter', sprintf('-id:"%s"', addcslashes($id, '"')));
         }
-        $params->set('rows', $limit);
-        $params->set('start', $offset);
+        $params->set('limit', $limit);
+        $params->set('offset', $offset);
         if (!$params->hasParam('sort')) {
             $params->add('sort', 'publishDateSort desc, title_sort asc');
         }
diff --git a/module/VuFindSearch/src/VuFindSearch/Backend/Solr/Connector.php b/module/VuFindSearch/src/VuFindSearch/Backend/Solr/Connector.php
index a8cc5a9e947..3dc0416f5d4 100644
--- a/module/VuFindSearch/src/VuFindSearch/Backend/Solr/Connector.php
+++ b/module/VuFindSearch/src/VuFindSearch/Backend/Solr/Connector.php
@@ -41,6 +41,7 @@
 use VuFindSearch\Backend\Exception\RequestErrorException;
 use VuFindSearch\Backend\Solr\Document\DocumentInterface;
 use VuFindSearch\Exception\InvalidArgumentException;
+use VuFindSearch\NestingParamBag;
 use VuFindSearch\ParamBag;
 
 use function call_user_func_array;
@@ -199,7 +200,7 @@ public function retrieve($id, ?ParamBag $params = null)
     {
         $params = $params ?: new ParamBag();
         $params
-            ->set('q', sprintf('%s:"%s"', $this->uniqueKey, addcslashes($id, '"')));
+            ->set('query', sprintf('%s:"%s"', $this->uniqueKey, addcslashes($id, '"')));
 
         $handler = $this->map->getHandler(__FUNCTION__);
         $this->map->prepare(__FUNCTION__, $params);
@@ -307,23 +308,23 @@ public function write(
      */
     public function query($handler, ParamBag $params, bool $cacheable = false)
     {
+        $params = NestingParamBag::from($params);
         $urlSuffix = '/' . $handler;
-        $paramString = implode('&', $params->request());
-        if (strlen($paramString) > self::MAX_GET_URL_LENGTH) {
-            $method = Request::METHOD_POST;
-            $callback = function ($client) use ($paramString): void {
-                $client->setRawBody($paramString);
-                $client->setEncType(HttpClient::ENC_URLENCODED);
-                $client->setHeaders(['Content-Length' => strlen($paramString)]);
-            };
-        } else {
-            $method = Request::METHOD_GET;
-            $urlSuffix .= '?' . $paramString;
-            $callback = null;
-        }
+        $body = json_encode($params, JSON_THROW_ON_ERROR);
+        $method = Request::METHOD_POST;
+        $callback = function ($client) use ($body): void {
+            $client->setRawBody($body);
+            $client->setEncType('application/json');
+            $client->setHeaders(['Content-Length' => strlen($body)]);
+        };
 
-        $this->debug(sprintf('Query %s', $paramString));
+        $this->debug(sprintf('Query body %s', $body));
         return $this->trySolrUrls($method, $urlSuffix, $callback, $cacheable);
+        // $responseBody = $this->trySolrUrls($method, $urlSuffix, $callback, $cacheable);
+        // if (strlen($body) > 500) {
+        //    $this->debug(sprintf('Response body to long query %s', $responseBody));
+        // }
+        // return $responseBody;
     }
 
     /**
diff --git a/module/VuFindSearch/src/VuFindSearch/Backend/Solr/QueryBuilder.php b/module/VuFindSearch/src/VuFindSearch/Backend/Solr/QueryBuilder.php
index 38eb0732141..b2ee6c0013d 100644
--- a/module/VuFindSearch/src/VuFindSearch/Backend/Solr/QueryBuilder.php
+++ b/module/VuFindSearch/src/VuFindSearch/Backend/Solr/QueryBuilder.php
@@ -31,6 +31,7 @@
 
 namespace VuFindSearch\Backend\Solr;
 
+use VuFindSearch\NestingParamBag;
 use VuFindSearch\ParamBag;
 use VuFindSearch\Query\AbstractQuery;
 use VuFindSearch\Query\Query;
@@ -129,16 +130,17 @@ public function __construct(
      * @param AbstractQuery $query  User query
      * @param ?ParamBag     $params Search backend parameters
      *
-     * @return ParamBag
+     * @return NestingParamBag
      */
-    public function build(AbstractQuery $query, ?ParamBag $params = null)
+    public function build(AbstractQuery $query, ?ParamBag $params = null): NestingParamBag
     {
-        $newParams = new ParamBag();
+        $newParams = new NestingParamBag();
 
         // Add spelling query if applicable -- note that we must set this up before
         // we process the main query in order to avoid unwanted extra syntax:
         if ($this->createSpellingQuery) {
-            $newParams->set(
+            $newParams->setNested(
+                'params',
                 'spellcheck.q',
                 $this->getLuceneHelper()->extractSearchTerms($query->getAllTerms())
             );
@@ -169,17 +171,17 @@ public function build(AbstractQuery $query, ?ParamBag $params = null)
                     // If a boost was added, we don't want to highlight based on
                     // the boost query, so we should use the non-boosted version:
                     if ($highlight && $oldString != $string) {
-                        $newParams->set('hl.q', $oldString);
+                        $newParams->setNested('params', 'hl.q', $oldString);
                     }
                 }
             } elseif ($handler->hasDismax()) {
-                $newParams->set('qf', implode(' ', $handler->getDismaxFields()));
-                $newParams->set('qt', $handler->getDismaxHandler());
+                $newParams->setNested('params', 'qf', implode(' ', $handler->getDismaxFields()));
+                $newParams->setNested('params', 'qt', $handler->getDismaxHandler());
                 foreach ($handler->getDismaxParams() as $param) {
-                    $newParams->add(reset($param), next($param));
+                    $newParams->addNested('params', reset($param), next($param));
                 }
                 if ($handler->hasFilterQuery()) {
-                    $newParams->add('fq', $handler->getFilterQuery());
+                    $newParams->add('filter', $handler->getFilterQuery());
                 }
             } else {
                 $string = $handler->createSimpleQueryString($string);
@@ -188,9 +190,9 @@ public function build(AbstractQuery $query, ?ParamBag $params = null)
         // Set an appropriate highlight field list when applicable:
         if ($highlight) {
             $filter = $handler ? $handler->getAllFields() : [];
-            $newParams->add('hl.fl', $this->getFieldsToHighlight($filter));
+            $newParams->addNested('params', 'hl.fl', $this->getFieldsToHighlight($filter));
         }
-        $newParams->set('q', $string);
+        $newParams->set('query', $string);
 
         // Handle any extra parameters:
         foreach ($this->globalExtraParams as $extraParam) {
@@ -203,7 +205,7 @@ public function build(AbstractQuery $query, ?ParamBag $params = null)
                 continue;
             }
             foreach ((array)$extraParam['value'] as $value) {
-                $newParams->add($extraParam['param'], $value);
+                $newParams->addNested('params', $extraParam['param'], $value);
             }
         }
 
diff --git a/module/VuFindSearch/src/VuFindSearch/Backend/Solr/Response/Json/RecordCollection.php b/module/VuFindSearch/src/VuFindSearch/Backend/Solr/Response/Json/RecordCollection.php
index 3829a56f6ab..8a4f7faed40 100644
--- a/module/VuFindSearch/src/VuFindSearch/Backend/Solr/Response/Json/RecordCollection.php
+++ b/module/VuFindSearch/src/VuFindSearch/Backend/Solr/Response/Json/RecordCollection.php
@@ -32,6 +32,7 @@
 use VuFindSearch\Response\AbstractRecordCollection;
 
 use function array_key_exists;
+use function is_array;
 
 /**
  * Simple JSON-based record collection.
@@ -55,7 +56,7 @@ class RecordCollection extends AbstractRecordCollection
         'responseHeader' => [],
         'response'       => ['numFound' => 0, 'start' => 0],
         'spellcheck'     => ['suggestions' => []],
-        'facet_counts'   => [],
+        'facets'   => [],
     ];
 
     /**
@@ -144,22 +145,25 @@ public function getFacets()
     {
         if (null === $this->facetFields) {
             $this->facetFields = [];
-            $facetFieldData = $this->response['facet_counts']['facet_fields'] ?? [];
+            $facetFieldData = $this->response['facets'] ?? [];
             foreach ($facetFieldData as $field => $facetData) {
-                $values = [];
-                foreach ($facetData as $value) {
-                    $values[$value[0]] = $value[1];
+                if (is_array($facetData)) {
+                    $values = [];
+                    foreach ($facetData['buckets'] as $bucket) {
+                        $values[$bucket['val']] = $bucket['count'];
+                    }
+                    $this->facetFields[$field] = $values;
                 }
-                $this->facetFields[$field] = $values;
-            }
-            $facetRangeData = $this->response['facet_counts']['facet_ranges'] ?? [];
-            foreach ($facetRangeData as $field => $facetData) {
-                $values = [];
-                foreach ($facetData['counts'] as $value) {
-                    $values[$value[0]] = $value[1];
-                }
-                $this->facetFields[$field] = $values;
             }
+            // TODO Fix this
+            // $facetRangeData = $this->response['facet_counts']['facet_ranges'] ?? [];
+            // foreach ($facetRangeData as $field => $facetData) {
+            //     $values = [];
+            //     foreach ($facetData['counts'] as $value) {
+            //         $values[$value[0]] = $value[1];
+            //     }
+            //     $this->facetFields[$field] = $values;
+            // }
         }
         return $this->facetFields;
     }
diff --git a/module/VuFindSearch/src/VuFindSearch/Backend/Solr/SimilarBuilder.php b/module/VuFindSearch/src/VuFindSearch/Backend/Solr/SimilarBuilder.php
index 851fcb6d81e..47d6b6d6904 100644
--- a/module/VuFindSearch/src/VuFindSearch/Backend/Solr/SimilarBuilder.php
+++ b/module/VuFindSearch/src/VuFindSearch/Backend/Solr/SimilarBuilder.php
@@ -33,6 +33,7 @@
 
 namespace VuFindSearch\Backend\Solr;
 
