Add schema validation to lowlevel server

README.md

@@ -829,6 +829,67 @@ if __name__ == "__main__":
 
 Caution: The `mcp run` and `mcp dev` tool doesn't support low-level server.
 
+#### Structured Output Support
+
+The low-level server supports structured output for tools, allowing you to return both human-readable content and machine-readable structured data. Tools can define an `outputSchema` to validate their structured output:
+
+```python
+from types import Any
+
+import mcp.types as types
+from mcp.server.lowlevel import Server
+
+server = Server("example-server")
+
+
+@server.list_tools()
+async def list_tools() -> list[types.Tool]:
+    return [
+        types.Tool(
+            name="calculate",
+            description="Perform mathematical calculations",
+            inputSchema={
+                "type": "object",
+                "properties": {
+                    "expression": {"type": "string", "description": "Math expression"}
+                },
+                "required": ["expression"],
+            },
+            outputSchema={
+                "type": "object",
+                "properties": {
+                    "result": {"type": "number"},
+                    "expression": {"type": "string"},
+                },
+                "required": ["result", "expression"],
+            },
+        )
+    ]
+
+
+@server.call_tool()
+async def call_tool(name: str, arguments: dict[str, Any]) -> dict[str, Any]:
+    if name == "calculate":
+        expression = arguments["expression"]
+        try:
+            result = eval(expression)  # Use a safe math parser
+            structured = {"result": result, "expression": expression}
+
+            # low-level server will validate structured output against the tool's
+            # output schema, and automatically serialize it into a TextContent block
+            # for backwards compatibility with pre-2025-06-18 clients.
+            return structured
+        except Exception as e:
+            raise ValueError(f"Calculation error: {str(e)}")
+```
+
+Tools can return data in three ways:
+1. **Content only**: Return a list of content blocks (default behavior before spec revision 2025-06-18)
+2. **Structured data only**: Return a dictionary that will be serialized to JSON (Introduced in spec revision 2025-06-18)
+3. **Both**: Return a tuple of (content, structured_data) preferred option to use for backwards compatibility
+
+When an `outputSchema` is defined, the server automatically validates the structured output against the schema. This ensures type safety and helps catch errors early.
+
 ### Writing MCP Clients
 
 The SDK provides a high-level client interface for connecting to MCP servers using various [transports](https://modelcontextprotocol.io/specification/2025-03-26/basic/transports):

examples/servers/structured_output_lowlevel.py

@@ -0,0 +1,98 @@
+#!/usr/bin/env python3
+"""
+Example low-level MCP server demonstrating structured output support.
+
+This example shows how to use the low-level server API to return
+structured data from tools, with automatic validation against output
+schemas.
+"""
+
+import asyncio
+from datetime import datetime
+from typing import Any
+
+import mcp.server.stdio
+import mcp.types as types
+from mcp.server.lowlevel import NotificationOptions, Server
+from mcp.server.models import InitializationOptions
+
+# Create low-level server instance
+server = Server("structured-output-lowlevel-example")
+
+
+@server.list_tools()
+async def list_tools() -> list[types.Tool]:
+    """List available tools with their schemas."""
+    return [
+        types.Tool(
+            name="get_weather",
+            description="Get weather information (simulated)",
+            inputSchema={
+                "type": "object",
+                "properties": {"city": {"type": "string", "description": "City name"}},
+                "required": ["city"],
+            },
+            outputSchema={
+                "type": "object",
+                "properties": {
+                    "temperature": {"type": "number"},
+                    "conditions": {"type": "string"},
+                    "humidity": {"type": "integer", "minimum": 0, "maximum": 100},
+                    "wind_speed": {"type": "number"},
+                    "timestamp": {"type": "string", "format": "date-time"},
+                },
+                "required": ["temperature", "conditions", "humidity", "wind_speed", "timestamp"],
+            },
+        ),
+    ]
+
+
+@server.call_tool()
+async def call_tool(name: str, arguments: dict[str, Any]) -> Any:
+    """
+    Handle tool call with structured output.
+    """
+
+    if name == "get_weather":
+        # city = arguments["city"]  # Would be used with real weather API
+
+        # Simulate weather data (in production, call a real weather API)
+        import random
+
+        weather_conditions = ["sunny", "cloudy", "rainy", "partly cloudy", "foggy"]
+
+        weather_data = {
+            "temperature": round(random.uniform(0, 35), 1),
+            "conditions": random.choice(weather_conditions),
+            "humidity": random.randint(30, 90),
+            "wind_speed": round(random.uniform(0, 30), 1),
+            "timestamp": datetime.now().isoformat(),
+        }
+
+        # Return structured data only
+        # The low-level server will serialize this to JSON content automatically
+        return weather_data
+
+    else:
+        raise ValueError(f"Unknown tool: {name}")
+
+
+async def run():
+    """Run the low-level server using stdio transport."""
+    async with mcp.server.stdio.stdio_server() as (read_stream, write_stream):
+        await server.run(
+            read_stream,
+            write_stream,
+            InitializationOptions(
+                server_name="structured-output-lowlevel-example",
+                server_version="0.1.0",
+                capabilities=server.get_capabilities(
+                    notification_options=NotificationOptions(),
+                    experimental_capabilities={},
+                ),
+            ),
+        )
+
+
+if __name__ == "__main__":
+    asyncio.run(run())

