diff --git a/src/JsonApi/State/JsonApiProvider.php b/src/JsonApi/State/JsonApiProvider.php
index 0976d9ce84..c09c2fdb68 100644
--- a/src/JsonApi/State/JsonApiProvider.php
+++ b/src/JsonApi/State/JsonApiProvider.php
@@ -15,6 +15,7 @@
 
 use ApiPlatform\Metadata\Operation;
 use ApiPlatform\State\ProviderInterface;
+use ApiPlatform\State\Util\RequestParser;
 
 final class JsonApiProvider implements ProviderInterface
 {
@@ -87,6 +88,16 @@ public function provide(Operation $operation, array $uriVariables = [], array $c
         }
 
         if ($filters) {
+            // ReadProvider skips its raw-query fallback when _api_filters is set,
+            // so preserve flat custom params here too. JSON:API transforms win.
+            $rawParams = $request->attributes->get('_api_query_parameters');
+            if (null === $rawParams) {
+                $queryString = RequestParser::getQueryString($request);
+                $rawParams = $queryString ? RequestParser::parseRequestParams($queryString) : [];
+                $request->attributes->set('_api_query_parameters', $rawParams);
+            }
+            $filters = array_replace($rawParams, $filters);
+
             $request->attributes->set('_api_filters', $filters);
         }
 
diff --git a/src/JsonApi/Tests/State/JsonApiProviderTest.php b/src/JsonApi/Tests/State/JsonApiProviderTest.php
index 6e5999bbdb..e315e8dbb2 100644
--- a/src/JsonApi/Tests/State/JsonApiProviderTest.php
+++ b/src/JsonApi/Tests/State/JsonApiProviderTest.php
@@ -55,4 +55,47 @@ public function testProvideMergesFlatPaginationWithBracketFilter(): void
             'pagination' => 'true',
         ], $request->attributes->get('_api_filters'));
     }
+
+    // #8216: flat custom params must survive when _api_filters is pre-set by JSON:API.
+    public function testProvidePreservesFlatCustomQueryParamsWithoutBracketFilter(): void
+    {
+        $request = Request::create('/sessions?city_id=3152&order[distance]=asc&page=1');
+        $request->setRequestFormat('jsonapi');
+
+        $operation = new Get(class: \stdClass::class, shortName: 'dummy');
+        $context = ['request' => $request];
+        $decorated = $this->createMock(ProviderInterface::class);
+        $decorated->expects($this->once())->method('provide')->with($operation, [], $context);
+
+        $provider = new JsonApiProvider($decorated);
+        $provider->provide($operation, [], $context);
+
+        $filters = $request->attributes->get('_api_filters');
+
+        $this->assertIsArray($filters);
+        $this->assertSame('3152', $filters['city_id'] ?? null);
+        $this->assertSame(['distance' => 'asc'], $filters['order'] ?? null);
+        $this->assertSame('1', $filters['page'] ?? null);
+    }
+
+    // _api_query_parameters set by an earlier listener / ParameterProvider must be reused.
+    public function testProvideHonoursPrePopulatedApiQueryParameters(): void
+    {
+        $request = Request::create('/sessions?page=1');
+        $request->setRequestFormat('jsonapi');
+        $request->attributes->set('_api_query_parameters', ['custom_override' => 'yes', 'page' => '1']);
+
+        $operation = new Get(class: \stdClass::class, shortName: 'dummy');
+        $context = ['request' => $request];
+        $decorated = $this->createMock(ProviderInterface::class);
+        $decorated->expects($this->once())->method('provide')->with($operation, [], $context);
+
+        $provider = new JsonApiProvider($decorated);
+        $provider->provide($operation, [], $context);
+
+        $filters = $request->attributes->get('_api_filters');
+
+        $this->assertSame('yes', $filters['custom_override'] ?? null);
+        $this->assertSame('1', $filters['page'] ?? null);
+    }
 }
diff --git a/tests/Functional/JsonApiFlatPaginationTest.php b/tests/Functional/JsonApiFlatPaginationTest.php
index 8495531a2f..8766b2e1fa 100644
--- a/tests/Functional/JsonApiFlatPaginationTest.php
+++ b/tests/Functional/JsonApiFlatPaginationTest.php
@@ -70,6 +70,26 @@ public function testFlatPageWithBracketFilterDrivesCurrentPage(): void
         $this->assertSame(5, $data['meta']['totalItems'] ?? null);
     }
 
+    // #8216: flat filter param combined with flat page must still drive filtering.
+    public function testFlatCustomParamWithFlatPagePreservesFilter(): void
+    {
+        if ($this->isMongoDB()) {
+            $this->markTestSkipped('Not tested with mongodb.');
+        }
+
+        $response = self::createClient()->request('GET', '/dummies?name=foo&page=1', [
+            'headers' => ['Accept' => 'application/vnd.api+json'],
+        ]);
+
+        $this->assertResponseIsSuccessful();
+        $this->assertResponseHeaderSame('content-type', 'application/vnd.api+json; charset=utf-8');
+
+        $data = $response->toArray();
+
+        $this->assertSame(1, $data['meta']['currentPage'] ?? null);
+        $this->assertSame(5, $data['meta']['totalItems'] ?? null);
+    }
+
     private function loadFixtures(): void
     {
         $manager = $this->getManager();
@@ -80,6 +100,10 @@ private function loadFixtures(): void
             $manager->persist($dummy);
         }
 
+        $bar = new Dummy();
+        $bar->setName('bar');
+        $manager->persist($bar);
+
         $manager->flush();
     }
 }