+use VuFindSearch\NestingParamBag;
 use VuFindSearch\ParamBag;
 
 use function sprintf;
@@ -113,28 +114,29 @@ public function __construct(
     /**
      * Return SOLR search parameters based on a record Id and params.
      *
-     * @param string $id Record Id
+     * @param string    $id     Record Id
+     * @param ?ParamBag $params Existing params
      *
      * @return ParamBag
      */
-    public function build($id)
+    public function build(string $id, ?ParamBag $params = null): ParamBag
     {
-        $params = new ParamBag();
+        $params = NestingParamBag::from($params);
         if ($this->useHandler) {
             $mltParams = $this->handlerParams
                 ? $this->handlerParams
                 : 'qf=title,title_short,callnumber-label,topic,language,author,'
                     . 'publishDate mintf=1 mindf=1';
-            $params->set('q', sprintf('{!mlt %s}%s', $mltParams, $id));
+            $params->set('query', sprintf('{!mlt %s}%s', $mltParams, $id));
         } else {
             $params->set(
-                'q',
+                'query',
                 sprintf('%s:"%s"', $this->uniqueKey, addcslashes($id, '"'))
             );
-            $params->set('qt', 'morelikethis');
+            $params->setNested('params', 'qt', 'morelikethis');
         }
-        if (null === $params->get('rows')) {
-            $params->set('rows', $this->count);
+        if (null === $params->get('limit')) {
+            $params->set('limit', $this->count);
         }
         return $params;
     }
diff --git a/module/VuFindSearch/src/VuFindSearch/Backend/Solr/SimilarBuilderInterface.php b/module/VuFindSearch/src/VuFindSearch/Backend/Solr/SimilarBuilderInterface.php
index 183015b8f40..014fb3d66d9 100644
--- a/module/VuFindSearch/src/VuFindSearch/Backend/Solr/SimilarBuilderInterface.php
+++ b/module/VuFindSearch/src/VuFindSearch/Backend/Solr/SimilarBuilderInterface.php
@@ -52,9 +52,10 @@ interface SimilarBuilderInterface
     /**
      * Build SOLR query based on VuFind query object.
      *
-     * @param string $id Record id
+     * @param string    $id     Record id
+     * @param ?ParamBag $params Existing params
      *
      * @return ParamBag
      */
-    public function build($id);
+    public function build(string $id, ?ParamBag $params): ParamBag;
 }
diff --git a/module/VuFindSearch/src/VuFindSearch/NestingParamBag.php b/module/VuFindSearch/src/VuFindSearch/NestingParamBag.php
new file mode 100644
index 00000000000..0e417f9bf39
--- /dev/null
+++ b/module/VuFindSearch/src/VuFindSearch/NestingParamBag.php
@@ -0,0 +1,320 @@
+<?php
+
+/**
+ * Bag of parameters or nested parameter bags.
+ *
+ * PHP version 8
+ *
+ * Copyright (C) Villanova University 2010.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2,
+ * as published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, see
+ * <https://www.gnu.org/licenses/>.
+ *
+ * @category VuFind
+ * @package  Search
+ * @author   Maccabee Levine <msl321@lehigh.edu>
+ * @license  http://opensource.org/licenses/gpl-2.0.php GNU General Public License
+ * @link     https://vufind.org
+ */
+
+namespace VuFindSearch;
+
+use JsonSerializable;
+
+use function count;
+use function is_array;
+
+/**
+ * Bag of (string) parameters or nested ParamBags.
+ *
+ * @category VuFind
+ * @package  Search
+ * @author   Maccabee Levine <msl321@lehigh.edu>
+ * @license  http://opensource.org/licenses/gpl-2.0.php GNU General Public License
+ * @link     https://vufind.org
+ */
+class NestingParamBag extends ParamBag implements JsonSerializable
+{
+    /**
+     * Transform any ParamBag into a NestingParamBag.
+     *
+     * @param ?ParamBag $original     The original ParamBag
+     * @param bool      $createIfNull Create an empty ParamBag if $original is null
+     *
+     * @return ?NestingParamBag
+     */
+    public static function from(?ParamBag $original, bool $createIfNull = true): ?NestingParamBag
+    {
+        if (!$original) {
+            return $createIfNull ? new NestingParamBag() : null;
+        }
+        if ($original instanceof NestingParamBag) {
+            return $original;
+        }
+        $bag = new NestingParamBag();
+        $bag->mergeWith($original);
+        return $bag;
+    }
+
+    /**
+     * Transform a potentially nested array of values into a NestingParamBag.
+     *
+     * @param array $values Source values
+     *
+     * @return NestingParamBag
+     */
+    public static function fromArray(array $values): NestingParamBag
+    {
+        $bag = new NestingParamBag();
+        foreach ($values as $name => $value) {
+            $bag->addMultiNested($name, $value);
+        }
+        return $bag;
+    }
+
+    /**
+     * Return nested parameter value.
+     *
+     * @param string $name       Parameter name
+     * @param string $nestedName Nested parameter name
+     *
+     * @return ?array Array of parameter values or NULL if not set
+     */
+    public function getNested(string $name, string $nestedName): ?array
+    {
+        $nestedBag = $this->get($name);
+        if (!$nestedBag) {
+            return null;
+        }
+        return $nestedBag[0]->get($nestedName);
+    }
+
+    /**
+     * Return true if the bag contains any value(s) for the specified parameters.
+     *
+     * @param string $name       Parameter name
+     * @param string $nestedName Nested parameter name
+     *
+     * @return bool
+     */
+    public function hasNestedParam(string $name, string $nestedName): bool
+    {
+        $nestedBag = $this->get($name);
+        if (!$nestedBag) {
+            return false;
+        }
+        return $nestedBag[0]->hasParam($nestedName);
+    }
+
+    /**
+     * Set a nested parameter.
+     *
+     * @param string $name        Parameter name
+     * @param string $nestedName  Nested parameter name
+     * @param string $nestedValue Nested parameter value
+     *
+     * @return void
+     */
+    public function setNested(string $name, string $nestedName, string $nestedValue): void
+    {
+        $nestedBag = $this->items[$name] ?? null;
+        if (!$nestedBag) {
+            $nestedBag = [new ParamBag()];
+            $this->set($name, $nestedBag);
+        }
+        $nestedBag[0]->set($nestedName, $nestedValue);
+    }
+
+    /**
+     * Add a nested parameter value.
+     *
+     * @param string $name        Parameter name
+     * @param string $nestedName  Nested parameter name
+     * @param string $nestedValue Nested parameter value
+     *
+     * @return void
+     */
+    public function addNested(string $name, string $nestedName, string $nestedValue): void
+    {
+        $nestedBag = $this->items[$name] ?? null;
+        if (!$nestedBag) {
+            $nestedBag = [new ParamBag()];
+            $this->set($name, $nestedBag);
+        }
+        $nestedBag[0]->add($nestedName, $nestedValue);
+    }
+
+    /**
+     * Parse n-deep arrays to add values.
+     *
+     * @param string $name  Parameter name
+     * @param mixed  $value A scalar value, or some n-deep array of arrays into parameters
+     *
+     * @return void
+     */
+    public function addMultiNested(string $name, mixed $value): void
+    {
+        if (is_array($value)) {
+            if (array_is_list($value)) {
+                foreach ($value as $valueItem) {
+                    $this->add($name, $valueItem);
+                }
+            } else {
+                $nestedBag = $this->items[$name][0] ?? null;
+                if (!$nestedBag || !$nestedBag instanceof NestingParamBag) {
+                    $nestedBag = NestingParamBag::from($nestedBag);
+                    $this->set($name, $nestedBag);
+                }
+                foreach ($value as $nestedName => $nestedValue) {
+                    $nestedBag->addMultiNested($nestedName, $nestedValue);
+                }
+            }
+        } else {
+            $this->add($name, $value);
+        }
+    }
+
+    /**
+     * Add parameter value.
+     *
+     * @param string $name        Parameter name
+     * @param mixed  $value       Parameter value
+     * @param bool   $deduplicate Deduplicate parameter values
+     *
+     * @return void
+     */
+    public function add($name, $value, $deduplicate = true): void
+    {
+        $existingValues = $this->items[$name] ?? [];
+        $this->validateCompatibleValues($existingValues, $value, $name);
+
+        if ($value && is_array($value)) {
+            // Merge as needed so there is only one ParamBag for any $name
+            if ($value[0] instanceof ParamBag) {
+                $existingValues[0]->mergeWith($value[0]);
+                return;
+            }
+        }
+
+        parent::add($name, $value, $deduplicate);
+    }
+
+    /**
+     * Validate that the new values can be added to the existing values.
+     *
+     * @param array   $existingValues The existing values
+     * @param mixed   $newValues      Values being added
+     * @param ?string $name           Name associated with these values
+     *
+     * @throws \Exception If the combination of values is not valid.
+     *
+     * @return void
+     */
+    protected function validateCompatibleValues(array $existingValues, mixed $newValues, ?string $name)
+    {
+        if (!is_array($newValues)) {
+            $newValues = [$newValues];
+        }
+        foreach ([$existingValues, $newValues] as $values) {
+            if (!$values) {
+                return;
+            }
+            $this->validateValues($values, $name);
+        }
+        // Either both arrays, or neither, should contain a ParamBag
+        if (
+            empty(array_filter($existingValues, fn ($value) => $value instanceof ParamBag)) !=
+            empty(array_filter($newValues, fn ($value) => $value instanceof ParamBag))
+        ) {
+            throw new \Exception('New values for name ' . ($name ?? '(unknown)')
+                . ' are not compatible with existing values; both or neither must be a ParamBag.');
+        }
+    }
+
+    /**
+     * Return a serializable object, for json_encode use into a POST body.
+     *
+     * @return mixed
+     */
+    public function jsonSerialize(): mixed
+    {
+        $serializable = $this->jsonSerializeItems($this->items);
+        return $serializable;
+    }
+
+    /**
+     * Parse ParamBag items into an array, recursively.
+     *
+     * @param array $items The array from a ParamBag
+     *
+     * @return array
+     */
+    protected function jsonSerializeItems($items): array
+    {
+        $jsonObject = [];
+        foreach ($items as $name => $values) {
+            if (is_array($values)) {
+                $this->validateValues($values, $name);
+
+                if (count($values) > 1) {
+                    $jsonObject[$name] = $values;
+                } elseif (count($values) == 1) {
+                    $value = $values[0];
+                    if ($value instanceof ParamBag) {
+                        $nestedValues = $value->getArrayCopy();
+                        $jsonObject[$name] = $this->jsonSerializeItems($nestedValues);
+                    } else {
+                        $jsonObject[$name] = $value;
+                    }
+                }
+            } else {
+                throw new \Exception('ParamBag values for ' . $name . ' is not an array.');
+            }
+        }
+        return $jsonObject;
+    }
+
+    /**
+     * Return array of params ready to be used in a HTTP request.
+     *
+     * Returns a numerical array with all request parameters as properly URL
+     * encoded key-value pairs.
+     *
+     * @return array
+     */
+    public function request()
+    {
+        throw new \Exception('Simple query parameters are not supported by NestingParamBag');
+    }
+
+    /**
+     * Validate the values in a ParamBag.
+     *
+     * @param array   $values The values
+     * @param ?string $name   Name associated with these values
+     *
+     * @throws \Exception If the combination of values is not valid.
+     *
+     * @return void
+     */
+    protected function validateValues(array $values, ?string $name): void
+    {
+        if (
+            count($values) > 1 &&
+            array_filter($values, fn ($value) => $value instanceof ParamBag)
+        ) {
+            throw new \Exception('More than one value for name ' . ($name ?? '(unknown)')
+                . 'including at least one ParamBag.');
+        }
+    }
+}
diff --git a/module/VuFindSearch/src/VuFindSearch/ParamBag.php b/module/VuFindSearch/src/VuFindSearch/ParamBag.php
index e5e480857b9..c7603ac536f 100644
--- a/module/VuFindSearch/src/VuFindSearch/ParamBag.php
+++ b/module/VuFindSearch/src/VuFindSearch/ParamBag.php
@@ -54,7 +54,7 @@ class ParamBag implements \Countable
      *
      * @var array
      */
-    protected $params = [];
+    protected $items = [];
 
