diff --git a/src/Plugins/Tia/CoverageMerger.php b/src/Plugins/Tia/CoverageMerger.php
index 40ea62c09..a83a5d2f1 100644
--- a/src/Plugins/Tia/CoverageMerger.php
+++ b/src/Plugins/Tia/CoverageMerger.php
@@ -7,6 +7,7 @@
 use Pest\Plugins\Tia;
 use Pest\Plugins\Tia\Contracts\State;
 use Pest\Support\Container;
+use Pest\TestSuite;
 use SebastianBergmann\CodeCoverage\CodeCoverage;
 use SebastianBergmann\CodeCoverage\Driver\Selector;
 use SebastianBergmann\CodeCoverage\Filter;
@@ -28,31 +29,22 @@ public static function applyIfMarked(string $reportPath): void
 
         $state->delete(Tia::KEY_COVERAGE_MARKER);
 
-        $cachedBytes = $state->read(Tia::KEY_COVERAGE_CACHE);
+        $cached = self::readCache($state);
 
-        if ($cachedBytes === null) {
+        if (! $cached instanceof CodeCoverage) {
             $current = self::requireCoverage($reportPath);
 
             if ($current instanceof CodeCoverage) {
                 self::primeUncoveredFiles($current);
-                $state->write(Tia::KEY_COVERAGE_CACHE, self::compress(serialize($current)));
+                $state->write(Tia::KEY_COVERAGE_CACHE, self::compress(self::serializeRelativeToProjectRoot($current)));
             }
 
             return;
         }
 
-        $decoded = self::decompress($cachedBytes);
-
-        if ($decoded === null) {
-            $state->delete(Tia::KEY_COVERAGE_CACHE);
-
-            return;
-        }
-
-        $cached = self::unserializeCoverage($decoded);
         $current = self::requireCoverage($reportPath);
 
-        if (! $cached instanceof CodeCoverage || ! $current instanceof CodeCoverage) {
+        if (! $current instanceof CodeCoverage) {
             return;
         }
 
@@ -63,13 +55,108 @@ public static function applyIfMarked(string $reportPath): void
 
         $cached->merge($current);
 
-        $serialised = serialize($cached);
-
         @file_put_contents(
             $reportPath,
-            '<?php return unserialize('.var_export($serialised, true).");\n",
+            '<?php return unserialize('.var_export(serialize($cached), true).");\n",
         );
-        $state->write(Tia::KEY_COVERAGE_CACHE, self::compress($serialised));
+        $state->write(Tia::KEY_COVERAGE_CACHE, self::compress(self::serializeRelativeToProjectRoot($cached)));
+    }
+
+    /**
+     * Reads the cached baseline coverage and maps it onto this machine's
+     * project root. An unreadable cache — or one recorded on another machine
+     * that cannot be mapped — is dropped so the current run can seed a fresh
+     * one, instead of poisoning the merged report with foreign paths.
+     */
+    private static function readCache(State $state): ?CodeCoverage
+    {
+        $bytes = $state->read(Tia::KEY_COVERAGE_CACHE);
+
+        if ($bytes === null) {
+            return null;
+        }
+
+        $decoded = self::decompress($bytes);
+        $cached = $decoded === null ? null : self::unserializeCoverage($decoded);
+
+        if ($cached instanceof CodeCoverage) {
+            $cached = self::rebaseOntoProjectRoot($cached);
+        }
+
+        if (! $cached instanceof CodeCoverage) {
+            $state->delete(Tia::KEY_COVERAGE_CACHE);
+
+            return null;
+        }
+
+        return $cached;
+    }
+
+    /**
+     * The cache stores file paths relative to the project root so that a
+     * baseline recorded on one machine (e.g. CI) stays mergeable on another.
+     * Relative entries are mapped onto the local root here; absolute entries
+     * under the local root are kept (caches written by previous versions on
+     * this machine), while absolute entries of another machine make the whole
+     * cache unusable — a report built from mixed roots contains no resolvable
+     * file at all.
+     */
+    private static function rebaseOntoProjectRoot(CodeCoverage $coverage): ?CodeCoverage
+    {
+        $root = self::projectRootPrefix();
+        $data = $coverage->getData(true);
+
+        foreach ($data->coveredFiles() as $file) {
+            if (self::isAbsolutePath($file)) {
+                if (! str_starts_with($file, $root)) {
+                    return null;
+                }
+
+                continue;
+            }
+
+            $data->renameFile($file, $root.str_replace('/', DIRECTORY_SEPARATOR, $file));
+        }
+
+        $coverage->clearCache();
+
+        return $coverage;
+    }
+
+    private static function serializeRelativeToProjectRoot(CodeCoverage $coverage): string
+    {
+        $root = self::projectRootPrefix();
+        $data = $coverage->getData(true);
+
+        foreach ($data->coveredFiles() as $file) {
+            if (! str_starts_with($file, $root)) {
+                continue;
+            }
+
+            $relative = str_replace(DIRECTORY_SEPARATOR, '/', substr($file, strlen($root)));
+
+            if ($relative === '') {
+                continue;
+            }
+
+            $data->renameFile($file, $relative);
+        }
+
+        $coverage->clearCache();
+
+        return serialize($coverage);
+    }
+
+    private static function projectRootPrefix(): string
+    {
+        return rtrim(TestSuite::getInstance()->rootPath, '/\\').DIRECTORY_SEPARATOR;
+    }
+
+    private static function isAbsolutePath(string $path): bool
+    {
+        return str_starts_with($path, '/')
+            || str_starts_with($path, '\\')
+            || preg_match('#^[A-Za-z]:[/\\\\]#', $path) === 1;
     }
 