pyproject.toml

@@ -31,6 +31,7 @@ dependencies = [
     "sse-starlette>=1.6.1",
     "pydantic-settings>=2.5.2",
     "uvicorn>=0.23.1; sys_platform != 'emscripten'",
+    "jsonschema==4.20.0",
 ]
 
 [project.optional-dependencies]

src/mcp/server/lowlevel/server.py

@@ -68,13 +68,15 @@ async def main():
 from __future__ import annotations as _annotations
 
 import contextvars
+import json
 import logging
 import warnings
 from collections.abc import AsyncIterator, Awaitable, Callable, Iterable
 from contextlib import AbstractAsyncContextManager, AsyncExitStack, asynccontextmanager
-from typing import Any, Generic
+from typing import Any, Generic, TypeAlias, cast
 
 import anyio
+import jsonschema
 from anyio.streams.memory import MemoryObjectReceiveStream, MemoryObjectSendStream
 from pydantic import AnyUrl
 from typing_extensions import TypeVar
@@ -94,6 +96,11 @@ async def main():
 LifespanResultT = TypeVar("LifespanResultT")
 RequestT = TypeVar("RequestT", default=Any)
 
+# type aliases for tool call results
+StructuredContent: TypeAlias = dict[str, Any]
+UnstructuredContent: TypeAlias = Iterable[types.ContentBlock]
+CombinationContent: TypeAlias = tuple[UnstructuredContent, StructuredContent]
+
 # This will be properly typed in each Server instance's context
 request_ctx: contextvars.ContextVar[RequestContext[ServerSession, Any, Any]] = contextvars.ContextVar("request_ctx")
 
@@ -143,6 +150,7 @@ def __init__(
         }
         self.notification_handlers: dict[type, Callable[..., Awaitable[None]]] = {}
         self.notification_options = NotificationOptions()
+        self._tool_cache: dict[str, types.Tool] = {}
         logger.debug("Initializing server %r", name)
 
     def create_initialization_options(
@@ -373,33 +381,120 @@ def decorator(func: Callable[[], Awaitable[list[types.Tool]]]):
 
             async def handler(_: Any):
                 tools = await func()
+                # Refresh the tool cache
+                self._tool_cache.clear()
+                for tool in tools:
+                    self._tool_cache[tool.name] = tool
                 return types.ServerResult(types.ListToolsResult(tools=tools))
 
