feat: Add output schema support to MCP tools

CHANGELOG.md

@@ -2,6 +2,11 @@
 
 All notable changes to `mcp/sdk` will be documented in this file.
 
+0.3
+-----
+
+* Add output schema support to MCP tools
+
 0.2.2
 -----
 

examples/server/env-variables/EnvToolHandler.php

@@ -23,7 +23,31 @@ final class EnvToolHandler
      *
      * @return array<string, string|int> the result, varying by APP_MODE
      */
-    #[McpTool(name: 'process_data_by_mode')]
+    #[McpTool(
+        name: 'process_data_by_mode',
+        outputSchema: [
+            'type' => 'object',
+            'properties' => [
+                'mode' => [
+                    'type' => 'string',
+                    'description' => 'The processing mode used',
+                ],
+                'processed_input' => [
+                    'type' => 'string',
+                    'description' => 'The processed input data',
+                ],
+                'original_input' => [
+                    'type' => 'string',
+                    'description' => 'The original input data (only in default mode)',
+                ],
+                'message' => [
+                    'type' => 'string',
+                    'description' => 'A descriptive message about the processing',
+                ],
+            ],
+            'required' => ['mode', 'message'],
+        ]
+    )]
     public function processData(string $input): array
     {
         $appMode = getenv('APP_MODE'); // Read from environment

src/Capability/Attribute/McpTool.php

@@ -21,18 +21,20 @@
 class McpTool
 {
     /**
-     * @param string|null           $name        The name of the tool (defaults to the method name)
-     * @param string|null           $description The description of the tool (defaults to the DocBlock/inferred)
-     * @param ToolAnnotations|null  $annotations Optional annotations describing tool behavior
-     * @param ?Icon[]               $icons       Optional list of icon URLs representing the tool
-     * @param ?array<string, mixed> $meta        Optional metadata
+     * @param string|null           $name         The name of the tool (defaults to the method name)
+     * @param string|null           $description  The description of the tool (defaults to the DocBlock/inferred)
+     * @param ToolAnnotations|null  $annotations  Optional annotations describing tool behavior
+     * @param ?Icon[]               $icons        Optional list of icon URLs representing the tool
+     * @param ?array<string, mixed> $meta         Optional metadata
+     * @param array<string, mixed>  $outputSchema Optional JSON Schema object for defining the expected output structure
      */
     public function __construct(
         public ?string $name = null,
         public ?string $description = null,
         public ?ToolAnnotations $annotations = null,
         public ?array $icons = null,
         public ?array $meta = null,
+        public ?array $outputSchema = null,
     ) {
     }
 }

src/Capability/Discovery/Discoverer.php

@@ -222,13 +222,15 @@ private function processMethod(\ReflectionMethod $method, array &$discoveredCoun
                     $name = $instance->name ?? ('__invoke' === $methodName ? $classShortName : $methodName);
                     $description = $instance->description ?? $this->docBlockParser->getSummary($docBlock) ?? null;
                     $inputSchema = $this->schemaGenerator->generate($method);
+                    $outputSchema = $this->schemaGenerator->generateOutputSchema($method);
                     $tool = new Tool(
                         $name,
                         $inputSchema,
                         $description,
                         $instance->annotations,
                         $instance->icons,
                         $instance->meta,
+                        $outputSchema,
                     );
                     $tools[$name] = new ToolReference($tool, [$className, $methodName], false);
                     ++$discoveredCount['tools'];

src/Capability/Discovery/SchemaGenerator.php

@@ -11,6 +11,7 @@
 
 namespace Mcp\Capability\Discovery;
 
+use Mcp\Capability\Attribute\McpTool;
 use Mcp\Capability\Attribute\Schema;
 use Mcp\Exception\InvalidArgumentException;
 use Mcp\Server\RequestContext;
@@ -81,6 +82,27 @@ public function generate(\ReflectionMethod|\ReflectionFunction $reflection): arr
         return $this->buildSchemaFromParameters($parametersInfo, $methodSchema);
     }
 