     private static function primeUncoveredFiles(CodeCoverage $coverage): void
diff --git a/tests/Unit/Plugins/Tia/CoverageMergerPortability.php b/tests/Unit/Plugins/Tia/CoverageMergerPortability.php
new file mode 100644
index 000000000..d0e6735a8
--- /dev/null
+++ b/tests/Unit/Plugins/Tia/CoverageMergerPortability.php
@@ -0,0 +1,192 @@
+<?php
+
+declare(strict_types=1);
+
+use Pest\Plugins\Tia;
+use Pest\Plugins\Tia\Contracts\State;
+use Pest\Plugins\Tia\CoverageMerger;
+use Pest\Plugins\Tia\FileState;
+use Pest\Support\Container;
+use Pest\TestSuite;
+use SebastianBergmann\CodeCoverage\CodeCoverage;
+use SebastianBergmann\CodeCoverage\Data\ProcessedCodeCoverageData;
+use SebastianBergmann\CodeCoverage\Data\RawCodeCoverageData;
+use SebastianBergmann\CodeCoverage\Driver\Driver;
+use SebastianBergmann\CodeCoverage\Filter;
+
+final class CoverageMergerPortabilityFakeDriver extends Driver
+{
+    public function name(): string
+    {
+        return 'fake';
+    }
+
+    public function version(): string
+    {
+        return '1.0.0';
+    }
+
+    public function start(): void {}
+
+    public function stop(): RawCodeCoverageData
+    {
+        return RawCodeCoverageData::fromXdebugWithoutPathCoverage([]);
+    }
+}
+
+/**
+ * @param  array<string, array<int, array<int, string>>>  $lineCoverage
+ * @param  array<string, array{size: string, status: string}>  $tests
+ */
+function coverageMergerPortabilityCoverage(array $lineCoverage, array $tests = []): CodeCoverage
+{
+    $coverage = new CodeCoverage(new CoverageMergerPortabilityFakeDriver, new Filter);
+
+    $data = new ProcessedCodeCoverageData;
+    $data->setLineCoverage($lineCoverage);
+
+    $coverage->setData($data);
+    $coverage->setTests($tests);
+
+    return $coverage;
+}
+
+function coverageMergerPortabilityReport(string $reportPath, CodeCoverage $coverage): void
+{
+    file_put_contents(
+        $reportPath,
+        '<?php return unserialize('.var_export(serialize($coverage), true).");\n",
+    );
+}
+
+beforeEach(function (): void {
+    $this->stateDir = sys_get_temp_dir().'/pest-tia-coverage-merger-'.bin2hex(random_bytes(4));
+    mkdir($this->stateDir, 0755, true);
+
+    $this->reportPath = $this->stateDir.'/coverage-report.php';
+
+    try {
+        $this->previousState = Container::getInstance()->get(State::class);
+    } catch (Throwable) {
+        $this->previousState = null;
+    }
+
+    Container::getInstance()->add(State::class, new FileState($this->stateDir));
+
+    $this->projectRoot = rtrim(TestSuite::getInstance()->rootPath, '/\\');
+});
+
+afterEach(function (): void {
+    if ($this->previousState instanceof State) {
+        Container::getInstance()->add(State::class, $this->previousState);
+    }
+
+    foreach (glob($this->stateDir.'/*') ?: [] as $file) {
+        @unlink($file);
+    }
+
+    @rmdir($this->stateDir);
+});
+
+it('discards a cache recorded on another machine and reseeds from the current run', function (): void {
+    $state = Container::getInstance()->get(State::class);
+    assert($state instanceof State);
+
+    $foreign = coverageMergerPortabilityCoverage([
+        '/home/runner/work/acme/acme/src/Example.php' => [10 => ['cached-test']],
+    ], ['cached-test' => ['size' => 'unknown', 'status' => 'success']]);
+
+    $state->write(Tia::KEY_COVERAGE_CACHE, (string) gzencode(serialize($foreign)));
+    $state->write(Tia::KEY_COVERAGE_MARKER, '');
+
+    $localFile = $this->projectRoot.DIRECTORY_SEPARATOR.'src'.DIRECTORY_SEPARATOR.'Pest.php';
+
+    coverageMergerPortabilityReport($this->reportPath, coverageMergerPortabilityCoverage([
+        $localFile => [5 => ['fresh-test']],
