[Server] Keep injectable parameters out of the published inputSchema

src/Capability/Discovery/SchemaGenerator.php

@@ -13,9 +13,9 @@
 
 use Mcp\Capability\Attribute\McpTool;
 use Mcp\Capability\Attribute\Schema;
+use Mcp\Capability\Registry\InjectableParameters;
 use Mcp\Exception\BadMethodCallException;
 use Mcp\Exception\InvalidArgumentException;
-use Mcp\Server\RequestContext;
 use phpDocumentor\Reflection\DocBlock\Tags\Param;
 
 /**
@@ -528,12 +528,10 @@ private function parseParametersInfo(\ReflectionMethod|\ReflectionFunction $refl
         foreach ($reflection->getParameters() as $rp) {
             $reflectionType = $rp->getType();
 
-            if ($reflectionType instanceof \ReflectionNamedType && !$reflectionType->isBuiltin()) {
-                $typeName = $reflectionType->getName();
-
-                if (is_a($typeName, RequestContext::class, true)) {
-                    continue;
-                }
+            if ($reflectionType instanceof \ReflectionNamedType && !$reflectionType->isBuiltin()
+                && InjectableParameters::supports($reflectionType->getName())
+            ) {
+                continue;
             }
 
             $paramName = $rp->getName();

src/Capability/Registry/InjectableParameters.php

@@ -0,0 +1,63 @@
+<?php
+
+/*
+ * This file is part of the official PHP MCP SDK.
+ *
+ * A collaboration between Symfony and the PHP Foundation.
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Mcp\Capability\Registry;
+
+use Mcp\Server\ClientGateway;
+use Mcp\Server\RequestContext;
+
+/**
+ * Single source of truth for handler parameter types the SDK injects itself.
+ *
+ * Both schema generation (which must exclude these parameters from the
+ * published inputSchema) and argument preparation (which must inject them)
+ * read this list, so the two cannot drift apart.
+ *
+ * @internal
+ */
+final class InjectableParameters
+{
+    private const TYPES = [
+        RequestContext::class,
+        ClientGateway::class,
+    ];
+
+    private function __construct()
+    {
+    }
+
+    public static function supports(string $typeName): bool
+    {
+        foreach (self::TYPES as $type) {
+            if (is_a($typeName, $type, true)) {
+                return true;
+            }
+        }
+
+        return false;
+    }
+
+    /**
+     * @param array<string, mixed> $arguments the raw argument bag including the internal "_session" and "_request" entries
+     */
+    public static function resolve(string $typeName, array $arguments): ?object
+    {
+        if (RequestContext::class === $typeName && isset($arguments['_session'], $arguments['_request'])) {
+            return new RequestContext($arguments['_session'], $arguments['_request']);
+        }
+
+        if (ClientGateway::class === $typeName && isset($arguments['_session'])) {
+            return new ClientGateway($arguments['_session']);
+        }
+
+        return null;
+    }
+}

src/Capability/Registry/ReferenceHandler.php

@@ -13,8 +13,6 @@
 
 use Mcp\Exception\InvalidArgumentException;
 use Mcp\Exception\RegistryException;
-use Mcp\Server\ClientGateway;
-use Mcp\Server\RequestContext;
 use Mcp\Server\Session\SessionInterface;
 use Psr\Container\ContainerInterface;
 
@@ -106,15 +104,9 @@ private function prepareArguments(\ReflectionFunctionAbstract $reflection, array
             // Check if parameter is a special injectable type
             $type = $parameter->getType();
             if ($type instanceof \ReflectionNamedType && !$type->isBuiltin()) {
-                $typeName = $type->getName();
-
-                if (RequestContext::class === $typeName && isset($arguments['_session'], $arguments['_request'])) {
-                    $finalArgs[$paramPosition] = new RequestContext($arguments['_session'], $arguments['_request']);
-                    continue;
-                }
-
-                if (ClientGateway::class === $typeName && isset($arguments['_session'])) {
-                    $finalArgs[$paramPosition] = new ClientGateway($arguments['_session']);
+                $injected = InjectableParameters::resolve($type->getName(), $arguments);
+                if (null !== $injected) {
+                    $finalArgs[$paramPosition] = $injected;
                     continue;
                 }
             }

tests/Integration/Fixture/sampling.php

@@ -16,6 +16,7 @@
 use Mcp\Exception\ClientException;
 use Mcp\Schema\Content\TextContent;
 use Mcp\Server;
+use Mcp\Server\ClientGateway;
 use Mcp\Server\RequestContext;
 use Mcp\Server\Transport\StdioTransport;
 