+    /**
+     * Generates a JSON Schema object (as a PHP array) for a method's or function's return type.
+     *
+     * Only returns an outputSchema if explicitly provided in the McpTool attribute.
+     * Per MCP spec, outputSchema should only be present when explicitly provided.
+     *
+     * @return array<string, mixed>|null
+     */
+    public function generateOutputSchema(\ReflectionMethod|\ReflectionFunction $reflection): ?array
+    {
+        // Only return outputSchema if explicitly provided in McpTool attribute
+        $mcpToolAttrs = $reflection->getAttributes(McpTool::class, \ReflectionAttribute::IS_INSTANCEOF);
+        if ($mcpToolAttrs) {
+            $mcpToolInstance = $mcpToolAttrs[0]->newInstance();
+
+            return $mcpToolInstance->outputSchema;
+        }
+
+        return null;
+    }
+
     /**
      * Extracts method-level or function-level Schema attribute.
      *

src/Capability/Registry/Loader/ArrayLoader.php

@@ -47,7 +47,8 @@ final class ArrayLoader implements LoaderInterface
      *     description: ?string,
      *     annotations: ?ToolAnnotations,
      *     icons: ?Icon[],
-     *     meta: ?array<string, mixed>
+     *     meta: ?array<string, mixed>,
+     *     outputSchema: ?array<string, mixed>
      * }[] $tools
      * @param array{
      *     handler: Handler,
@@ -117,6 +118,7 @@ public function load(RegistryInterface $registry): void
                     annotations: $data['annotations'] ?? null,
                     icons: $data['icons'] ?? null,
                     meta: $data['meta'] ?? null,
+                    outputSchema: $data['outputSchema'] ?? null,
                 );
                 $registry->registerTool($tool, $data['handler'], true);
 

src/Capability/Registry/ToolReference.php

@@ -111,4 +111,33 @@ public function formatResult(mixed $toolExecutionResult): array
 
         return [new TextContent($jsonResult)];
     }
+
+    /**
+     * Extracts structured content from a tool result using the output schema.
+     *
+     * @param mixed $toolExecutionResult the raw value returned by the tool's PHP method
+     *
+     * @return array<string, mixed>|null the structured content, or null if not extractable
+     *
+     * @throws \JsonException if JSON encoding fails for non-Content array/object results
+     */
+    public function extractStructuredContent(mixed $toolExecutionResult): ?array
+    {
+        if (\is_array($toolExecutionResult)) {
+            return $toolExecutionResult;
+        }
+
+        if (\is_object($toolExecutionResult) && !($toolExecutionResult instanceof Content)) {
+            $jsonResult = json_encode(
+                $toolExecutionResult,
+                \JSON_PRETTY_PRINT | \JSON_UNESCAPED_SLASHES | \JSON_UNESCAPED_UNICODE | \JSON_THROW_ON_ERROR | \JSON_INVALID_UTF8_SUBSTITUTE
+            );
+
+            return json_decode(
+                $jsonResult, true, 512, \JSON_THROW_ON_ERROR
+            );
+        }
+
+        return null;
+    }
 }

src/Schema/Result/CallToolResult.php

@@ -83,6 +83,7 @@ public static function error(array $content, ?array $meta = null): self
      *     content: array<mixed>,
      *     isError?: bool,
      *     _meta?: array<string, mixed>,
+     *     structuredContent?: array<string, mixed>
      * } $data
      */
     public static function fromArray(array $data): self