+    ], ['fresh-test' => ['size' => 'unknown', 'status' => 'success']]));
+
+    CoverageMerger::applyIfMarked($this->reportPath);
+
+    expect($state->exists(Tia::KEY_COVERAGE_MARKER))->toBeFalse();
+
+    $cachedBytes = $state->read(Tia::KEY_COVERAGE_CACHE);
+    expect($cachedBytes)->not->toBeNull();
+
+    $cache = unserialize((string) gzdecode((string) $cachedBytes));
+    expect($cache)->toBeInstanceOf(CodeCoverage::class);
+    assert($cache instanceof CodeCoverage);
+
+    expect($cache->getData(true)->coveredFiles())->toBe(['src/Pest.php']);
+});
+
+it('rebases relative cached paths onto the local project root before merging', function (): void {
+    $state = Container::getInstance()->get(State::class);
+    assert($state instanceof State);
+
+    $cached = coverageMergerPortabilityCoverage([
+        'src/Pest.php' => [10 => ['cached-test']],
+    ], ['cached-test' => ['size' => 'unknown', 'status' => 'success']]);
+
+    $state->write(Tia::KEY_COVERAGE_CACHE, (string) gzencode(serialize($cached)));
+    $state->write(Tia::KEY_COVERAGE_MARKER, '');
+
+    $cachedFile = $this->projectRoot.DIRECTORY_SEPARATOR.'src'.DIRECTORY_SEPARATOR.'Pest.php';
+    $freshFile = $this->projectRoot.DIRECTORY_SEPARATOR.'src'.DIRECTORY_SEPARATOR.'Panic.php';
+
+    coverageMergerPortabilityReport($this->reportPath, coverageMergerPortabilityCoverage([
+        $freshFile => [21 => ['fresh-test']],
+    ], ['fresh-test' => ['size' => 'unknown', 'status' => 'success']]));
+
+    CoverageMerger::applyIfMarked($this->reportPath);
+
+    $merged = require $this->reportPath;
+    expect($merged)->toBeInstanceOf(CodeCoverage::class);
+    assert($merged instanceof CodeCoverage);
+
+    $lineCoverage = $merged->getData(true)->lineCoverage();
+    expect($lineCoverage)->toHaveKeys([$cachedFile, $freshFile])
+        ->and($lineCoverage[$cachedFile][10])->toBe(['cached-test'])
+        ->and($lineCoverage[$freshFile][21])->toBe(['fresh-test']);
+
+    $cache = unserialize((string) gzdecode((string) $state->read(Tia::KEY_COVERAGE_CACHE)));
+    assert($cache instanceof CodeCoverage);
+
+    expect($cache->getData(true)->coveredFiles())->toBe(['src/Panic.php', 'src/Pest.php']);
+});
+
+it('still merges a same-machine cache that stores absolute paths', function (): void {
+    $state = Container::getInstance()->get(State::class);
+    assert($state instanceof State);
+
+    $localFile = $this->projectRoot.DIRECTORY_SEPARATOR.'src'.DIRECTORY_SEPARATOR.'Pest.php';
+
+    $cached = coverageMergerPortabilityCoverage([
+        $localFile => [10 => ['cached-test'], 12 => ['cached-test']],
+    ], ['cached-test' => ['size' => 'unknown', 'status' => 'success']]);
+
+    $state->write(Tia::KEY_COVERAGE_CACHE, (string) gzencode(serialize($cached)));
+    $state->write(Tia::KEY_COVERAGE_MARKER, '');
+
+    coverageMergerPortabilityReport($this->reportPath, coverageMergerPortabilityCoverage([
+        $localFile => [11 => ['fresh-test']],
+    ], ['fresh-test' => ['size' => 'unknown', 'status' => 'success']]));
+
+    CoverageMerger::applyIfMarked($this->reportPath);
+
+    $merged = require $this->reportPath;
+    assert($merged instanceof CodeCoverage);
+
+    $lines = $merged->getData(true)->lineCoverage()[$localFile];
+    ksort($lines);
+
+    expect($lines)->toBe([
+        10 => ['cached-test'],
+        11 => ['fresh-test'],
+        12 => ['cached-test'],
+    ]);
+
+    $cache = unserialize((string) gzdecode((string) $state->read(Tia::KEY_COVERAGE_CACHE)));
+    assert($cache instanceof CodeCoverage);
+
+    expect($cache->getData(true)->coveredFiles())->toBe(['src/Pest.php']);
+});