diff --git a/ext/standard/ClassExistsJitHelper.php b/ext/standard/ClassExistsJitHelper.php
new file mode 100644
index 000000000..bda2c4e56
--- /dev/null
+++ b/ext/standard/ClassExistsJitHelper.php
@@ -0,0 +1,32 @@
+<?php
+
+declare(strict_types=1);
+
+namespace PHPCompiler\ext\standard;
+
+use PHPCompiler\Web\Superglobals;
+
+/**
+ * class_exists() for compiled JIT/AOT modules (#16185, php-in-PHP).
+ *
+ * SSOT: {@see VmReflection::classExists()}
+ * php-src: ext/standard/basic_functions.c — PHP_FUNCTION(class_exists)
+ */
+final class ClassExistsJitHelper
+{
+    public static function existsArgv(string $name): bool
+    {
+        $ctx = Superglobals::getActiveContext();
+        if (null === $ctx) {
+            throw new \LogicException(
+                'ClassExistsJitHelper::existsArgv() requires an active VM context in this compiler build'
+            );
+        }
+
+        return VmReflection::classExists(
+            $ctx,
+            VmReflection::normalizeGlobalIntrospectionName($name),
+            true
+        );
+    }
+}
diff --git a/ext/standard/InterfaceExistsJitHelper.php b/ext/standard/InterfaceExistsJitHelper.php
new file mode 100644
index 000000000..8ff669745
--- /dev/null
+++ b/ext/standard/InterfaceExistsJitHelper.php
@@ -0,0 +1,32 @@
+<?php
+
+declare(strict_types=1);
+
+namespace PHPCompiler\ext\standard;
+
+use PHPCompiler\Web\Superglobals;
+
+/**
+ * interface_exists() for compiled JIT/AOT modules (#16185, php-in-PHP).
+ *
+ * SSOT: {@see VmReflection::interfaceExists()}
+ * php-src: ext/standard/basic_functions.c — PHP_FUNCTION(interface_exists)
+ */
+final class InterfaceExistsJitHelper
+{
+    public static function existsArgv(string $name): bool
+    {
+        $ctx = Superglobals::getActiveContext();
+        if (null === $ctx) {
+            throw new \LogicException(
+                'InterfaceExistsJitHelper::existsArgv() requires an active VM context in this compiler build'
+            );
+        }
+
+        return VmReflection::interfaceExists(
+            $ctx,
+            VmReflection::normalizeGlobalIntrospectionName($name),
+            true
+        );
+    }
+}
diff --git a/ext/standard/JitClassExists.php b/ext/standard/JitClassExists.php
index cf257a627..409850b89 100644
--- a/ext/standard/JitClassExists.php
+++ b/ext/standard/JitClassExists.php
@@ -4,46 +4,20 @@
 
 namespace PHPCompiler\ext\standard;
 
-use PHPCompiler\JIT\Builtin\GlobalIntrospectionNameRuntime;
-use PHPCompiler\JIT\Builtin\StringCaseCompare;
+use PHPCompiler\JIT\Builtin\StringClassExists;
 use PHPCompiler\JIT\Context;
-use PHPLLVM\Builder;
 use PHPLLVM\Value;
 
-/** LLVM lowering for class_exists() (issues #1214, #1056). */
+/**
+ * JIT/AOT helper for class_exists() via ClassExistsJitHelper PHP (#1214, #16185).
+ *
+ * {@see self::stringDataPtr()} remains for other JIT class-name scans.
+ */
 final class JitClassExists
 {
     public static function invoke(Context $context, Value $nameStr): Value
     {
-        GlobalIntrospectionNameRuntime::ensureLinked($context);
-        StringCaseCompare::ensureStrcasecmpLinked($context);
-        $nameStr = GlobalIntrospectionNameRuntime::normalizeString($context, $nameStr);
-        $i1 = $context->getTypeFromString('int1');
-        $i32 = $context->getTypeFromString('int32');
-        $exists = $i1->constInt(0, false);
-        $strcasecmpFn = $context->lookupFunction('strcasecmp');
-        $nameData = self::stringDataPtr($context, $nameStr);
-
-        $candidates = $context->type->object->allDeclaredClassLowerNames();
-        if (null !== $context->runtime->vmContext) {
-            $candidates = array_values(array_unique(array_merge(
-                $candidates,
-                array_keys($context->runtime->vmContext->classes)
-            )));
-        }
-
-        foreach ($candidates as $lc) {
-            if (\PHPCompiler\VM\ResourceSupport::isHiddenPseudoClassLc($lc)) {
-                continue;
-            }
-            $candidate = $context->builder->load($context->constantStringFromString($lc));
-            $candidateData = self::stringDataPtr($context, $candidate);
-            $cmp = $context->builder->call($strcasecmpFn, $nameData, $candidateData);
-            $match = $context->builder->icmp(Builder::INT_EQ, $cmp, $i32->constInt(0, false));
-            $exists = $context->builder->or($exists, $match);
-        }
-
-        return $exists;
+        return StringClassExists::invoke($context, $nameStr);
     }
 