src/Schema/Tool.php

@@ -24,28 +24,37 @@
  *     properties: array<string, mixed>,
  *     required: string[]|null
  * }
+ * @phpstan-type ToolOutputSchema array{
+ *     type: 'object',
+ *     properties?: array<string, mixed>,
+ *     required?: string[]|null,
+ *     additionalProperties?: bool|array<string, mixed>,
+ *     description?: string
+ * }
  * @phpstan-type ToolData array{
  *     name: string,
  *     inputSchema: ToolInputSchema,
  *     description?: string|null,
  *     annotations?: ToolAnnotationsData,
  *     icons?: IconData[],
- *     _meta?: array<string, mixed>
+ *     _meta?: array<string, mixed>,
+ *     outputSchema?: ToolOutputSchema
  * }
  *
  * @author Kyrian Obikwelu <[email protected]>
  */
 class Tool implements \JsonSerializable
 {
     /**
-     * @param string                $name        the name of the tool
-     * @param ?string               $description A human-readable description of the tool.
-     *                                           This can be used by clients to improve the LLM's understanding of
-     *                                           available tools. It can be thought of like a "hint" to the model.
-     * @param ToolInputSchema       $inputSchema a JSON Schema object (as a PHP array) defining the expected 'arguments' for the tool
-     * @param ?ToolAnnotations      $annotations optional additional tool information
-     * @param ?Icon[]               $icons       optional icons representing the tool
-     * @param ?array<string, mixed> $meta        Optional metadata
+     * @param string                $name         the name of the tool
+     * @param ?string               $description  A human-readable description of the tool.
+     *                                            This can be used by clients to improve the LLM's understanding of
+     *                                            available tools. It can be thought of like a "hint" to the model.
+     * @param ToolInputSchema       $inputSchema  a JSON Schema object (as a PHP array) defining the expected 'arguments' for the tool
+     * @param ?ToolAnnotations      $annotations  optional additional tool information
+     * @param ?Icon[]               $icons        optional icons representing the tool
+     * @param ?array<string, mixed> $meta         Optional metadata
+     * @param ToolOutputSchema|null $outputSchema optional JSON Schema object (as a PHP array) defining the expected output structure
      */
     public function __construct(
         public readonly string $name,
@@ -54,6 +63,7 @@ public function __construct(
         public readonly ?ToolAnnotations $annotations,
         public readonly ?array $icons = null,
         public readonly ?array $meta = null,
+        public readonly ?array $outputSchema = null,
     ) {
         if (!isset($inputSchema['type']) || 'object' !== $inputSchema['type']) {
             throw new InvalidArgumentException('Tool inputSchema must be a JSON Schema of type "object".');
@@ -78,13 +88,23 @@ public static function fromArray(array $data): self
             $data['inputSchema']['properties'] = new \stdClass();
         }
 
+        if (isset($data['outputSchema']) && \is_array($data['outputSchema'])) {
+            if (!isset($data['outputSchema']['type']) || 'object' !== $data['outputSchema']['type']) {
+                throw new InvalidArgumentException('Tool outputSchema must be of type "object".');
+            }
+            if (isset($data['outputSchema']['properties']) && \is_array($data['outputSchema']['properties']) && empty($data['outputSchema']['properties'])) {
+                $data['outputSchema']['properties'] = new \stdClass();
+            }
+        }
+
         return new self(
             $data['name'],
             $data['inputSchema'],
             isset($data['description']) && \is_string($data['description']) ? $data['description'] : null,
             isset($data['annotations']) && \is_array($data['annotations']) ? ToolAnnotations::fromArray($data['annotations']) : null,
             isset($data['icons']) && \is_array($data['icons']) ? array_map(Icon::fromArray(...), $data['icons']) : null,
-            isset($data['_meta']) && \is_array($data['_meta']) ? $data['_meta'] : null
+            isset($data['_meta']) && \is_array($data['_meta']) ? $data['_meta'] : null,
+            isset($data['outputSchema']) && \is_array($data['outputSchema']) ? $data['outputSchema'] : null,
         );
     }
 
@@ -95,7 +115,8 @@ public static function fromArray(array $data): self
      *     description?: string,
      *     annotations?: ToolAnnotations,
      *     icons?: Icon[],
-     *     _meta?: array<string, mixed>
+     *     _meta?: array<string, mixed>,
+     *     outputSchema?: ToolOutputSchema
      * }
      */
     public function jsonSerialize(): array
@@ -116,6 +137,9 @@ public function jsonSerialize(): array
         if (null !== $this->meta) {
             $data['_meta'] = $this->meta;
         }
+        if (null !== $this->outputSchema) {
+            $data['outputSchema'] = $this->outputSchema;
+        }
 
