[Server] Log failures in CompletionCompleteHandler

src/Server/Builder.php

@@ -1079,7 +1079,7 @@ private function resolve(): array
 
         $requestHandlers = array_merge($this->requestHandlers, [
             new Handler\Request\CallToolHandler($registry, $referenceHandler, $logger),
-            new Handler\Request\CompletionCompleteHandler($registry, $container),
+            new Handler\Request\CompletionCompleteHandler($registry, $container, $logger),
             new Handler\Request\GetPromptHandler($registry, $referenceHandler, $logger),
             new Handler\Request\InitializeHandler($configuration),
             new Handler\Request\ListPromptsHandler($registry, $this->paginationLimit),

src/Server/Handler/Request/CompletionCompleteHandler.php

@@ -25,6 +25,8 @@
 use Mcp\Schema\Result\CompletionCompleteResult;
 use Mcp\Server\Session\SessionInterface;
 use Psr\Container\ContainerInterface;
+use Psr\Log\LoggerInterface;
+use Psr\Log\NullLogger;
 
 /**
  * Handles completion/complete requests.
@@ -38,6 +40,7 @@ final class CompletionCompleteHandler implements RequestHandlerInterface
     public function __construct(
         private readonly RegistryInterface $registry,
         private readonly ?ContainerInterface $container = null,
+        private readonly LoggerInterface $logger = new NullLogger(),
     ) {
     }
 
@@ -87,8 +90,12 @@ public function handle(Request $request, SessionInterface $session): Response|Er
         } catch (PromptNotFoundException|ResourceNotFoundException $e) {
             // The reference names something the server does not have, which is
             // a bad parameter rather than a missing resource.
+            $this->logger->warning(\sprintf('Completion requested for unknown reference: %s', $e->getMessage()), ['exception' => $e]);
+
             return Error::forInvalidParams($e->getMessage(), $request->getId());
         } catch (\Throwable $e) {
+            $this->logger->error(\sprintf('Error while handling completion request: %s', $e->getMessage()), ['exception' => $e]);
+
             return Error::forInternalError('Error while handling completion request', $request->getId());
         }
     }

tests/Unit/Server/Handler/Request/CompletionCompleteHandlerTest.php

@@ -11,9 +11,11 @@
 
 namespace Mcp\Tests\Unit\Server\Handler\Request;
 
+use Mcp\Capability\Completion\ProviderInterface;
 use Mcp\Capability\Registry\ResourceReference;
 use Mcp\Capability\Registry\ResourceTemplateReference;
 use Mcp\Capability\RegistryInterface;
+use Mcp\Schema\JsonRpc\Error;
 use Mcp\Schema\JsonRpc\Response;
 use Mcp\Schema\Request\CompletionCompleteRequest;
 use Mcp\Schema\ResourceTemplate;
@@ -23,6 +25,7 @@
 use Mcp\Tests\Unit\Capability\Attribute\CompletionProviderFixture;
 use PHPUnit\Framework\MockObject\MockObject;
 use PHPUnit\Framework\TestCase;
+use Psr\Log\LoggerInterface;
 
 class CompletionCompleteHandlerTest extends TestCase
 {
@@ -79,6 +82,42 @@ public function testReturnsCompletionsForResourceTemplate(): void
         $this->assertEquals(new CompletionCompleteResult(['alpha'], 1, false), $response->result);
     }
 
+    public function testLogsProviderFailureBeforeReturningInternalError(): void
+    {
+        $uri = 'file://users/alice';
+        $request = $this->createCompletionRequest($uri, ['name' => 'id', 'value' => 'al']);
+
+        $exception = new \RuntimeException('Provider blew up');
+        $provider = $this->createMock(ProviderInterface::class);
+        $provider->method('getCompletions')->willThrowException($exception);
+
+        $templateReference = new ResourceTemplateReference(
+            new ResourceTemplate('file://users/{id}', 'user'),
+            static fn () => null,
+            ['id' => $provider],
+        );
+
+        $this->registry
+            ->method('getResource')
+            ->with($uri)
+            ->willReturn($templateReference);
+
+        $logger = $this->createMock(LoggerInterface::class);
+        $logger
+            ->expects($this->once())
+            ->method('error')
+            ->with(
+                'Error while handling completion request: Provider blew up',
+                ['exception' => $exception],
+            );
+
+        $handler = new CompletionCompleteHandler($this->registry, null, $logger);
+        $response = $handler->handle($request, $this->session);
+
+        $this->assertInstanceOf(Error::class, $response);
+        $this->assertSame('Error while handling completion request', $response->message);
+    }
+
     /**
      * @param array{ name: string, value: string } $argument
      */