     /**
      * Constructor.
@@ -79,7 +79,7 @@ public function __construct(array $initial = [])
      */
     public function get($name)
     {
-        return $this->params[$name] ?? null;
+        return $this->items[$name] ?? null;
     }
 
     /**
@@ -89,7 +89,7 @@ public function get($name)
      */
     public function count(): int
     {
-        return count($this->params);
+        return count($this->items);
     }
 
     /**
@@ -101,7 +101,7 @@ public function count(): int
      */
     public function hasParam($name)
     {
-        return isset($this->params[$name]);
+        return isset($this->items[$name]);
     }
 
     /**
@@ -128,7 +128,7 @@ public function contains($name, $value)
      */
     public function set($name, $value)
     {
-        $this->params[$name] = is_array($value) ? $value : [$value];
+        $this->items[$name] = is_array($value) ? $value : [$value];
     }
 
     /**
@@ -140,8 +140,8 @@ public function set($name, $value)
      */
     public function remove($name)
     {
-        if (isset($this->params[$name])) {
-            unset($this->params[$name]);
+        if (isset($this->items[$name])) {
+            unset($this->items[$name]);
         }
     }
 
@@ -156,22 +156,22 @@ public function remove($name)
      */
     public function add($name, $value, $deduplicate = true)
     {
-        if (!isset($this->params[$name])) {
-            $this->params[$name] = [];
+        if (!isset($this->items[$name])) {
+            $this->items[$name] = [];
         }
         if (is_array($value)) {
-            $this->params[$name] = array_merge_recursive($this->params[$name], $value);
+            $this->items[$name] = array_merge_recursive($this->items[$name], $value);
         } else {
-            $this->params[$name][] = $value;
+            $this->items[$name][] = $value;
         }
         if ($deduplicate) {
             // Avoid deduplicating associative array params (like Primo filterList):
-            foreach ($this->params[$name] as $key => $current) {
+            foreach ($this->items[$name] as $key => $current) {
                 if (!is_numeric($key) || is_array($current)) {
                     return;
                 }
             }
-            $this->params[$name] = array_values(array_unique($this->params[$name]));
+            $this->items[$name] = array_values(array_unique($this->items[$name]));
         }
     }
 
@@ -184,7 +184,7 @@ public function add($name, $value, $deduplicate = true)
      */
     public function mergeWith(ParamBag $bag)
     {
-        foreach ($bag->params as $key => $value) {
+        foreach ($bag->items as $key => $value) {
             if (!empty($value)) {
                 $this->add($key, $value);
             }
@@ -212,7 +212,7 @@ public function mergeWithAll(array $bags)
      */
     public function getArrayCopy()
     {
-        return $this->params;
+        return $this->items;
     }
 