         return $data;
     }

src/Server/Builder.php

@@ -87,7 +87,8 @@ final class Builder
      *     description: ?string,
      *     annotations: ?ToolAnnotations,
      *     icons: ?Icon[],
-     *     meta: ?array<string, mixed>
+     *     meta: ?array<string, mixed>,
+     *     outputSchema: ?array<string, mixed>,
      * }[]
      */
     private array $tools = [];
@@ -330,6 +331,7 @@ public function setProtocolVersion(ProtocolVersion $protocolVersion): self
      * @param array<string, mixed>|null $inputSchema
      * @param ?Icon[]                   $icons
      * @param array<string, mixed>|null $meta
+     * @param array<string, mixed>|null $outputSchema
      */
     public function addTool(
         callable|array|string $handler,
@@ -339,6 +341,7 @@ public function addTool(
         ?array $inputSchema = null,
         ?array $icons = null,
         ?array $meta = null,
+        ?array $outputSchema = null,
     ): self {
         $this->tools[] = compact(
             'handler',
@@ -348,6 +351,7 @@ public function addTool(
             'inputSchema',
             'icons',
             'meta',
+            'outputSchema',
         );
 
         return $this;

src/Server/Handler/Request/CallToolHandler.php

@@ -65,13 +65,16 @@ public function handle(Request $request, SessionInterface $session): Response|Er
 
             $result = $this->referenceHandler->handle($reference, $arguments);
 
+            $structuredContent = null;
             if (!$result instanceof CallToolResult) {
-                $result = new CallToolResult($reference->formatResult($result));
+                $structuredContent = $reference->extractStructuredContent($result);
+                $result = new CallToolResult($reference->formatResult($result), structuredContent: $structuredContent);
             }
 
             $this->logger->debug('Tool executed successfully', [
                 'name' => $toolName,
                 'result_type' => \gettype($result),
+                'structured_content' => $structuredContent,
             ]);
 
             return new Response($request->getId(), $result);

tests/Conformance/Elements.php

@@ -11,13 +11,13 @@
 
 namespace Mcp\Tests\Conformance;
 
-use Mcp\Schema\Content\Content;
 use Mcp\Schema\Content\EmbeddedResource;
 use Mcp\Schema\Content\ImageContent;
 use Mcp\Schema\Content\PromptMessage;
 use Mcp\Schema\Content\TextContent;
 use Mcp\Schema\Content\TextResourceContents;
 use Mcp\Schema\Enum\Role;
+use Mcp\Schema\Result\CallToolResult;
 use Mcp\Server\Protocol;
 use Mcp\Server\RequestContext;
 
@@ -26,20 +26,17 @@ final class Elements
     public const TEST_IMAGE_BASE64 = 'iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAADUlEQVR42mP8z8DwHwAFBQIAX8jx0gAAAABJRU5ErkJggg==';
     public const TEST_AUDIO_BASE64 = 'UklGRiYAAABXQVZFZm10IBAAAAABAAEAQB8AAAB9AAACABAAZGF0YQIAAAA=';
 
-    /**
-     * @return Content[]
-     */
-    public function toolMultipleTypes(): array
+    public function toolMultipleTypes(): CallToolResult
     {
-        return [
+        return new CallToolResult([
             new TextContent('Multiple content types test:'),
             new ImageContent(self::TEST_IMAGE_BASE64, 'image/png'),
             EmbeddedResource::fromText(
                 'test://mixed-content-resource',
                 '{ "test" = "data", "value" = 123 }',
                 'application/json',
             ),
-        ];
+        ]);
     }
 
     public function toolWithLogging(RequestContext $context): string

tests/Inspector/Http/snapshots/HttpComplexToolSchemaTest-tools_call-schedule_event_all_day_reminder.json

@@ -5,5 +5,18 @@
       "text": "{\n    \"success\": true,\n    \"message\": \"Event \\\"Project Deadline\\\" scheduled successfully for \\\"2024-12-15\\\".\",\n    \"event_details\": {\n        \"title\": \"Project Deadline\",\n        \"date\": \"2024-12-15\",\n        \"type\": \"reminder\",\n        \"time\": \"All day\",\n        \"priority\": \"Normal\",\n        \"attendees\": [],\n        \"invites_will_be_sent\": false\n    }\n}"
     }
   ],
+  "structuredContent": {
+    "success": true,
+    "message": "Event \"Project Deadline\" scheduled successfully for \"2024-12-15\".",
+    "event_details": {
+      "title": "Project Deadline",
+      "date": "2024-12-15",
+      "type": "reminder",
+      "time": "All day",
+      "priority": "Normal",
+      "attendees": [],
+      "invites_will_be_sent": false
+    }
+  },
   "isError": false
 }