     public static function stringDataPtr(Context $context, Value $strPtr): Value
diff --git a/ext/standard/JitInterfaceExists.php b/ext/standard/JitInterfaceExists.php
index 1627ed021..afd4f07fa 100644
--- a/ext/standard/JitInterfaceExists.php
+++ b/ext/standard/JitInterfaceExists.php
@@ -2,17 +2,19 @@
 
 declare(strict_types=1);
 
+/**
+ * JIT/AOT helper for interface_exists() via InterfaceExistsJitHelper PHP (#1371, #16185).
+ */
+
 namespace PHPCompiler\ext\standard;
 
-use PHPCompiler\JIT\Builtin\StringCaseCompare;
+use PHPCompiler\JIT\Builtin\StringInterfaceExists;
 use PHPCompiler\JIT\Context;
 use PHPCompiler\JIT\JitStringArg;
 use PHPCompiler\JIT\ReflectionBuiltinHelper;
 use PHPCompiler\JIT\Variable as JITVariable;
-use PHPLLVM\Builder;
 use PHPLLVM\Value;
 
-/** LLVM lowering for interface_exists() (issue #1371, #2312). */
 final class JitInterfaceExists
 {
     public static function invoke(Context $context, JITVariable $nameArg): Value
@@ -22,37 +24,9 @@ public static function invoke(Context $context, JITVariable $nameArg): Value
             return ReflectionBuiltinHelper::interfaceExistsLiteral($context, $literal);
         }
 