@@ -38,5 +39,20 @@ static function (RequestContext $context, string $text): string {
         name: 'summarize',
         description: 'Summarizes text by asking the client to sample.',
     )
+    ->addTool(
+        static function (ClientGateway $client, string $text): string {
+            try {
+                $result = $client->sample($text, maxTokens: 64);
+            } catch (ClientException $e) {
+                return $e->getMessage();
+            }
+
+            assert($result->content instanceof TextContent);
+
+            return sprintf('%s said: %s', $result->model, $result->content->text);
+        },
+        name: 'summarize_via_gateway',
+        description: 'Summarizes text through a directly injected gateway.',
+    )
     ->build()
     ->run(new StdioTransport());

tests/Integration/SamplingTest.php

@@ -56,6 +56,29 @@ public function testPromptReachesTheClient(): void
         $this->assertSame(64, $seen[0]->maxTokens);
     }
 
+    #[TestDox('a gateway parameter is injected, not published in the schema')]
+    public function testGatewayParameterIsInjectedNotPublished(): void
+    {
+        $client = $this->connect('sampling', $this->clientSampling());
+
+        $tool = null;
+        foreach ($client->listTools()->tools as $candidate) {
+            if ('summarize_via_gateway' === $candidate->name) {
+                $tool = $candidate;
+            }
+        }
+
+        $this->assertNotNull($tool);
+        $this->assertArrayNotHasKey('client', $tool->inputSchema['properties']);
+        $this->assertArrayHasKey('text', $tool->inputSchema['properties']);
+        $this->assertSame(['text'], $tool->inputSchema['required']);
+
+        $result = $client->callTool('summarize_via_gateway', ['text' => 'a long report']);
+
+        $this->assertInstanceOf(TextContent::class, $result->content[0]);
+        $this->assertSame('test-model said: a long report', $result->content[0]->text);
+    }
+
     #[TestDox('a client that cannot sample refuses instead of stalling the tool')]
     public function testClientWithoutSamplingRefuses(): void
     {

tests/Unit/Capability/Discovery/SchemaGeneratorFixture.php

@@ -13,6 +13,8 @@
 
 use Mcp\Capability\Attribute\McpTool;
 use Mcp\Capability\Attribute\Schema;
+use Mcp\Server\ClientGateway;
+use Mcp\Server\RequestContext;
 use Mcp\Tests\Unit\Fixtures\Enum\BackedIntEnum;
 use Mcp\Tests\Unit\Fixtures\Enum\BackedStringEnum;
 use Mcp\Tests\Unit\Fixtures\Enum\UnitEnum;
@@ -519,6 +521,13 @@ public function withParameterNamedRequest(string $_request): void
     {
     }
 
+    /**
+     * @param string $query The search query
+     */
+    public function withInjectableParameters(string $query, ClientGateway $gateway, RequestContext $context, int $limit = 10): void
+    {
+    }
+
     // ===== OUTPUT SCHEMA FIXTURES =====
     #[McpTool(
         outputSchema: [

tests/Unit/Capability/Discovery/SchemaGeneratorTest.php

@@ -413,6 +413,17 @@ public function testInfersParameterTypeAsAnyIfOnlyConstraintsAreGiven(): void
         $this->assertEquals(['inferredParam'], $schema['required']);
     }
 
+    public function testExcludesInjectableParameterTypesFromSchema(): void
+    {
+        $method = new \ReflectionMethod(SchemaGeneratorFixture::class, 'withInjectableParameters');
+        $schema = $this->schemaGenerator->generate($method);
+        $this->assertArrayNotHasKey('gateway', $schema['properties']);
+        $this->assertArrayNotHasKey('context', $schema['properties']);
+        $this->assertEquals(['type' => 'string', 'description' => 'The search query'], $schema['properties']['query']);
+        $this->assertEquals(['type' => 'integer', 'default' => 10], $schema['properties']['limit']);
+        $this->assertEquals(['query'], $schema['required']);
+    }
+
     public static function methodsWithForbiddenParameter(): array
     {
         return [

tests/Unit/Capability/Registry/ReferenceHandlerTest.php

@@ -108,6 +108,32 @@ public function read(string $uri, ClientGateway $gateway): mixed
         $this->assertInstanceOf(ClientGateway::class, $resourceHandler->receivedGateway);
     }
 
+    public function testHandleInjectsClientGatewayIntoReflectedHandler(): void
+    {
+        $session = $this->createMock(SessionInterface::class);
+        $session->method('getId')->willReturn(Uuid::v4());
+
+        $handler = new class {
+            public ?ClientGateway $receivedGateway = null;
+
+            public function search(string $query, ClientGateway $gateway): string
+            {
+                $this->receivedGateway = $gateway;
+
+                return 'found: '.$query;
+            }
+        };
+
+        $result = (new ReferenceHandler())->handle(new ElementReference([$handler, 'search']), [
+            '_session' => $session,
+            '_request' => new \stdClass(),
+            'query' => 'foo',
+        ]);
+
+        $this->assertSame('found: foo', $result);
+        $this->assertInstanceOf(ClientGateway::class, $handler->receivedGateway);
+    }
+
     public function testHandleStillReflectsOrdinaryClosuresAndDoesNotInjectArgumentBag(): void
     {
         $session = $this->createMock(SessionInterface::class);