     /**
@@ -224,8 +224,8 @@ public function getArrayCopy()
      */
     public function exchangeArray(array $input)
     {
-        $current = $this->params;
-        $this->params = [];
+        $current = $this->items;
+        $this->items = [];
         foreach ($input as $key => $value) {
             $this->set($key, $value);
         }
@@ -243,7 +243,7 @@ public function exchangeArray(array $input)
     public function request()
     {
         $request = [];
-        foreach ($this->params as $name => $values) {
+        foreach ($this->items as $name => $values) {
             if (!empty($values)) {
                 $request = array_merge(
                     $request,
diff --git a/module/VuFindSearch/tests/fixtures/blender/response/solr/search-author.json b/module/VuFindSearch/tests/fixtures/blender/response/solr/search-author.json
index a3d4705a94b..503ee20791c 100644
--- a/module/VuFindSearch/tests/fixtures/blender/response/solr/search-author.json
+++ b/module/VuFindSearch/tests/fixtures/blender/response/solr/search-author.json
@@ -181,62 +181,69 @@
         "work_keys_str_mv":["AT willbbrokeafterwordbypssherman ofmoneyandslashes"],
         "_version_":1726988259027845120}]
   },
-  "facet_counts":{
-    "facet_queries":{},
-    "facet_fields":{
-      "institution":
-      [
-        ["MyInstitution",240]],
-      "building":
-      [
-        ["0/Main/", 231],
-        ["1/Main/Sub/", 7],
-        ["0/Sub/",1],
-        ["1/Sub/Foo/",1]],
-      "format":
-      [
-        ["Book Chapter",177],
-        ["Book",63]],
-      "author_facet":
-      [
-        ["Granda, Isiah",5],
-        ["Hartfield, Shelba",5],
-        ["Oram, Donn",5],
-        ["Overall, Lula",5],
-        ["Winrow, Sanjuana",5],
-        ["Abner, Shelli",4],
-        ["Crafts, Claudette",4],
-        ["Erickson, Joette",4],
-        ["Harrill, Mariko",4],
-        ["Leyva, Marie",4],
-        ["Mackson, Yolanda",4],
-        ["Oros, Cierra",4],
-        ["Pakele, Marina",4],
-        ["Saar, Danna",4],
-        ["Walcott, Porfirio",4],
-        ["Murden, Claud",3],
-        ["Nelms, Lanelle",3],
-        ["Alcina, Rashad",2],
-        ["Arechiga, Moon",2],
-        ["Beckett, Tamala",2],
-        ["Behringer, Anitra",2],
-        ["Beveridge, Albina",2],
-        ["Blacker, Minerva",2],
-        ["Bogard, Noma",2],
-        ["Bornstein, Dia",2],
-        ["Boyland, Bebe",2],
-        ["Bucher, Margit",2],
-        ["Cantrell, Cornelius",2],
-        ["Cornforth, Desiree",2],
-        ["Counter, Modesto",2]],
-      "language":
-      [
-        ["English",7],
-        ["Italian",2],
-        ["Latin",2]]},
-    "facet_ranges":{},
-    "facet_intervals":{},
-    "facet_heatmaps":{}},
+  "facets": {
+    "count": 240,
+    "institution": {
+      "buckets": [
+        { "val": "MyInstitution", "count": 240 }
+      ]
+    },
+    "building": {
+      "buckets": [
+        { "val": "0/Main/", "count": 231 },
+        { "val": "1/Main/Sub/", "count": 7 },
+        { "val": "0/Sub/", "count": 1 },
+        { "val": "1/Sub/Foo/", "count": 1 }
+      ]
+    },
+    "format": {
+      "buckets": [
+        { "val": "Book Chapter", "count": 177 },
+        { "val": "Book", "count": 63 }
+      ]
+    },
+    "author_facet": {
+      "buckets": [
+        { "val": "Granda, Isiah", "count": 5 },
+        { "val": "Hartfield, Shelba", "count": 5 },
+        { "val": "Oram, Donn", "count": 5 },
+        { "val": "Overall, Lula", "count": 5 },
+        { "val": "Winrow, Sanjuana", "count": 5 },
+        { "val": "Abner, Shelli", "count": 4 },
+        { "val": "Crafts, Claudette", "count": 4 },
+        { "val": "Erickson, Joette", "count": 4 },
+        { "val": "Harrill, Mariko", "count": 4 },
+        { "val": "Leyva, Marie", "count": 4 },
+        { "val": "Mackson, Yolanda", "count": 4 },
+        { "val": "Oros, Cierra", "count": 4 },
+        { "val": "Pakele, Marina", "count": 4 },
+        { "val": "Saar, Danna", "count": 4 },
+        { "val": "Walcott, Porfirio", "count": 4 },
+        { "val": "Murden, Claud", "count": 3 },
+        { "val": "Nelms, Lanelle", "count": 3 },
+        { "val": "Alcina, Rashad", "count": 2 },
+        { "val": "Arechiga, Moon", "count": 2 },
+        { "val": "Beckett, Tamala", "count": 2 },
+        { "val": "Behringer, Anitra", "count": 2 },
+        { "val": "Beveridge, Albina", "count": 2 },
+        { "val": "Blacker, Minerva", "count": 2 },
+        { "val": "Bogard, Noma", "count": 2 },
+        { "val": "Bornstein, Dia", "count": 2 },
+        { "val": "Boyland, Bebe", "count": 2 },
+        { "val": "Bucher, Margit", "count": 2 },
+        { "val": "Cantrell, Cornelius", "count": 2 },
+        { "val": "Cornforth, Desiree", "count": 2 },
+        { "val": "Counter, Modesto", "count": 2 }
+      ]
+    },
+    "language": {
+      "buckets": [
+        { "val": "English", "count": 7 },
+        { "val": "Italian", "count": 2 },
+        { "val": "Latin", "count": 2 }
+      ]
+    }
+  },
   "spellcheck":{
     "suggestions":
     [],
diff --git a/module/VuFindSearch/tests/fixtures/blender/response/solr/search-title.json b/module/VuFindSearch/tests/fixtures/blender/response/solr/search-title.json
index c3d42d663e1..39695663d6a 100644
--- a/module/VuFindSearch/tests/fixtures/blender/response/solr/search-title.json
+++ b/module/VuFindSearch/tests/fixtures/blender/response/solr/search-title.json
@@ -409,62 +409,69 @@
         "work_keys_str_mv":["AT demiankatz octothorpeswhynot"],
         "_version_":1726988259035185152}]
   },
-  "facet_counts":{
-    "facet_queries":{},
-    "facet_fields":{
-      "institution":
-      [
-        ["MyInstitution",240]],
-      "building":
-      [
-        ["0/Main/", 231],
-        ["1/Main/Sub/", 7],
-        ["0/Sub/",1],
-        ["1/Sub/Foo/",1]],
-      "format":
-      [
-        ["Book Chapter",177],
-        ["Book",63]],
-      "author_facet":
-      [
-        ["Granda, Isiah",5],
-        ["Hartfield, Shelba",5],
-        ["Oram, Donn",5],
-        ["Overall, Lula",5],
-        ["Winrow, Sanjuana",5],
-        ["Abner, Shelli",4],
-        ["Crafts, Claudette",4],
-        ["Erickson, Joette",4],
-        ["Harrill, Mariko",4],
-        ["Leyva, Marie",4],
-        ["Mackson, Yolanda",4],
-        ["Oros, Cierra",4],
-        ["Pakele, Marina",4],
-        ["Saar, Danna",4],
-        ["Walcott, Porfirio",4],
-        ["Murden, Claud",3],
-        ["Nelms, Lanelle",3],
-        ["Alcina, Rashad",2],
-        ["Arechiga, Moon",2],
-        ["Beckett, Tamala",2],
-        ["Behringer, Anitra",2],
-        ["Beveridge, Albina",2],
-        ["Blacker, Minerva",2],
-        ["Bogard, Noma",2],
-        ["Bornstein, Dia",2],
-        ["Boyland, Bebe",2],
-        ["Bucher, Margit",2],
-        ["Cantrell, Cornelius",2],
-        ["Cornforth, Desiree",2],
-        ["Counter, Modesto",2]],
-      "language":
-      [
-        ["English",7],
-        ["Italian",2],
-        ["Latin",2]]},
-    "facet_ranges":{},
-    "facet_intervals":{},
-    "facet_heatmaps":{}},
+"facets": {
+    "count": 240,
+    "institution": {
+      "buckets": [
+        { "val": "MyInstitution", "count": 240 }
+      ]
+    },
+    "building": {
+      "buckets": [
+        { "val": "0/Main/", "count": 231 },
+        { "val": "1/Main/Sub/", "count": 7 },
+        { "val": "0/Sub/", "count": 1 },
+        { "val": "1/Sub/Foo/", "count": 1 }
+      ]
+    },
+    "format": {
+      "buckets": [
+        { "val": "Book Chapter", "count": 177 },
+        { "val": "Book", "count": 63 }
+      ]
+    },
+    "author_facet": {
+      "buckets": [
+        { "val": "Granda, Isiah", "count": 5 },
+        { "val": "Hartfield, Shelba", "count": 5 },
+        { "val": "Oram, Donn", "count": 5 },
+        { "val": "Overall, Lula", "count": 5 },
+        { "val": "Winrow, Sanjuana", "count": 5 },
+        { "val": "Abner, Shelli", "count": 4 },
+        { "val": "Crafts, Claudette", "count": 4 },
+        { "val": "Erickson, Joette", "count": 4 },
+        { "val": "Harrill, Mariko", "count": 4 },
+        { "val": "Leyva, Marie", "count": 4 },
+        { "val": "Mackson, Yolanda", "count": 4 },
+        { "val": "Oros, Cierra", "count": 4 },
+        { "val": "Pakele, Marina", "count": 4 },
+        { "val": "Saar, Danna", "count": 4 },
+        { "val": "Walcott, Porfirio", "count": 4 },
+        { "val": "Murden, Claud", "count": 3 },
+        { "val": "Nelms, Lanelle", "count": 3 },
+        { "val": "Alcina, Rashad", "count": 2 },
+        { "val": "Arechiga, Moon", "count": 2 },
+        { "val": "Beckett, Tamala", "count": 2 },
+        { "val": "Behringer, Anitra", "count": 2 },
+        { "val": "Beveridge, Albina", "count": 2 },
+        { "val": "Blacker, Minerva", "count": 2 },
+        { "val": "Bogard, Noma", "count": 2 },
+        { "val": "Bornstein, Dia", "count": 2 },
+        { "val": "Boyland, Bebe", "count": 2 },
+        { "val": "Bucher, Margit", "count": 2 },
+        { "val": "Cantrell, Cornelius", "count": 2 },
+        { "val": "Cornforth, Desiree", "count": 2 },
+        { "val": "Counter, Modesto", "count": 2 }
+      ]
+    },
+    "language": {
+      "buckets": [
+        { "val": "English", "count": 7 },
+        { "val": "Italian", "count": 2 },
+        { "val": "Latin", "count": 2 }
+      ]
+    }
+  },
   "spellcheck":{
     "suggestions":
     [],
diff --git a/module/VuFindSearch/tests/fixtures/blender/response/solr/search.json b/module/VuFindSearch/tests/fixtures/blender/response/solr/search.json
index 0017bec6519..206d4aa72cc 100644
--- a/module/VuFindSearch/tests/fixtures/blender/response/solr/search.json
+++ b/module/VuFindSearch/tests/fixtures/blender/response/solr/search.json
@@ -1844,63 +1844,70 @@
           "Site 324"],
         "_version_":1726988325127979008}]
   },
-  "facet_counts":{
-    "facet_queries":{},
-    "facet_fields":{
-      "institution":
-      [
-        ["MyInstitution",240]],
-      "building":
-      [
-        ["0/Main/", 231],
-        ["1/Main/Sub/", 7],
-        ["0/Sub/",1],
-        ["1/Sub/Foo/",1]],
-      "format":
-      [
-        ["Book Chapter",177],
-        ["Book",63]],
-      "author_facet":
-      [
-        ["Granda, Isiah",5],
-        ["Hartfield, Shelba",5],
-        ["Oram, Donn",5],
-        ["Overall, Lula",5],
-        ["Winrow, Sanjuana",5],
-        ["Abner, Shelli",4],
-        ["Crafts, Claudette",4],
-        ["Erickson, Joette",4],
-        ["Harrill, Mariko",4],
-        ["Leyva, Marie",4],
-        ["Mackson, Yolanda",4],
-        ["Oros, Cierra",4],
-        ["Pakele, Marina",4],
-        ["Saar, Danna",4],
-        ["Walcott, Porfirio",4],
-        ["Murden, Claud",3],
-        ["Nelms, Lanelle",3],
-        ["Alcina, Rashad",2],
-        ["Arechiga, Moon",2],
-        ["Beckett, Tamala",2],
-        ["Behringer, Anitra",2],
-        ["Beveridge, Albina",2],
-        ["Blacker, Minerva",2],
-        ["Bogard, Noma",2],
-        ["Bornstein, Dia",2],
-        ["Boyland, Bebe",2],
-        ["Bucher, Margit",2],
-        ["Cantrell, Cornelius",2],
-        ["Cornforth, Desiree",2],
-        ["Counter, Modesto",2]],
-      "language":
-      [
-        ["English",7],
-        ["Italian",2],
-        ["Latin",2]]},
-    "facet_ranges":{},
-    "facet_intervals":{},
-    "facet_heatmaps":{}},
+  "facets": {
+    "count": 240,
+    "institution": {
+      "buckets": [
+        { "val": "MyInstitution", "count": 240 }
+      ]
+    },
+    "building": {
+      "buckets": [
+        { "val": "0/Main/", "count": 231 },
+        { "val": "1/Main/Sub/", "count": 7 },
+        { "val": "0/Sub/", "count": 1 },
+        { "val": "1/Sub/Foo/", "count": 1 }
+      ]
+    },
+    "format": {
+      "buckets": [
+        { "val": "Book Chapter", "count": 177 },
+        { "val": "Book", "count": 63 }
+      ]
+    },
+    "author_facet": {
+      "buckets": [
+        { "val": "Granda, Isiah", "count": 5 },
+        { "val": "Hartfield, Shelba", "count": 5 },
+        { "val": "Oram, Donn", "count": 5 },
+        { "val": "Overall, Lula", "count": 5 },
+        { "val": "Winrow, Sanjuana", "count": 5 },
+        { "val": "Abner, Shelli", "count": 4 },
+        { "val": "Crafts, Claudette", "count": 4 },
+        { "val": "Erickson, Joette", "count": 4 },
+        { "val": "Harrill, Mariko", "count": 4 },
+        { "val": "Leyva, Marie", "count": 4 },
+        { "val": "Mackson, Yolanda", "count": 4 },
+        { "val": "Oros, Cierra", "count": 4 },
+        { "val": "Pakele, Marina", "count": 4 },
+        { "val": "Saar, Danna", "count": 4 },
+        { "val": "Walcott, Porfirio", "count": 4 },
+        { "val": "Murden, Claud", "count": 3 },
+        { "val": "Nelms, Lanelle", "count": 3 },
+        { "val": "Alcina, Rashad", "count": 2 },
+        { "val": "Arechiga, Moon", "count": 2 },
+        { "val": "Beckett, Tamala", "count": 2 },
+        { "val": "Behringer, Anitra", "count": 2 },
+        { "val": "Beveridge, Albina", "count": 2 },
+        { "val": "Blacker, Minerva", "count": 2 },
+        { "val": "Bogard, Noma", "count": 2 },
+        { "val": "Bornstein, Dia", "count": 2 },
+        { "val": "Boyland, Bebe", "count": 2 },
+        { "val": "Bucher, Margit", "count": 2 },
+        { "val": "Cantrell, Cornelius", "count": 2 },
+        { "val": "Cornforth, Desiree", "count": 2 },
+        { "val": "Counter, Modesto", "count": 2 }
+      ]
+    },
+    "language": {
+      "buckets": [
+        { "val": "English", "count": 7 },
+        { "val": "Italian", "count": 2 },
+        { "val": "Latin", "count": 2 }
+      ]
+    }
+  },
   "spellcheck":{
     "suggestions":
     [],
-    "correctlySpelled":true}}
+    "correctlySpelled":true}}
\ No newline at end of file
diff --git a/module/VuFindSearch/tests/fixtures/solr/response/facet b/module/VuFindSearch/tests/fixtures/solr/response/facet
index 6738f16783b..1b094136a81 100644
--- a/module/VuFindSearch/tests/fixtures/solr/response/facet
+++ b/module/VuFindSearch/tests/fixtures/solr/response/facet
@@ -1,43 +1,47 @@
-HTTP/1.1 200 OK
-Content-Security-Policy: default-src 'none'; base-uri 'none'; connect-src 'self'; form-action 'self'; font-src 'self'; frame-ancestors 'none'; img-src 'self'; media-src 'self'; style-src 'self' 'unsafe-inline'; script-src 'self'; worker-src 'self';
-X-Content-Type-Options: nosniff
-X-Frame-Options: SAMEORIGIN
-X-XSS-Protection: 1; mode=block
-Last-Modified: Fri, 08 Apr 2022 10:21:31 GMT
-ETag: "MWMwMDAwMDAwMDAwMDAwMFNvbHI="
-Content-Type: application/json;charset=utf-8
-Vary: Accept-Encoding, User-Agent
-Content-Length: 779
-
+HTTP/1.1 200 OK
+Content-Security-Policy: default-src 'none'; base-uri 'none'; connect-src 'self'; form-action 'self'; font-src 'self'; frame-ancestors 'none'; img-src 'self'; media-src 'self'; style-src 'self' 'unsafe-inline'; script-src 'self'; worker-src 'self';
+X-Content-Type-Options: nosniff
+X-Frame-Options: SAMEORIGIN
+X-XSS-Protection: 1; mode=block
+Last-Modified: Fri, 08 Apr 2022 10:21:31 GMT
+ETag: "MWMwMDAwMDAwMDAwMDAwMFNvbHI="
+Content-Type: application/json;charset=utf-8
+Vary: Accept-Encoding, User-Agent
+Content-Length: 779
+
 {
   "responseHeader":{
     "status":0,
-    "QTime":0,
+    "QTime":3,
     "params":{
-      "facet.limit":"5",
-      "q":"*:*",
-      "facet.field":"topic_facet",
-      "json.nl":"arrarr",
-      "fl":"*",
-      "start":"0",
-      "facet.mincount":"1",
-      "sort":"title_sort asc",
-      "rows":"0",
-      "facet":"true",
-      "wt":"json",
-      "facet.sort":"count"}},
-  "response":{"numFound":317,"start":0,"numFoundExact":true,"docs":[]
+      "json":"{\r\n    \"query\": \"*:*\",\r\n    \"limit\" :0,\r\n    \"facet\": {\r\n        \"topic_facet\": {\r\n            \"type\": \"terms\",\r\n            \"field\": \"topic_facet\",\r\n            \"limit\": 5\r\n        }\r\n    }\r\n}"
+    }
   },
-  "facet_counts":{
-    "facet_queries":{},
-    "facet_fields":{
-      "topic_facet":
-      [
-        ["Research",16],
-        ["Psychotherapy",8],
-        ["Adult children of aging parents",7],
-        ["Automobile drivers' tests",7],
-        ["Fathers and daughters",7]]},
-    "facet_ranges":{},
-    "facet_intervals":{},
-    "facet_heatmaps":{}}}
+  "response":{
+    "numFound":328,
+    "start":0,
+    "numFoundExact":true,
+    "docs":[ ]
+  },
+  "facets":{
+    "count":328,
+    "topic_facet":{
+      "buckets":[{
+        "val":"Research",
+        "count":16
+      },{
+        "val":"Psychotherapy",
+        "count":8
+      },{
+        "val":"Adult children of aging parents",
+        "count":7
+      },{
+        "val":"Automobile drivers' tests",
+        "count":7
+      },{
+        "val":"Fathers and daughters",
+        "count":7
+      }]
+    }
+  }
+}
\ No newline at end of file
diff --git a/module/VuFindSearch/tests/unit-tests/src/VuFindTest/Backend/Blender/BackendTest.php b/module/VuFindSearch/tests/unit-tests/src/VuFindTest/Backend/Blender/BackendTest.php
index 7374dc7181d..036341bc74a 100644
--- a/module/VuFindSearch/tests/unit-tests/src/VuFindTest/Backend/Blender/BackendTest.php
+++ b/module/VuFindSearch/tests/unit-tests/src/VuFindTest/Backend/Blender/BackendTest.php
@@ -350,54 +350,54 @@ public static function getSearchTestData(): array
         );
 
