diff --git a/src/Laravel/Routing/IriConverter.php b/src/Laravel/Routing/IriConverter.php
index cb80414da9..e0bafb8887 100644
--- a/src/Laravel/Routing/IriConverter.php
+++ b/src/Laravel/Routing/IriConverter.php
@@ -103,7 +103,7 @@ public function getIriFromResource(object|string $resource, int $referenceType =
             $operation = $this->operationMetadataFactory->create($context['item_uri_template']);
         }
 
-        $localOperationCacheKey = ($operation?->getName() ?? '').$resourceClass.(\is_string($resource) ? '_c' : '_i');
+        $localOperationCacheKey = ($operation?->getName() ?? '').$resourceClass.(\is_string($resource) ? '_s' : '_o').($operation instanceof CollectionOperationInterface ? '_c' : '_i');
         if ($operation && isset($this->localOperationCache[$localOperationCacheKey])) {
             return $this->generateRoute($resource, $referenceType, $this->localOperationCache[$localOperationCacheKey], $context, $this->localIdentifiersExtractorOperationCache[$localOperationCacheKey] ?? null);
         }
diff --git a/src/Laravel/Tests/Unit/Routing/IriConverterTest.php b/src/Laravel/Tests/Unit/Routing/IriConverterTest.php
new file mode 100644
index 0000000000..c88de228bd
--- /dev/null
+++ b/src/Laravel/Tests/Unit/Routing/IriConverterTest.php
@@ -0,0 +1,90 @@
+<?php
+
+/*
+ * This file is part of the API Platform project.
+ *
+ * (c) Kévin Dunglas <dunglas@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+declare(strict_types=1);
+
+namespace ApiPlatform\Laravel\Tests\Unit\Routing;
+
+use ApiPlatform\Laravel\Routing\IriConverter;
+use ApiPlatform\Metadata\ApiResource;
+use ApiPlatform\Metadata\Get;
+use ApiPlatform\Metadata\GetCollection;
+use ApiPlatform\Metadata\IdentifiersExtractorInterface;
+use ApiPlatform\Metadata\Operation\Factory\OperationMetadataFactoryInterface;
+use ApiPlatform\Metadata\Operations;
+use ApiPlatform\Metadata\Resource\Factory\ResourceMetadataCollectionFactoryInterface;
+use ApiPlatform\Metadata\Resource\ResourceMetadataCollection;
+use ApiPlatform\Metadata\ResourceClassResolverInterface;
+use ApiPlatform\Metadata\UrlGeneratorInterface;
+use ApiPlatform\State\ProviderInterface;
+use PHPUnit\Framework\TestCase;
+use Symfony\Component\Routing\RouterInterface;
+use Workbench\App\Models\Book;
+
+class IriConverterTest extends TestCase
+{
+    public function testLocalCacheKeyDistinguishesItemAndCollectionForStringResource(): void
+    {
+        $collectionOpName = 'collection_op';
+        $itemOpName = 'item_op';
+
+        $collectionOp = (new GetCollection())->withName($collectionOpName)->withClass(Book::class);
+        $itemOp = (new Get())->withName($itemOpName)->withClass(Book::class);
+
+        $router = $this->createMock(RouterInterface::class);
+        $router->expects($this->exactly(2))
+            ->method('generate')
+            ->willReturnCallback(function (string $routeName, array $params) use ($collectionOpName, $itemOpName): string {
+                if ($collectionOpName === $routeName) {
+                    return '/api/books';
+                }
+                if ($itemOpName === $routeName) {
+                    return '/api/books/'.$params['id'];
+                }
+                $this->fail(\sprintf('Unexpected route name "%s".', $routeName));
+            });
+
+        $metadataCollection = new ResourceMetadataCollection(Book::class, [
+            (new ApiResource())->withOperations(new Operations([
+                $collectionOpName => $collectionOp,
+                $itemOpName => $itemOp,
+            ])),
+        ]);
+
+        $resourceMetadataFactory = $this->createStub(ResourceMetadataCollectionFactoryInterface::class);
+        $resourceMetadataFactory->method('create')->willReturn($metadataCollection);
+
+        $resourceClassResolver = $this->createStub(ResourceClassResolverInterface::class);
+        $resourceClassResolver->method('isResourceClass')->willReturn(true);
+
+        $iriConverter = new IriConverter(
+            $this->createStub(ProviderInterface::class),
+            $this->createStub(OperationMetadataFactoryInterface::class),
+            $router,
+            $this->createStub(IdentifiersExtractorInterface::class),