-        $nameStr = JitStringArg::lower($context, $nameArg, 'interface_exists() interface name');
-        StringCaseCompare::ensureStrcasecmpLinked($context);
-        $i1 = $context->getTypeFromString('int1');
-        $i32 = $context->getTypeFromString('int32');
-        $exists = $i1->constInt(0, false);
-        $strcasecmpFn = $context->lookupFunction('strcasecmp');
-        $nameData = JitClassExists::stringDataPtr($context, $nameStr);
-
-        $candidates = [];
-        foreach ($context->type->object->allDeclaredClassLowerNames() as $lc) {
-            if ($context->type->object->isInterfaceClassLc($lc)) {
-                $candidates[] = $lc;
-            }
-        }
-        if (null !== $context->runtime->vmContext) {
-            foreach ($context->runtime->vmContext->classes as $lc => $entry) {
-                if ($entry->isInterface) {
-                    $candidates[] = $lc;
-                }
-            }
-            $candidates = array_values(array_unique($candidates));
-        }
-
-        foreach ($candidates as $lc) {
-            $candidate = $context->builder->load($context->constantStringFromString($lc));
-            $candidateData = JitClassExists::stringDataPtr($context, $candidate);
-            $cmp = $context->builder->call($strcasecmpFn, $nameData, $candidateData);
-            $match = $context->builder->icmp(Builder::INT_EQ, $cmp, $i32->constInt(0, false));
-            $exists = $context->builder->or($exists, $match);
-        }
-
-        return $exists;
+        return StringInterfaceExists::invoke(
+            $context,
+            JitStringArg::lower($context, $nameArg, 'interface_exists() interface name')
+        );
     }
 }
diff --git a/lib/JIT/Builtin/StringClassExists.php b/lib/JIT/Builtin/StringClassExists.php
new file mode 100644
index 000000000..5c2511dae
--- /dev/null
+++ b/lib/JIT/Builtin/StringClassExists.php
@@ -0,0 +1,98 @@
+<?php
+
+declare(strict_types=1);
+
+namespace PHPCompiler\JIT\Builtin;
+
+use PHPCompiler\JIT\Context;
+use PHPCompiler\JIT\JitNestedHelperCoerce;
+use PHPCompiler\JIT\JitVmHelperLink;
+use PHPLLVM\Value;
+
+/**
+ * JIT/AOT link for class_exists() via ClassExistsJitHelper PHP (#16185).
+ *
+ * Replaces inline strcasecmp LLVM loop in ext/standard/JitClassExists.php.
+ * SSOT: {@see \PHPCompiler\ext\standard\ClassExistsJitHelper}.
+ * php-src: ext/standard/basic_functions.c — PHP_FUNCTION(class_exists)
+ */
+final class StringClassExists
+{
+    private const ABI = '__phpc_jit_class_exists';
+
+    private const HELPER_PATH = '/ext/standard/ClassExistsJitHelper.php';
+
+    private const INVOKE_HELPER = 'PHPCompiler\\ext\\standard\\ClassExistsJitHelper::existsArgv';
+
+    /** @var list<string> */
+    private const COMPILED_HELPERS = [
+        self::INVOKE_HELPER,
+    ];
+
+    public static function ensureLinked(Context $context): void
+    {
+        self::implement($context);
+    }
+
+    public static function ensureStandaloneBodies(Context $context): void
+    {
+        self::ensureLinked($context);
+    }
+
+    public static function invoke(Context $context, Value $nameStr): Value
+    {
+        self::ensureLinked($context);
+
+        return $context->builder->call(
+            $context->lookupFunction(self::ABI),
+            $nameStr
+        );
+    }
+
+    private static function implement(Context $context): void
+    {
+        $probe = $context->module->getNamedFunction(self::ABI);
+        if (null !== $probe && $probe->countBasicBlocks() > 0) {
+            $context->registerFunction(self::ABI, $probe);
+
+            return;
+        }
+
+        $savedBlock = null;
+        try {
+            $savedBlock = $context->builder->getInsertBlock();
+        } catch (\Throwable) {
+        }
+
+        JitVmHelperLink::ensureCompiled($context, self::HELPER_PATH, self::COMPILED_HELPERS, '#16185');
+
+        $strPtr = $context->getTypeFromString('__string__*');
+        $i1 = $context->getTypeFromString('int1');
+        $fn = null !== $probe
+            ? $probe
+            : $context->module->addFunction(
+                self::ABI,
+                $context->context->functionType($i1, false, $strPtr)
+            );
+
+        $entry = $fn->appendBasicBlock('class_exists_bridge_entry');
+        $context->builder->positionAtEnd($entry);
+
+        $helperFn = JitVmHelperLink::lookupCompiled($context, self::INVOKE_HELPER, '#16185');
+        $raw = JitNestedHelperCoerce::callHelper(
+            $context,
+            $helperFn,
+            [$fn->getParam(0)]
+        );
+        $exists = JitNestedHelperCoerce::coerceHelperScalarResult($context, $raw, $i1);
+        $context->builder->returnValue($exists);
+
+        $context->registerFunction(self::ABI, $fn);