         return [
-            [
+            'limit 0' => [
                 0,
                 0,
                 [],
             ],
-            [
+            'limit 20' => [
                 0,
                 20,
                 array_slice($expectedRecords, 0, 20),
             ],
-            [
+            'offset 1, limit 20' => [
                 1,
                 20,
                 array_slice($expectedRecords, 1, 20),
             ],
-            [
+            'offset 2, limit 20' => [
                 2,
                 20,
                 array_slice($expectedRecords, 2, 20),
             ],
-            [
+            'offset 3, limit 20' => [
                 3,
                 20,
                 array_slice($expectedRecords, 3, 20),
             ],
-            [
+            'offset 19, limit 20' => [
                 19,
                 20,
                 array_slice($expectedRecords, 19, 20),
             ],
-            [
+            'offset 0, limit 40' => [
                 0,
                 40,
                 array_slice($expectedRecords, 0, 40),
             ],
-            [
+            'offset 0, limit 40, no boost config' => [
                 0,
                 40,
                 array_slice($expectedRecordsNoBoost, 0, 40),
                 $noBoostConfig,
             ],
-            [
+            'offset 0, limit 40, adaptive config' => [
                 0,
                 40,
                 array_slice($expectedRecordsAdaptive, 0, 40),
                 $adaptiveConfig,
             ],
-            [
+            'solr only' => [
                 0,
                 20,
                 array_slice($solrRecords, 0, 20),
@@ -406,7 +406,7 @@ public static function getSearchTestData(): array
                 240,
                 null,
             ],
-            [
+            'not EDS' => [
                 0,
                 20,
                 array_slice($solrRecords, 0, 20),
@@ -415,7 +415,7 @@ public static function getSearchTestData(): array
                 240,
                 0,
             ],
-            [
+            'EDS only' => [
                 0,
                 20,
                 array_slice($edsRecords, 0, 20),
@@ -424,7 +424,7 @@ public static function getSearchTestData(): array
                 0,
                 65924,
             ],
-            [
+            'both backends' => [
                 0,
                 40,
                 array_slice($expectedRecords, 0, 40),
@@ -434,7 +434,7 @@ public static function getSearchTestData(): array
                     . '(blender_backend:"Solr" OR blender_backend:"EDS")',
                 ],
             ],
-            [
+            'no Solr or EDS backend' => [
                 0,
                 20,
                 [],
@@ -443,7 +443,7 @@ public static function getSearchTestData(): array
                 0,
                 0,
             ],
-            [
+            'title query' => [
                 0,
                 20,
                 $expectedRecordsTitleSearch,
@@ -453,7 +453,7 @@ public static function getSearchTestData(): array
                 65924,
                 new Query('foo', 'Title'),
             ],
-            [
+            'author query' => [
                 0,
                 20,
                 $expectedRecordsAuthorSearch,
@@ -1102,15 +1102,15 @@ protected function getSolrConnector(
                 throw new BackendException('Simulated Solr failure');
             }
             if (null === $fixture) {
-                $field = $params->get('qf')[0] ?? '';
+                $field = $params->getNested('params', 'qf')[0] ?? '';
                 $type = '';
                 if (in_array($field, ['title', 'author'])) {
                     $type = "-$field";
                 }
                 $fixture = "blender/response/solr/search$type.json";
             }
-            $start = $params->get('start')[0];
-            $rows = $params->get('rows')[0];
+            $start = $params->get('offset')[0];
+            $rows = $params->get('limit')[0];
             $results = $this->getJsonFixture($fixture, 'VuFindSearch');
             $results['response']['docs'] = array_slice(
                 $results['response']['docs'],
diff --git a/module/VuFindSearch/tests/unit-tests/src/VuFindTest/Backend/Solr/BackendTest.php b/module/VuFindSearch/tests/unit-tests/src/VuFindTest/Backend/Solr/BackendTest.php
index 9a6d50f139f..ec9592c05f2 100644
--- a/module/VuFindSearch/tests/unit-tests/src/VuFindTest/Backend/Solr/BackendTest.php
+++ b/module/VuFindSearch/tests/unit-tests/src/VuFindTest/Backend/Solr/BackendTest.php
@@ -40,11 +40,10 @@
 use VuFindSearch\Backend\Solr\Document\CommitDocument;
 use VuFindSearch\Backend\Solr\HandlerMap;
 use VuFindSearch\Backend\Solr\Response\Json\RecordCollection;
+use VuFindSearch\NestingParamBag;
 use VuFindSearch\ParamBag;
 use VuFindSearch\Query\Query;
 
-use function count;
-
 /**
  * Unit tests for SOLR backend.
  *
@@ -430,9 +429,9 @@ public function testInjectResponseWriterThrownOnIncompabileResponseWriter(): voi
         $this->expectException(\VuFindSearch\Exception\InvalidArgumentException::class);
         $this->expectExceptionMessage('Invalid response writer type: xml');
 
-        $conn = $this->getConnectorMock();
+        $conn = $this->getConnectorMock(['retrieve']);
         $back = new Backend($conn);
-        $back->retrieve('foobar', new ParamBag(['wt' => ['xml']]));
+        $back->retrieve('foobar', NestingParamBag::fromArray(['params' => ['wt' => 'xml']]));
     }
 
     /**
@@ -447,7 +446,7 @@ public function testInjectResponseWriterThrownOnIncompabileNamedListSetting(): v
 
         $conn = $this->getConnectorMock();
         $back = new Backend($conn);
-        $back->retrieve('foobar', new ParamBag(['json.nl' => ['bad']]));
+        $back->retrieve('foobar', NestingParamBag::fromArray(['params' => ['json.nl' => ['bad']]]));
     }
 