             self.request_handlers[types.ListToolsRequest] = handler
             return func
 
         return decorator
 
-    def call_tool(self):
+    def _make_error_result(self, error_message: str) -> types.ServerResult:
+        """Create a ServerResult with an error CallToolResult."""
+        return types.ServerResult(
+            types.CallToolResult(
+                content=[types.TextContent(type="text", text=error_message)],
+                isError=True,
+            )
+        )
+
+    async def _get_cached_tool_definition(self, tool_name: str) -> types.Tool | None:
+        """Get tool definition from cache, refreshing if necessary.
+
+        Returns the Tool object if found, None otherwise.
+        """
+        if tool_name not in self._tool_cache:
+            if types.ListToolsRequest in self.request_handlers:
+                logger.debug("Tool cache miss for %s, refreshing cache", tool_name)
+                await self.request_handlers[types.ListToolsRequest](None)
+
+        tool = self._tool_cache.get(tool_name)
+        if tool is None:
+            logger.warning("Tool '%s' not listed, no validation will be performed", tool_name)
+
+        return tool
+
+    def call_tool(self, *, validate_input: bool = True):
+        """Register a tool call handler.
+
+        Args:
+            validate_input: If True, validates input against inputSchema. Default is True.
+
+        The handler validates input against inputSchema (if validate_input=True), calls the tool function,
+        and builds a CallToolResult with the results:
+        - Unstructured content (iterable of ContentBlock): returned in content
+        - Structured content (dict): returned in structuredContent, serialized JSON text returned in content
+        - Both: returned in content and structuredContent
+
+        If outputSchema is defined, validates structuredContent or errors if missing.
+        """
+
         def decorator(
             func: Callable[
                 ...,
-                Awaitable[Iterable[types.ContentBlock]],
+                Awaitable[UnstructuredContent | StructuredContent | CombinationContent],
             ],
         ):
             logger.debug("Registering handler for CallToolRequest")
 
             async def handler(req: types.CallToolRequest):
                 try:
-                    results = await func(req.params.name, (req.params.arguments or {}))
-                    return types.ServerResult(types.CallToolResult(content=list(results), isError=False))
-                except Exception as e:
+                    tool_name = req.params.name
+                    arguments = req.params.arguments or {}
+                    tool = await self._get_cached_tool_definition(tool_name)
+
+                    # input validation
+                    if validate_input and tool:
+                        try:
+                            jsonschema.validate(instance=arguments, schema=tool.inputSchema)
+                        except jsonschema.ValidationError as e:
+                            return self._make_error_result(f"Input validation error: {e.message}")
+
+                    # tool call
+                    results = await func(tool_name, arguments)
+
+                    # output normalization
+                    unstructured_content: UnstructuredContent
+                    maybe_structured_content: StructuredContent | None
+                    if isinstance(results, tuple) and len(results) == 2:
+                        # tool returned both structured and unstructured content
+                        unstructured_content, maybe_structured_content = cast(CombinationContent, results)
+                    elif isinstance(results, dict):
+                        # tool returned structured content only
+                        maybe_structured_content = cast(StructuredContent, results)
+                        unstructured_content = [types.TextContent(type="text", text=json.dumps(results, indent=2))]
+                    elif hasattr(results, "__iter__"):
+                        # tool returned unstructured content only
+                        unstructured_content = cast(UnstructuredContent, results)
+                        maybe_structured_content = None
+                    else:
+                        return self._make_error_result(f"Unexpected return type from tool: {type(results).__name__}")
+
+                    # output validation
+                    if tool and tool.outputSchema is not None:
+                        if maybe_structured_content is None:
+                            return self._make_error_result(
+                                "Output validation error: outputSchema defined but no structured output returned"
+                            )
+                        else:
+                            try:
+                                jsonschema.validate(instance=maybe_structured_content, schema=tool.outputSchema)
+                            except jsonschema.ValidationError as e:
+                                return self._make_error_result(f"Output validation error: {e.message}")
+
+                    # result
                     return types.ServerResult(
                         types.CallToolResult(
-                            content=[types.TextContent(type="text", text=str(e))],
-                            isError=True,
+                            content=list(unstructured_content),
+                            structuredContent=maybe_structured_content,
+                            isError=False,
                         )
                     )
+                except Exception as e:
+                    return self._make_error_result(str(e))
 
             self.request_handlers[types.CallToolRequest] = handler
             return func

src/mcp/types.py

@@ -839,6 +839,11 @@ class Tool(BaseMetadata):
     """A human-readable description of the tool."""
     inputSchema: dict[str, Any]
     """A JSON Schema object defining the expected parameters for the tool."""
+    outputSchema: dict[str, Any] | None = None
+    """
+    An optional JSON Schema object defining the structure of the tool's output 
+    returned in the structuredContent field of a CallToolResult.
+    """
     annotations: ToolAnnotations | None = None
     """Optional additional tool information."""
     meta: dict[str, Any] | None = Field(alias="_meta", default=None)
@@ -874,6 +879,8 @@ class CallToolResult(Result):
     """The server's response to a tool call."""
 
     content: list[ContentBlock]
+    structuredContent: dict[str, Any] | None = None
+    """An optional JSON object that represents the structured result of the tool call."""
     isError: bool = False