+
+        if (null !== $savedBlock) {
+            $context->builder->positionAtEnd($savedBlock);
+        } else {
+            $context->builder->clearInsertionPosition();
+        }
+    }
+}
diff --git a/lib/JIT/Builtin/StringInterfaceExists.php b/lib/JIT/Builtin/StringInterfaceExists.php
new file mode 100644
index 000000000..d95d9f1bd
--- /dev/null
+++ b/lib/JIT/Builtin/StringInterfaceExists.php
@@ -0,0 +1,98 @@
+<?php
+
+declare(strict_types=1);
+
+namespace PHPCompiler\JIT\Builtin;
+
+use PHPCompiler\JIT\Context;
+use PHPCompiler\JIT\JitNestedHelperCoerce;
+use PHPCompiler\JIT\JitVmHelperLink;
+use PHPLLVM\Value;
+
+/**
+ * JIT/AOT link for interface_exists() via InterfaceExistsJitHelper PHP (#16185).
+ *
+ * Replaces inline strcasecmp LLVM loop in ext/standard/JitInterfaceExists.php.
+ * SSOT: {@see \PHPCompiler\ext\standard\InterfaceExistsJitHelper}.
+ * php-src: ext/standard/basic_functions.c — PHP_FUNCTION(interface_exists)
+ */
+final class StringInterfaceExists
+{
+    private const ABI = '__phpc_jit_interface_exists';
+
+    private const HELPER_PATH = '/ext/standard/InterfaceExistsJitHelper.php';
+
+    private const INVOKE_HELPER = 'PHPCompiler\\ext\\standard\\InterfaceExistsJitHelper::existsArgv';
+
+    /** @var list<string> */
+    private const COMPILED_HELPERS = [
+        self::INVOKE_HELPER,
+    ];
+
+    public static function ensureLinked(Context $context): void
+    {
+        self::implement($context);
+    }
+
+    public static function ensureStandaloneBodies(Context $context): void
+    {
+        self::ensureLinked($context);
+    }
+
+    public static function invoke(Context $context, Value $nameStr): Value
+    {
+        self::ensureLinked($context);
+
+        return $context->builder->call(
+            $context->lookupFunction(self::ABI),
+            $nameStr
+        );
+    }
+
+    private static function implement(Context $context): void
+    {
+        $probe = $context->module->getNamedFunction(self::ABI);
+        if (null !== $probe && $probe->countBasicBlocks() > 0) {
+            $context->registerFunction(self::ABI, $probe);
+
+            return;
+        }
+
+        $savedBlock = null;
+        try {
+            $savedBlock = $context->builder->getInsertBlock();
+        } catch (\Throwable) {
+        }
+
+        JitVmHelperLink::ensureCompiled($context, self::HELPER_PATH, self::COMPILED_HELPERS, '#16185');
+
+        $strPtr = $context->getTypeFromString('__string__*');
+        $i1 = $context->getTypeFromString('int1');
+        $fn = null !== $probe
+            ? $probe
+            : $context->module->addFunction(
+                self::ABI,
+                $context->context->functionType($i1, false, $strPtr)
+            );
+
+        $entry = $fn->appendBasicBlock('interface_exists_bridge_entry');
+        $context->builder->positionAtEnd($entry);
+
+        $helperFn = JitVmHelperLink::lookupCompiled($context, self::INVOKE_HELPER, '#16185');
+        $raw = JitNestedHelperCoerce::callHelper(
+            $context,
+            $helperFn,
+            [$fn->getParam(0)]
+        );
+        $exists = JitNestedHelperCoerce::coerceHelperScalarResult($context, $raw, $i1);
+        $context->builder->returnValue($exists);
+
+        $context->registerFunction(self::ABI, $fn);
+
+        if (null !== $savedBlock) {
+            $context->builder->positionAtEnd($savedBlock);
+        } else {
+            $context->builder->clearInsertionPosition();
+        }
+    }
+}
diff --git a/test/selfhost/compiler_lib_spine_smoke/main.php b/test/selfhost/compiler_lib_spine_smoke/main.php
index 40dff40d2..f7f1f628c 100644
--- a/test/selfhost/compiler_lib_spine_smoke/main.php
+++ b/test/selfhost/compiler_lib_spine_smoke/main.php
@@ -788,6 +788,7 @@
 require_once __DIR__.'/../../../ext/standard/JitChunkSplit.php';
 require_once __DIR__.'/../../../ext/standard/JitClassAlias.php';
 require_once __DIR__.'/../../../ext/standard/JitClassConstants.php';
+require_once __DIR__.'/../../../ext/standard/ClassExistsJitHelper.php';
 require_once __DIR__.'/../../../ext/standard/JitClassExists.php';
 require_once __DIR__.'/../../../ext/standard/JitClassImplements.php';
 require_once __DIR__.'/../../../ext/standard/JitClassParents.php';
@@ -973,6 +974,7 @@
 require_once __DIR__.'/../../../ext/standard/JitIniParseQuantity.php';
 require_once __DIR__.'/../../../ext/standard/JitIniValueArg.php';
 require_once __DIR__.'/../../../ext/standard/JitIntdiv.php';
+require_once __DIR__.'/../../../ext/standard/InterfaceExistsJitHelper.php';
 require_once __DIR__.'/../../../ext/standard/JitInterfaceExists.php';
 require_once __DIR__.'/../../../ext/standard/JitIptcEmbed.php';
 require_once __DIR__.'/../../../ext/standard/JitIptcParse.php';