     /**
@@ -489,30 +488,27 @@ public static function getIdsProvider(): \Iterator
     /**
      * Test getting multiple IDs.
      *
-     * @param ?string $flIn          Additional field list in input (null = none)
-     * @param string  $expectedFlOut Expected field list in output
+     * @param ?string $fieldsIn          Additional field list in input (null = none)
+     * @param string  $expectedFieldsOut Expected field list in output
      *
      * @return void
      */
     #[\PHPUnit\Framework\Attributes\DataProvider('getIdsProvider')]
-    public function testGetIds(?string $flIn, string $expectedFlOut): void
+    public function testGetIds(?string $fieldsIn, string $expectedFieldsOut): void
     {
-        $paramBagChecker = function (ParamBag $params) use ($expectedFlOut) {
+        $paramBagChecker = function (ParamBag $params) use ($expectedFieldsOut) {
             $expected = [
-                'wt' => ['json'],
-                'json.nl' => ['arrarr'],
-                'fl' => [$expectedFlOut],
-                'rows' => [10],
-                'start' => [0],
-                'q' => ['foo'],
+                'params' => [
+                    'wt' => 'json',
+                    'json.nl' => 'arrarr',
+                ],
+                'fields' => $expectedFieldsOut,
+                'limit' => 10,
+                'offset' => 0,
+                'query' => 'foo',
             ];
-            $paramsArr = $params->getArrayCopy();
-            foreach ($expected as $key => $vals) {
-                if (count(array_diff($vals, $paramsArr[$key] ?? [])) !== 0) {
-                    return false;
-                }
-            }
-            return true;
+            $paramsArr = NestingParamBag::from($params)->jsonSerialize();
+            return $expected == $paramsArr;
         };
         // TODO: currently this test is concerned with ensuring that the right
         // parameters are sent to Solr; it may be worth adding a more realistic
@@ -524,8 +520,8 @@ public function testGetIds(?string $flIn, string $expectedFlOut): void
         $back = new Backend($conn);
         $query = new Query('foo');
         $params = new ParamBag();
-        if ($flIn) {
-            $params->set('fl', $flIn);
+        if ($fieldsIn) {
+            $params->set('fields', $fieldsIn);
         }
         $result = $back->getIds($query, 0, 10, $params);
         $this->assertInstanceOf(RecordCollection::class, $result);
@@ -579,7 +575,7 @@ public function testRefineAlphaBrowseExceptionWithNonBrowseString(): void
     public function testRandom(): void
     {
         // Test that random sort parameter is added:
-        $params = $this->getMockBuilder(\VuFindSearch\ParamBag::class)
+        $params = $this->getMockBuilder(\VuFindSearch\NestingParamBag::class)
             ->onlyMethods(['set'])->getMock();
         $params->expects($this->once())->method('set')
             ->with('sort', $this->matchesRegularExpression('/[0-9]+_random asc/'));
diff --git a/module/VuFindSearch/tests/unit-tests/src/VuFindTest/Backend/Solr/ConnectorTest.php b/module/VuFindSearch/tests/unit-tests/src/VuFindTest/Backend/Solr/ConnectorTest.php
index f59214eb4ed..ccb88f52cad 100644
--- a/module/VuFindSearch/tests/unit-tests/src/VuFindTest/Backend/Solr/ConnectorTest.php
+++ b/module/VuFindSearch/tests/unit-tests/src/VuFindTest/Backend/Solr/ConnectorTest.php
@@ -281,7 +281,7 @@ public function testClientCreation()
         $httpService = $this->createMock(\VuFindHttp\HttpService::class);
         $httpService->expects($this->once())
             ->method('createClient')
-            ->with('http://localhost/select?q=id%3A%221%22')
+            ->with('http://localhost/select')
             ->willReturn($this->createClient());
         $connector = new Connector(
             'http://localhost',
diff --git a/module/VuFindSearch/tests/unit-tests/src/VuFindTest/Backend/Solr/QueryBuilderTest.php b/module/VuFindSearch/tests/unit-tests/src/VuFindTest/Backend/Solr/QueryBuilderTest.php
index d92a099f12a..9edda0894f9 100644
--- a/module/VuFindSearch/tests/unit-tests/src/VuFindTest/Backend/Solr/QueryBuilderTest.php
+++ b/module/VuFindSearch/tests/unit-tests/src/VuFindTest/Backend/Solr/QueryBuilderTest.php
@@ -105,8 +105,8 @@ public function testNormalization(string $input, string $output): void
         $qb = new QueryBuilder();
         $q = new Query($input);
         $response = $qb->build($q);
-        $processedQ = $response->get('q');
-        $this->assertEquals($output, $processedQ[0]);
+        $processedQuery = $response->get('query');
+        $this->assertEquals($output, $processedQuery[0]);
     }
 
     /**
@@ -172,8 +172,8 @@ protected function runBasicQuestionTest($qb, $handler, $test)
         $response = $qb->build($q);
         // Make sure the query builder had no side effects on the query object:
         $this->assertEquals($before, $q->getString());
-        $processedQ = $response->get('q');
-        $this->assertEquals($basicOutput, $processedQ[0]);
+        $processedQuery = $response->get('query');
+        $this->assertEquals($basicOutput, $processedQuery[0]);
     }
 
     /**
@@ -200,8 +200,8 @@ protected function runAdvancedQuestionTest($qb, $handler, $test)
         }
         $advancedQ = new QueryGroup('AND', [new Query($input, 'test')]);
         $advResponse = $qb->build($advancedQ);
-        $advProcessedQ = $advResponse->get('q');
-        $this->assertEquals($advOutput, $advProcessedQ[0]);
+        $advProcessedQuery = $advResponse->get('query');
+        $this->assertEquals($advOutput, $advProcessedQuery[0]);
     }
 
     /**
@@ -291,13 +291,13 @@ public function testExactQueryHandler()
         // non-quoted search uses main DismaxFields
         $q = new Query('q', 'test');
         $response = $qb->build($q);
-        $qf = $response->get('qf');
+        $qf = $response->getNested('params', 'qf');
         $this->assertEquals('a b', $qf[0]);
 
         // quoted search uses ExactSettings>DismaxFields
         $q = new Query('"q"', 'test');
         $response = $qb->build($q);
-        $qf = $response->get('qf');
+        $qf = $response->getNested('params', 'qf');
         $this->assertEquals('c d', $qf[0]);
     }
 
@@ -315,8 +315,8 @@ public function testQueryHandlerWithFilterQueryAndDisMax()
         );
         $q = new Query('q', 'test');
         $response = $qb->build($q);
-        $fq = $response->get('fq');
-        $this->assertEquals('a:filter', $fq[0]);
+        $filter = $response->get('filter');
+        $this->assertEquals('a:filter', $filter[0]);
     }
 
     /**
@@ -333,8 +333,8 @@ public function testQueryHandlerWithFilterQueryAndNoDisMax()
         );
         $q = new Query('q', 'test');
         $response = $qb->build($q);
-        $q = $response->get('q');
-        $this->assertEquals('((q) AND (a:filter))', $q[0]);
+        $query = $response->get('query');
+        $this->assertEquals('((q) AND (a:filter))', $query[0]);
     }
 
     /**
@@ -352,8 +352,8 @@ public function testMatchAllQueryWithFilterQueryAndNoDisMax()
         );
         $q = new Query('*:*', 'test');
         $response = $qb->build($q);
-        $q = $response->get('q');
-        $this->assertEquals('a:filter', $q[0]);
+        $query = $response->get('query');
+        $this->assertEquals('a:filter', $query[0]);
     }
 
     /**
@@ -380,10 +380,10 @@ public function testHlQ()
 
         $qb->setFieldsToHighlight('*');
         $response = $qb->build($q);
-        $hlq = $response->get('hl.q');
-        $q = $response->get('q');
+        $hlq = $response->getNested('params', 'hl.q');
+        $query = $response->get('query');
         $this->assertEquals('(my friend*)', $hlq[0]);
-        $this->assertEquals('((my friend*)) AND (*:* OR boost)', $q[0]);
+        $this->assertEquals('((my friend*)) AND (*:* OR boost)', $query[0]);
     }
 
     /**
@@ -422,7 +422,7 @@ public function testSetFieldsToHighlight()
         foreach ($tests as $input => $output) {
             $qb->setFieldsToHighlight($input);
             $response = $qb->build($q);
-            $hlfl = $response->get('hl.fl');
+            $hlfl = $response->getNested('params', 'hl.fl');
             $this->assertEquals($output, $hlfl[0] ?? null);
         }
     }
@@ -448,13 +448,13 @@ public function testSetCreateSpellingQuery()
         // No spellcheck.q if spellcheck query disabled:
         $qb->setCreateSpellingQuery(false);
         $response1 = $qb->build($q);
-        $spQ1 = $response1->get('spellcheck.q');
+        $spQ1 = $response1->getNested('params', 'spellcheck.q');
         $this->assertFalse(isset($spQ1[0]));
 
         // spellcheck.q if spellcheck query enabled:
         $qb->setCreateSpellingQuery(true);
         $response2 = $qb->build($q);
-        $spQ2 = $response2->get('spellcheck.q');
+        $spQ2 = $response2->getNested('params', 'spellcheck.q');
         $this->assertEquals('my friend', $spQ2[0]);
     }
 
@@ -481,11 +481,11 @@ public function testQueryGroup()
         $q = new QueryGroup('OR', [$q1, $q2]);
 
         $response = $qb->build($q);
-        $processedQ = $response->get('q');
+        $processedQuery = $response->get('query');
         $this->assertEquals(
             '((_query_:"{!dismax qf=\"field_a\" mm=\\\'100%\\\'}value1") OR '
             . '(_query_:"{!dismax qf=\"field_b\" mm=\\\'100%\\\'}value2"))',
-            $processedQ[0]
+            $processedQuery[0]
         );
     }
 
@@ -516,11 +516,11 @@ public function testQueryGroupWithAdvancedSyntax()
         $q = new QueryGroup('OR', [$q1, $q2]);
 
         $response = $qb->build($q);
-        $processedQ = $response->get('q');
+        $processedQuery = $response->get('query');
         $this->assertEquals(
             '((field_a:(value*)^100 OR field_c:(value*)^200) OR '
             . '(_query_:"{!dismax qf=\"field_b\" mm=\\\'100%\\\'}value2"))',
-            $processedQ[0]
+            $processedQuery[0]
         );
     }
 
@@ -544,8 +544,8 @@ public function testMultipleQuotedPhrases()
         $q = new Query('"foo" "bar" "baz"', 'a');
 
         $response = $qb->build($q);
-        $processedQ = $response->get('q');
-        $this->assertEquals('(field_a:("foo" OR "bar" OR "baz"))', $processedQ[0]);
+        $processedQuery = $response->get('query');
+        $this->assertEquals('(field_a:("foo" OR "bar" OR "baz"))', $processedQuery[0]);
     }
 
     /**
@@ -568,8 +568,8 @@ public function testMixedQuotedPhrases()
         $q = new Query('708396 "708398" 708399 "708400"', 'a');
 
         $response = $qb->build($q);
-        $processedQ = $response->get('q');
-        $this->assertEquals('(field_a:(708396 OR "708398" OR 708399 OR "708400"))', $processedQ[0]);
+        $processedQuery = $response->get('query');
+        $this->assertEquals('(field_a:(708396 OR "708398" OR 708399 OR "708400"))', $processedQuery[0]);
     }
 
     /**
@@ -592,8 +592,8 @@ public function testMixedQuotedPhrasesWithEscapedQuote()
         $q = new Query('708396 "708398" 708399 "foo\"bar"', 'a');
 
         $response = $qb->build($q);
-        $processedQ = $response->get('q');
-        $this->assertEquals('(field_a:(708396 OR "708398" OR 708399 OR "foo\"bar"))', $processedQ[0]);
+        $processedQuery = $response->get('query');
+        $this->assertEquals('(field_a:(708396 OR "708398" OR 708399 OR "foo\"bar"))', $processedQuery[0]);
     }
 
     /**
@@ -805,7 +805,7 @@ public function testIndividualQueryHandlerWithGlobalExtraParams(
         $qb = new QueryBuilder($specs);
         $response = $qb->build($q1, $params1);
         foreach ($expected1 as $field => $expected) {
-            $values = $response->get($field);
+            $values = $response->getNested('params', $field);
             $this->assertEquals(
                 $expected,
                 $values,
@@ -814,7 +814,7 @@ public function testIndividualQueryHandlerWithGlobalExtraParams(
         }
         $response = $qb->build($q2, $params2);
         foreach ($expected2 as $field => $expected) {
-            $values = $response->get($field);
+            $values = $response->getNested('params', $field);
             $this->assertEquals(
                 $expected,
                 $values,
@@ -927,7 +927,7 @@ public function testGroupedQueryHandlerWithGlobalExtraParams(
         $qb = new QueryBuilder($specs);
         $response = $qb->build($group);
         foreach ($expectedFields as $field => $expected) {
-            $values = $response->get($field);
+            $values = $response->getNested('params', $field);
             $this->assertEquals(
                 $expected,
                 $values
@@ -945,7 +945,7 @@ public function testNegatedQuery()
         $group = new QueryGroup('NOT', [new Query('q')]);
         $qb = new QueryBuilder([]);
         $response = $qb->build($group);
-        $this->assertEquals(['(*:* NOT (q))'], $response->get('q'));
+        $this->assertEquals(['(*:* NOT (q))'], $response->get('query'));
     }
 