+            $resourceClassResolver,
+            $resourceMetadataFactory,
+        );
+
+        $this->assertSame('/api/books', $iriConverter->getIriFromResource(
+            Book::class,
+            UrlGeneratorInterface::ABS_PATH,
+            new GetCollection(),
+        ));
+
+        $this->assertSame('/api/books/1', $iriConverter->getIriFromResource(
+            Book::class,
+            UrlGeneratorInterface::ABS_PATH,
+            new Get(),
+            ['uri_variables' => ['id' => 1]],
+        ));
+    }
+}
diff --git a/src/Symfony/Routing/IriConverter.php b/src/Symfony/Routing/IriConverter.php
index 0a69b77be4..5a381802b7 100644
--- a/src/Symfony/Routing/IriConverter.php
+++ b/src/Symfony/Routing/IriConverter.php
@@ -122,7 +122,7 @@ public function getIriFromResource(object|string $resource, int $referenceType =
             $operation = $this->operationMetadataFactory->create($context['item_uri_template']);
         }
 
-        $localOperationCacheKey = ($operation?->getName() ?? '').$resourceClass.((\is_string($resource) || $operation instanceof CollectionOperationInterface) ? '_c' : '_i');
+        $localOperationCacheKey = ($operation?->getName() ?? '').$resourceClass.(\is_string($resource) ? '_s' : '_o').($operation instanceof CollectionOperationInterface ? '_c' : '_i');
         if ($operation && isset($this->localOperationCache[$localOperationCacheKey])) {
             return $this->generateSymfonyRoute($resource, $referenceType, $this->localOperationCache[$localOperationCacheKey], $context, $this->localIdentifiersExtractorOperationCache[$localOperationCacheKey] ?? null);
         }
diff --git a/tests/Functional/Json/RelationTest.php b/tests/Functional/Json/RelationTest.php
new file mode 100644
index 0000000000..cbe39825f7
--- /dev/null
+++ b/tests/Functional/Json/RelationTest.php
@@ -0,0 +1,69 @@
+<?php
+
+/*
+ * This file is part of the API Platform project.
+ *
+ * (c) Kévin Dunglas <dunglas@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+declare(strict_types=1);
+
+namespace ApiPlatform\Tests\Functional\Json;
+
+use ApiPlatform\Symfony\Bundle\Test\ApiTestCase;
+use ApiPlatform\Tests\Fixtures\TestBundle\Entity\RelatedDummy;
+use ApiPlatform\Tests\Fixtures\TestBundle\Entity\ThirdLevel;
+use ApiPlatform\Tests\RecreateSchemaTrait;
+use ApiPlatform\Tests\SetupClassResourcesTrait;
+
+final class RelationTest extends ApiTestCase
+{
+    use RecreateSchemaTrait;
+    use SetupClassResourcesTrait;
+
+    protected static ?bool $alwaysBootKernel = false;
+
+    public static function getResources(): array
+    {
+        return [ThirdLevel::class, RelatedDummy::class];
+    }
+
+    protected function setUp(): void
+    {
+        parent::setUp();
+
+        if ($this->isMongoDB()) {
+            $this->markTestSkipped('Not tested with MongoDB.');
+        }
+
+        $this->recreateSchema([ThirdLevel::class, RelatedDummy::class]);
+    }
+
+    public function testCreateRelatedDummyWithPlainIdentifierForRelation(): void
+    {
+        // Creates a ThirdLevel; PurgeHttpCacheListener::postFlush caches the GetCollection
+        // operation under the '' + ThirdLevel + '_c' slot of IriConverter::$localOperationCache.
+        self::createClient()->request('POST', '/third_levels', [
+            'headers' => ['Content-Type' => 'application/json'],
+            'json' => ['level' => 3],
+        ]);
+        $this->assertResponseStatusCodeSame(201);
+
+        // RelatedDummyPlainIdentifierDenormalizer calls getIriFromResource(ThirdLevel::class, new Get(), …).
+        // Without the fix the '_c' slot collision returns the GetCollection op, producing
+        // "/third_levels?id=1" instead of "/third_levels/1".
+        $response = self::createClient()->request('POST', '/related_dummies', [
+            'headers' => ['Content-Type' => 'application/json'],
+            'json' => ['thirdLevel' => '1'],
+        ]);
+        $this->assertResponseStatusCodeSame(201);
+
+        $data = $response->toArray(false);
+        $this->assertArrayHasKey('thirdLevel', $data);
+        $this->assertIsArray($data['thirdLevel']);
+        $this->assertSame('/third_levels/1', $data['thirdLevel']['@id']);
+    }
+}