@@ -3072,6 +3074,7 @@
 require_once __DIR__.'/../../../lib/JIT/Builtin/StringHttpBuildQuery.php';
 require_once __DIR__.'/../../../lib/JIT/Builtin/StringIdate.php';
 require_once __DIR__.'/../../../lib/JIT/Builtin/StringInfo.php';
+require_once __DIR__.'/../../../lib/JIT/Builtin/StringInterfaceExists.php';
 require_once __DIR__.'/../../../lib/JIT/Builtin/StringJsonDecode.php';
 require_once __DIR__.'/../../../lib/JIT/Builtin/StringJsonDecodeInventoryStubs.php';
 require_once __DIR__.'/../../../lib/JIT/Builtin/StringJsonEncode.php';
@@ -3087,6 +3090,7 @@
 require_once __DIR__.'/../../../lib/JIT/Builtin/StringMicrotime.php';
 require_once __DIR__.'/../../../lib/JIT/Builtin/StringMktime.php';
 require_once __DIR__.'/../../../lib/JIT/Builtin/StringCaseCompare.php';
+require_once __DIR__.'/../../../lib/JIT/Builtin/StringClassExists.php';
 require_once __DIR__.'/../../../lib/JIT/Builtin/StringNCompare.php';
 require_once __DIR__.'/../../../lib/JIT/Builtin/StringNaturalCompare.php';
 require_once __DIR__.'/../../../lib/JIT/Builtin/StringNetInterfacesJit.php';
diff --git a/test/unit/ClassInterfaceExistsRuntimeShrinkTest.php b/test/unit/ClassInterfaceExistsRuntimeShrinkTest.php
new file mode 100644
index 000000000..34396765c
--- /dev/null
+++ b/test/unit/ClassInterfaceExistsRuntimeShrinkTest.php
@@ -0,0 +1,65 @@
+<?php
+
+declare(strict_types=1);
+
+namespace PHPCompiler\Test\Unit;
+
+use PHPUnit\Framework\TestCase;
+
+/** class_exists()/interface_exists() JIT routes through JitHelper PHP not inline LLVM (#16185). */
+final class ClassInterfaceExistsRuntimeShrinkTest extends TestCase
+{
+    public function testJitClassExistsDelegatesToStringClassExistsBridge(): void
+    {
+        $source = (string) file_get_contents(__DIR__.'/../../ext/standard/JitClassExists.php');
+        $this->assertStringContainsString('StringClassExists::invoke', $source);
+        $this->assertStringNotContainsString("lookupFunction('strcasecmp'", $source);
+        $this->assertStringContainsString('stringDataPtr', $source);
+        $this->assertLessThan(35, \substr_count($source, "\n") + 1);
+    }
+
+    public function testJitInterfaceExistsDelegatesToStringInterfaceExistsBridge(): void
+    {
+        $source = (string) file_get_contents(__DIR__.'/../../ext/standard/JitInterfaceExists.php');
+        $this->assertStringContainsString('StringInterfaceExists::invoke', $source);
+        $this->assertStringNotContainsString("lookupFunction('strcasecmp'", $source);
+        $this->assertStringNotContainsString('JitClassExists::stringDataPtr', $source);
+        $this->assertLessThan(35, \substr_count($source, "\n") + 1);
+    }
+
+    public function testStringClassExistsUsesJitHelperNotStrcasecmpLoop(): void
+    {
+        $source = (string) file_get_contents(__DIR__.'/../../lib/JIT/Builtin/StringClassExists.php');
+        $this->assertStringContainsString('ClassExistsJitHelper', $source);
+        $this->assertStringContainsString('JitVmHelperLink', $source);
+        $this->assertStringNotContainsString("lookupFunction('strcasecmp'", $source);
+    }
+
+    public function testStringInterfaceExistsUsesJitHelperNotStrcasecmpLoop(): void
+    {
+        $source = (string) file_get_contents(__DIR__.'/../../lib/JIT/Builtin/StringInterfaceExists.php');
+        $this->assertStringContainsString('InterfaceExistsJitHelper', $source);
+        $this->assertStringContainsString('JitVmHelperLink', $source);
+        $this->assertStringNotContainsString("lookupFunction('strcasecmp'", $source);
+    }
+
+    public function testJitHelpersDelegateToVmReflection(): void
+    {
+        $classHelper = (string) file_get_contents(__DIR__.'/../../ext/standard/ClassExistsJitHelper.php');
+        $this->assertStringContainsString('VmReflection::classExists', $classHelper);
+        $this->assertStringContainsString('Superglobals::getActiveContext', $classHelper);
+
+        $interfaceHelper = (string) file_get_contents(__DIR__.'/../../ext/standard/InterfaceExistsJitHelper.php');
+        $this->assertStringContainsString('VmReflection::interfaceExists', $interfaceHelper);
+        $this->assertStringContainsString('Superglobals::getActiveContext', $interfaceHelper);
+    }
+
+    public function testSpineBundleIncludesClassInterfaceExistsJitHelpers(): void
+    {
+        $spine = (string) file_get_contents(__DIR__.'/../../test/selfhost/compiler_lib_spine_smoke/main.php');
+        $this->assertStringContainsString('ClassExistsJitHelper.php', $spine);
+        $this->assertStringContainsString('InterfaceExistsJitHelper.php', $spine);
+        $this->assertStringContainsString('StringClassExists.php', $spine);
+        $this->assertStringContainsString('StringInterfaceExists.php', $spine);
+    }
+}