     /**
@@ -962,7 +962,7 @@ public function testNegatedAndQuery()
         $response = $qb->build($group);
         $this->assertEquals(
             ['((*:* NOT (q1 OR q2)) AND (q3 AND q4))'],
-            $response->get('q')
+            $response->get('query')
         );
     }
 
@@ -980,7 +980,7 @@ public function testNegatedOrQuery()
         $response = $qb->build($group);
         $this->assertEquals(
             ['((*:* NOT (q1 OR q2)) OR (q3 AND q4))'],
-            $response->get('q')
+            $response->get('query')
         );
     }
 
@@ -1016,8 +1016,8 @@ public function testDismaxMunge()
             [$input, $output] = $test;
             $q = new Query($input, 'test');
             $response = $qb->build($q);
-            $processedQ = $response->get('q');
-            $this->assertEquals($output, $processedQ[0]);
+            $processedQuery = $response->get('query');
+            $this->assertEquals($output, $processedQuery[0]);
         }
     }
 }
diff --git a/module/VuFindSearch/tests/unit-tests/src/VuFindTest/Backend/Solr/Response/Json/RecordCollectionTest.php b/module/VuFindSearch/tests/unit-tests/src/VuFindTest/Backend/Solr/Response/Json/RecordCollectionTest.php
index 99df0042642..4ff4dce96f3 100644
--- a/module/VuFindSearch/tests/unit-tests/src/VuFindTest/Backend/Solr/Response/Json/RecordCollectionTest.php
+++ b/module/VuFindSearch/tests/unit-tests/src/VuFindTest/Backend/Solr/Response/Json/RecordCollectionTest.php
@@ -243,12 +243,13 @@ public function testFacets()
     {
         $coll = new RecordCollection(
             [
-                'facet_counts' => [
-                    'facet_fields' => [
-                        'format' => [
-                            ['Book', 123],
-                            ['Journal', 234],
-                            ['Map', 1],
+                'facets' => [
+                    'count' => 358,
+                    'format' => [
+                        'buckets' => [
+                            ['val' => 'Book', 'count' => 123],
+                            ['val' => 'Journal', 'count' => 234],
+                            ['val' => 'Map', 'count' => 1],
                         ],
                     ],
                 ],
diff --git a/module/VuFindSearch/tests/unit-tests/src/VuFindTest/Backend/Solr/SimilarBuilderTest.php b/module/VuFindSearch/tests/unit-tests/src/VuFindTest/Backend/Solr/SimilarBuilderTest.php
index 674a5ef5b6d..e2326d22be3 100644
--- a/module/VuFindSearch/tests/unit-tests/src/VuFindTest/Backend/Solr/SimilarBuilderTest.php
+++ b/module/VuFindSearch/tests/unit-tests/src/VuFindTest/Backend/Solr/SimilarBuilderTest.php
@@ -54,11 +54,11 @@ public function testDefaultParams()
     {
         $sb = new SimilarBuilder();
         $response = $sb->build('testrecord');
-        $rows = $response->get('rows');
-        $this->assertEquals(5, $rows[0]);
-        $q = $response->get('q');
-        $this->assertEquals('id:"testrecord"', $q[0]);
-        $qt = $response->get('qt');
+        $limit = $response->get('limit');
+        $this->assertEquals(5, $limit[0]);
+        $query = $response->get('query');
+        $this->assertEquals('id:"testrecord"', $query[0]);
+        $qt = $response->getNested('params', 'qt');
         $this->assertEquals('morelikethis', $qt[0]);
     }
 
@@ -71,8 +71,8 @@ public function testAlternativeIdField()
     {
         $sb = new SimilarBuilder(null, 'key');
         $response = $sb->build('testrecord');
-        $q = $response->get('q');
-        $this->assertEquals('key:"testrecord"', $q[0]);
+        $query = $response->get('query');
+        $this->assertEquals('key:"testrecord"', $query[0]);
     }
 
     /**
@@ -89,27 +89,27 @@ public function testMltConfig()
         ];
         $sb = new SimilarBuilder(new \VuFind\Config\Config($config));
         $response = $sb->build('testrecord');
-        $rows = $response->get('rows');
-        $this->assertEquals(10, $rows[0]);
+        $limit = $response->get('limit');
+        $this->assertEquals(10, $limit[0]);
 
         $config['MoreLikeThis']['useMoreLikeThisHandler'] = true;
         $sb = new SimilarBuilder(new \VuFind\Config\Config($config));
         $response = $sb->build('testrecord');
-        $rows = $response->get('rows');
-        $this->assertEquals(10, $rows[0]);
-        $q = $response->get('q');
+        $limit = $response->get('limit');
+        $this->assertEquals(10, $limit[0]);
+        $query = $response->get('query');
         $this->assertEquals(
             '{!mlt qf=title,title_short,callnumber-label,topic,language,author,'
             . 'publishDate mintf=1 mindf=1}testrecord',
-            $q[0]
+            $query[0]
         );
-        $qt = $response->get('qt');
+        $qt = $response->getNested('params', 'qt');
         $this->assertEquals(null, $qt);
 
