[Server] Allow configuring the schema validator

CHANGELOG.md

@@ -5,6 +5,8 @@ All notable changes to `mcp/sdk` will be documented in this file.
 0.9.0
 -----
 
+* [BC Break] `SchemaValidator` takes an optional `Opis\JsonSchema\Validator` as its first constructor argument, moving `$logger` to second. Pass `logger:` by name.
+* Add `Builder::setSchemaValidator()` to configure the validator used for `tools/call` input, e.g. with a resolver for external `$ref` schemas.
 * [BC Break] Remove the `providerClass` argument of `#[CompletionProvider]`. Use `provider:`, which takes the same class-string and is now the first positional argument.
 * Add `HttpTransport::getSessionId()` to read the server-minted `Mcp-Session-Id`: a request-scoped caller can persist it and pass it back through the constructor's `$headers` on a later transport. Always `null` on `2026-07-28`, which removed protocol-level sessions.
 

src/Capability/Discovery/SchemaValidator.php

@@ -30,9 +30,8 @@
  */
 class SchemaValidator
 {
-    private ?Validator $jsonSchemaValidator = null;
-
     public function __construct(
+        private ?Validator $jsonSchemaValidator = null,
         private LoggerInterface $logger = new NullLogger(),
     ) {
     }

src/Server/Builder.php

@@ -16,6 +16,7 @@
 use Mcp\Capability\Discovery\Discoverer;
 use Mcp\Capability\Discovery\DiscovererInterface;
 use Mcp\Capability\Discovery\SchemaGeneratorInterface;
+use Mcp\Capability\Discovery\SchemaValidator;
 use Mcp\Capability\Registry;
 use Mcp\Capability\Registry\Container;
 use Mcp\Capability\Registry\ElementReference;
@@ -105,6 +106,8 @@ final class Builder
 
     private ?SchemaGeneratorInterface $schemaGenerator = null;
 
+    private ?SchemaValidator $schemaValidator = null;
+
     private ?ReferenceHandlerInterface $referenceHandler = null;
 
     private ?DiscovererInterface $discoverer = null;
@@ -582,6 +585,13 @@ public function setSchemaGenerator(SchemaGeneratorInterface $schemaGenerator): s
         return $this;
     }
 
+    public function setSchemaValidator(SchemaValidator $schemaValidator): self
+    {
+        $this->schemaValidator = $schemaValidator;
+
+        return $this;
+    }
+
     public function setReferenceHandler(ReferenceHandlerInterface $referenceHandler): self
     {
         $this->referenceHandler = $referenceHandler;
@@ -1084,7 +1094,7 @@ private function resolve(): array
         $referenceHandler = $this->referenceHandler ?? new ReferenceHandler($container);
 
         $requestHandlers = array_merge($this->requestHandlers, [
-            new Handler\Request\CallToolHandler($registry, $referenceHandler, $logger),
+            new Handler\Request\CallToolHandler($registry, $referenceHandler, $logger, $this->schemaValidator),
             new Handler\Request\CompletionCompleteHandler($registry, $container, $logger),
             new Handler\Request\GetPromptHandler($registry, $referenceHandler, $logger),
             new Handler\Request\InitializeHandler($configuration),

src/Server/Handler/Request/CallToolHandler.php

@@ -48,7 +48,7 @@ public function __construct(
         private readonly LoggerInterface $logger = new NullLogger(),
         ?SchemaValidator $schemaValidator = null,
     ) {
-        $this->schemaValidator = $schemaValidator ?? new SchemaValidator($logger);
+        $this->schemaValidator = $schemaValidator ?? new SchemaValidator(logger: $logger);
     }
 
     public function supports(Request $request): bool

tests/Unit/Server/BuilderTest.php

@@ -98,6 +98,23 @@ public function testCustomReferenceHandlerIsUsedForToolCalls(): void
         $this->assertSame('intercepted', $result);
     }
 
+    #[TestDox('Custom SchemaValidator is used when calling a tool')]
+    public function testCustomSchemaValidatorIsUsedForToolCalls(): void
+    {
+        $schemaValidator = $this->createMock(\Mcp\Capability\Discovery\SchemaValidator::class);
+        $schemaValidator->expects($this->once())
+            ->method('validateAgainstJsonSchema')
+            ->willReturn([]);
+
+        $server = Server::builder()
+            ->setServerInfo('test', '1.0.0')
+            ->setSchemaValidator($schemaValidator)
+            ->addTool(static fn (): string => 'validated', name: 'test_tool', description: 'A test tool')
+            ->build();
+
+        $this->assertSame('validated', $this->callTool($server, 'test_tool'));
+    }
+
     #[TestDox('A pre-built instance handler with constructor dependencies is registered and invoked on that instance')]
     public function testPreBuiltInstanceHandlerIsInvokedOnTheGivenInstance(): void
     {