         $config['MoreLikeThis']['params'] = 'qf=title,topic';
         $sb = new SimilarBuilder(new \VuFind\Config\Config($config));
         $response = $sb->build('testrecord');
-        $q = $response->get('q');
-        $this->assertEquals('{!mlt qf=title,topic}testrecord', $q[0]);
+        $query = $response->get('query');
+        $this->assertEquals('{!mlt qf=title,topic}testrecord', $query[0]);
     }
 }
diff --git a/module/VuFindSearch/tests/unit-tests/src/VuFindTest/NestingParamBagTest.php b/module/VuFindSearch/tests/unit-tests/src/VuFindTest/NestingParamBagTest.php
new file mode 100644
index 00000000000..0495644c6fb
--- /dev/null
+++ b/module/VuFindSearch/tests/unit-tests/src/VuFindTest/NestingParamBagTest.php
@@ -0,0 +1,463 @@
+<?php
+
+/**
+ * Unit tests for NestingParamBag.
+ *
+ * PHP version 8
+ *
+ * Copyright (C) Villanova University 2025.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2,
+ * as published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, see
+ * <https://www.gnu.org/licenses/>.
+ *
+ * @category VuFind
+ * @package  Search
+ * @author   Maccabee Levine <msl321@lehigh.edu>
+ * @license  http://opensource.org/licenses/gpl-2.0.php GNU General Public License
+ * @link     https://vufind.org
+ */
+
+namespace VuFindTest;
+
+use PHPUnit\Framework\TestCase;
+use VuFindSearch\NestingParamBag;
+use VuFindSearch\ParamBag;
+
+use function is_string;
+
+/**
+ * Unit tests for NestingParamBag.
+ *
+ * @category VuFind
+ * @package  Search
+ * @author   Maccabee Levine <msl321@lehigh.edu>
+ * @license  http://opensource.org/licenses/gpl-2.0.php GNU General Public License
+ * @link     https://vufind.org
+ */
+class NestingParamBagTest extends TestCase
+{
+    /**
+     * Data provider for testFrom
+     *
+     * @return \Iterator
+     */
+    public static function fromProvider(): \Iterator
+    {
+        yield 'simple' => [
+            new ParamBag([ 'filter' => 'format:Book', 'debugQuery' => true ]),
+            [],
+            [ 'filter' => ['format:Book'], 'debugQuery' => [ true ], ],
+        ];
+        yield 'null, default create' => [
+            null,
+            [],
+            [],
+        ];
+        yield 'null, create true' => [
+            null,
+            [ true ],
+            [],
+        ];
+        yield 'null, create false' => [
+            null,
+            [ false ],
+            null,
+        ];
+    }
+
+    /**
+     * Test static from() function.
+     *
+     * @param ?ParamBag $original          Original ParamBag
+     * @param array     $createIfNullParam Array containing nothing or the $createIfNull bool
+     * @param ?array    $expectedContent   Expected content of the NestingParamBag
+     *
+     * @return void
+     */
+    #[\PHPUnit\Framework\Attributes\DataProvider('fromProvider')]
+    public function testFrom(?ParamBag $original, array $createIfNullParam, ?array $expectedContent): void
+    {
+        $params = NestingParamBag::from($original, ...$createIfNullParam);
+        $this->assertSame($expectedContent, $params?->getArrayCopy());
+    }
+
+    /**
+     * Data provider for testFromArray
+     *
+     * @return \Iterator
+     */
+    public static function fromArrayProvider(): \Iterator
+    {
+        yield 'simple' => [
+            [ 'filter' => 'format:Book', 'rows' => 10 ],
+            new NestingParamBag([ 'filter' => [ 'format:Book' ], 'rows' => [ 10 ] ]),
+        ];
+        yield 'two dimensions' => [
+            [ 'filter' => 'format:Book', 'params' => [ 'sow' => false, 'timeAllowed' => -1 ] ],
+            new NestingParamBag([
+                'filter' => [ 'format:Book' ],
+                 'params' => new NestingParamBag(['sow' => false, 'timeAllowed' => -1]),
+            ]),
+        ];
+    }
+
+    /**
+     * Test static fromArray() function.
+     *
+     * @param array           $values   Input values
+     * @param NestingParamBag $expected Expected result
+     *
+     * @return void
+     */
+    #[\PHPUnit\Framework\Attributes\DataProvider('fromArrayProvider')]
+    public function testFromArray(array $values, NestingParamBag $expected): void
+    {
+        $params = NestingParamBag::fromArray($values);
+        $this->assertEquals($expected, $params);
+    }
+
+    /**
+     * Data provider for getNested method
+     *
+     * @return \Iterator
+     */
+    public static function getNestedProvider(): \Iterator
+    {
+        yield 'present' => [ self::buildInputParams(), 'params', 'sow', [ false ] ];
+        yield 'absent' => [ self::buildInputParams(), 'params', 'debugQuery', null ];
+    }
+
+    /**
+     * Test getNested method.
+     *
+     * @param NestingParamBag $params     Bag of nested parameters
+     * @param string          $name       Parameter name
+     * @param string          $nestedName Nested parameter name
+     * @param ?array          $expected   Expected array of parameter values or NULL if not set
+     *
+     * @return void
+     */
+    #[\PHPUnit\Framework\Attributes\DataProvider('getNestedProvider')]
+    public function testGetNested(NestingParamBag $params, string $name, string $nestedName, ?array $expected): void
+    {
+        $this->assertSame($expected, $params->getNested($name, $nestedName));
+    }
+
+    /**
+     * Test hasNestedParam method.
+     *
+     * @param NestingParamBag $params        Bag of nested parameters
+     * @param string          $name          Parameter name
+     * @param string          $nestedName    Nested parameter name
+     * @param ?array          $expectedArray Expected array of parameter values or NULL if not set
+     *
+     * @return void
+     */
+    #[\PHPUnit\Framework\Attributes\DataProvider('getNestedProvider')]
+    public function testHasNested(
+        NestingParamBag $params,
+        string $name,
+        string $nestedName,
+        ?array $expectedArray
+    ): void {
+        $expected = !empty($expectedArray);
+        $this->assertSame($expected, $params->hasNestedParam($name, $nestedName));
+    }
+
+    /**
+     * Data provider for setNested method
+     *
+     * @return \Iterator
+     */
+    public static function setNestedProvider(): \Iterator
+    {
+        yield 'add value' => [
+            self::buildInputParams(),
+            'params', 'spellcheck', true,
+             new NestingParamBag([
+                'filter' => [ 'format:Book' ],
+                'params' => new NestingParamBag(['sow' => false, 'timeAllowed' => -1, 'spellcheck' => true]),
+             ]),
+        ];
+        yield 'overwrite value' => [
+            self::buildInputParams(),
+            'params', 'sow', true,
+             new NestingParamBag([
+                'filter' => [ 'format:Book' ],
+                'params' => new NestingParamBag(['sow' => true, 'timeAllowed' => -1]),
+             ]),
+        ];
+        yield 'new top-level name' => [
+            self::buildInputParams(),
+            'facet', 'topic_facet', 'foo',
+             new NestingParamBag([
+                'filter' => [ 'format:Book' ],
+                'params' => new NestingParamBag(['sow' => false, 'timeAllowed' => -1]),
+                'facet' => new ParamBag(['topic_facet' => 'foo']),
+             ]),
+        ];
+    }
+
+    /**
+     * Test setNestedParam method.
+     *
+     * @param NestingParamBag $params     Bag of nested parameters
+     * @param string          $name       Parameter name
+     * @param string          $nestedName Nested parameter name
+     * @param mixed           $value      Nested parameter value
+     * @param NestingParamBag $expected   Expected resulting bag of nested parameters
+     *
+     * @return void
+     */
+    #[\PHPUnit\Framework\Attributes\DataProvider('setNestedProvider')]
+    public function testSetNested(
+        NestingParamBag $params,
+        string $name,
+        string $nestedName,
+        mixed $value,
+        NestingParamBag $expected
+    ): void {
+        $params->setNested($name, $nestedName, $value);
+        $this->assertEquals($expected, $params);
+    }
+
+    /**
+     * Data provider for addNested method
+     *
+     * @return \Iterator
+     */
+    public static function addNestedProvider(): \Iterator
+    {
+        yield 'existing name' => [
+            self::buildInputParams(),
+            'params', 'sow', true,
+             new NestingParamBag([
+                'filter' => [ 'format:Book' ],
+                'params' => new NestingParamBag(['sow' => [false, true], 'timeAllowed' => -1]),
+             ]),
+        ];
+        yield 'new nested name' => [
+            self::buildInputParams(),
+            'params', 'spellcheck', true,
+             new NestingParamBag([
+                'filter' => [ 'format:Book' ],
+                'params' => new NestingParamBag(['sow' => [false], 'timeAllowed' => -1, 'spellcheck' => true]),
+             ]),
+        ];
+        yield 'new top-level name' => [
+            self::buildInputParams(),
+            'facet', 'topic_facet', 'foo',
+             new NestingParamBag([
+                'filter' => [ 'format:Book' ],
+                'params' => new NestingParamBag(['sow' => false, 'timeAllowed' => -1]),
+                'facet' => new ParamBag(['topic_facet' => 'foo']),
+             ]),
+        ];
+    }
+
+    /**
+     * Test addNestedParam method.
+     *
+     * @param NestingParamBag $params     Bag of nested parameters
+     * @param string          $name       Parameter name
+     * @param string          $nestedName Nested parameter name
+     * @param mixed           $value      Nested parameter value
+     * @param NestingParamBag $expected   Expected resulting bag of nested parameters
+     *
+     * @return void
+     */
+    #[\PHPUnit\Framework\Attributes\DataProvider('addNestedProvider')]
+    public function testAddNested(
+        NestingParamBag $params,
+        string $name,
+        string $nestedName,
+        mixed $value,
+        NestingParamBag $expected
+    ): void {
+        $params->addNested($name, $nestedName, $value);
+        $this->assertEquals($expected, $params);
+    }
+
+    /**
+     * Data provider for addMultiNested method
+     *
+     * @return \Iterator
+     */
+    public static function addMultiNestedProvider(): \Iterator
+    {
+        yield 'second level' => [
+            self::buildInputParams(),
+            'params',
+            ['spellcheck' => true],
+             new NestingParamBag([
+                'filter' => [ 'format:Book' ],
+                'params' => new NestingParamBag(['sow' => false, 'timeAllowed' => -1, 'spellcheck' => true]),
+             ]),
+        ];
+        yield 'deeper' => [
+            self::buildInputParams(),
+            'facet',
+            ['topic_facet' => ['type' => 'terms', 'field' => 'topic_facet', 'limit' => 30,
+                'domain' => ['excludeTags' => 'topic_facet_filter']]],
+             new NestingParamBag([
+                'filter' => [ 'format:Book' ],
+                'params' => new NestingParamBag(['sow' => false, 'timeAllowed' => -1]),
+                'facet' => new NestingParamBag(['topic_facet' => new NestingParamBag(
+                    ['type' => 'terms', 'field' => 'topic_facet', 'limit' => 30, 'domain' => new NestingParamBag(
+                        ['excludeTags' => 'topic_facet_filter']
+                    )]
+                )]),
+             ]),
+        ];
+    }
+
+    /**
+     * Test addMultiNested method.
+     *
+     * @param NestingParamBag $params   Bag of nested parameters
+     * @param string          $name     Parameter name
+     * @param array           $value    Nested array of parameter values
+     * @param NestingParamBag $expected Expected resulting bag of nested parameters
+     *
+     * @return void
+     */
+    #[\PHPUnit\Framework\Attributes\DataProvider('addMultiNestedProvider')]
+    public function testAddMultiNested(
+        NestingParamBag $params,
+        string $name,
+        mixed $value,
+        NestingParamBag $expected
+    ): void {
+        $params->addMultiNested($name, $value);
+        $this->assertEquals($expected, $params);
+    }
+
+    /**
+     * Data provider for add method
+     *
+     * @return \Iterator
+     */
+    public static function addProvider(): \Iterator
+    {
+        yield 'new name' => [
+            self::buildInputParams(),
+            'sort', 'title', true,
+             new NestingParamBag([
+                'filter' => [ 'format:Book' ],
+                'params' => new NestingParamBag(['sow' => false, 'timeAllowed' => -1]),
+                'sort' => 'title',
+             ]),
+        ];
+        yield 'existing name' => [
+            self::buildInputParams(),
+            'filter', 'location:Main Library', true,
+             new NestingParamBag([
+                'filter' => [ 'format:Book', 'location:Main Library' ],
+                'params' => new NestingParamBag(['sow' => false, 'timeAllowed' => -1]),
+             ]),
+        ];
+        yield 'adding scalar where existing is a ParamBag' => [
+            self::buildInputParams(),
+            'params', true, true,
+            'New values for name params are not compatible with existing values; both or neither must be a ParamBag.',
+        ];
+        yield 'adding ParamBag where existing is a scalar' => [
+            self::buildInputParams(),
+            'filter', new ParamBag(['foo' => 'bar']), true,
+            'New values for name filter are not compatible with existing values; both or neither must be a ParamBag.',
+        ];
+    }
+
+    /**
+     * Test add method.
+     *
+     * @param NestingParamBag        $params      Bag of nested parameters
+     * @param string                 $name        Parameter name
+     * @param mixed                  $value       Parameter values
+     * @param bool                   $deduplicate Whether to de-duplicate
+     * @param NestingParamBag|string $expected    Expected resulting bag of nested parameters or exception message
+     *
+     * @return void
+     */
+    #[\PHPUnit\Framework\Attributes\DataProvider('addProvider')]
+    public function testAdd(
+        NestingParamBag $params,
+        string $name,
+        mixed $value,
+        bool $deduplicate,
+        NestingParamBag|string $expected
+    ): void {
+        if (is_string($expected)) {
+            $this->expectExceptionMessage($expected);
+        }
+        $params->add($name, $value, $deduplicate);
+        if ($expected instanceof NestingParamBag) {
+            $this->assertEquals($expected, $params);
+        }
+    }
+
+    /**
+     * Data provider for testJsonSerialize method
+     *
+     * @return \Iterator
+     */
+    public static function jsonSerializeProvider(): \Iterator
+    {
+        yield 'basic' => [
+            self::buildInputParams(),
+            [
+                'filter' => 'format:Book',
+                'params' => ['sow' => false, 'timeAllowed' => -1],
+            ],
+        ];
+        yield 'multiple values for same name' => [
+            new NestingParamBag([
+                'filter' => [ 'format:Book', 'location:Main Library'],
+                'params' => new NestingParamBag(['sow' => false, 'timeAllowed' => -1]),
+            ]),
+            [
+                'filter' => ['format:Book', 'location:Main Library'],
+                'params' => ['sow' => false, 'timeAllowed' => -1],
+            ],
+
+        ];
+    }
+
+    /**
+     * Test add method.
+     *
+     * @param NestingParamBag $params   Bag of nested parameters
+     * @param array           $expected Expected array of JSON-serialized values
+     *
+     * @return void
+     */
+    #[\PHPUnit\Framework\Attributes\DataProvider('jsonSerializeProvider')]
+    public function testJsonSerialize(NestingParamBag $params, array $expected): void
+    {
+        $serialized = $params->jsonSerialize();
+        $this->assertEquals($expected, $serialized);
+    }
+
+    /**
+     * Build a NestingParamBag for testing.
+     *
+     * @return NestingParamBag
+     */
+    protected static function buildInputParams(): NestingParamBag
+    {
+        return new NestingParamBag([
+            'filter' => [ 'format:Book' ],
+            'params' => new NestingParamBag(['sow' => false, 'timeAllowed' => -1]),
+        ]);
+    }
+}