mcp-python-sdk/src/mcp/server/lowlevel/server.py

"""
MCP Server Module

This module provides a framework for creating an MCP (Model Context Protocol) server.
It allows you to easily define and handle various types of requests and notifications
in an asynchronous manner.

Usage:
1. Create a Server instance:
   server = Server("your_server_name")

2. Define request handlers using decorators:
   @server.list_prompts()
   async def handle_list_prompts() -> list[types.Prompt]:
       # Implementation

   @server.get_prompt()
   async def handle_get_prompt(
       name: str, arguments: dict[str, str] | None
   ) -> types.GetPromptResult:
       # Implementation

   @server.list_tools()
   async def handle_list_tools() -> list[types.Tool]:
       # Implementation

   @server.call_tool()
   async def handle_call_tool(
       name: str, arguments: dict | None
   ) -> list[types.TextContent | types.ImageContent | types.EmbeddedResource]:
       # Implementation

   @server.list_resource_templates()
   async def handle_list_resource_templates() -> list[types.ResourceTemplate]:
       # Implementation

3. Define notification handlers if needed:
   @server.progress_notification()
   async def handle_progress(
       progress_token: str | int, progress: float, total: float | None,
       message: str | None
   ) -> None:
       # Implementation

4. Run the server:
   async def main():
       async with mcp.server.stdio.stdio_server() as (read_stream, write_stream):
           await server.run(
               read_stream,
               write_stream,
               InitializationOptions(
                   server_name="your_server_name",
                   server_version="your_version",
                   capabilities=server.get_capabilities(
                       notification_options=NotificationOptions(),
                       experimental_capabilities={},
                   ),
               ),
           )

   asyncio.run(main())

The Server class provides methods to register handlers for various MCP requests and
notifications. It automatically manages the request context and handles incoming
messages from the client.
"""

from __future__ import annotations as _annotations

import contextvars
import logging
import warnings
from collections.abc import AsyncIterator, Awaitable, Callable, Iterable
from contextlib import AbstractAsyncContextManager, AsyncExitStack, asynccontextmanager
from typing import Any, Generic

import anyio
from anyio.streams.memory import MemoryObjectReceiveStream, MemoryObjectSendStream
from pydantic import AnyUrl
from typing_extensions import TypeVar

import mcp.types as types
from mcp.server.lowlevel.helper_types import ReadResourceContents
from mcp.server.models import InitializationOptions
from mcp.server.session import ServerSession
from mcp.server.stdio import stdio_server as stdio_server
from mcp.shared.context import RequestContext
from mcp.shared.exceptions import McpError
from mcp.shared.message import ServerMessageMetadata, SessionMessage
from mcp.shared.session import RequestResponder

logger = logging.getLogger(__name__)

LifespanResultT = TypeVar("LifespanResultT")
RequestT = TypeVar("RequestT", default=Any)

# This will be properly typed in each Server instance's context
request_ctx: contextvars.ContextVar[RequestContext[ServerSession, Any, Any]] = (
    contextvars.ContextVar("request_ctx")
)


class NotificationOptions:
    def __init__(
        self,
        prompts_changed: bool = False,
        resources_changed: bool = False,
        tools_changed: bool = False,
    ):
        self.prompts_changed = prompts_changed
        self.resources_changed = resources_changed
        self.tools_changed = tools_changed


@asynccontextmanager
async def lifespan(server: Server[LifespanResultT, RequestT]) -> AsyncIterator[object]:
    """Default lifespan context manager that does nothing.

    Args:
        server: The server instance this lifespan is managing

    Returns:
        An empty context object
    """
    yield {}


class Server(Generic[LifespanResultT, RequestT]):
    def __init__(
        self,
        name: str,
        version: str | None = None,
        instructions: str | None = None,
        lifespan: Callable[
            [Server[LifespanResultT, RequestT]],
            AbstractAsyncContextManager[LifespanResultT],
        ] = lifespan,
    ):
        self.name = name
        self.version = version
        self.instructions = instructions
        self.lifespan = lifespan
        self.request_handlers: dict[
            type, Callable[..., Awaitable[types.ServerResult]]
        ] = {
            types.PingRequest: _ping_handler,
        }
        self.notification_handlers: dict[type, Callable[..., Awaitable[None]]] = {}
        self.notification_options = NotificationOptions()
        logger.debug(f"Initializing server '{name}'")

    def create_initialization_options(
        self,
        notification_options: NotificationOptions | None = None,
        experimental_capabilities: dict[str, dict[str, Any]] | None = None,
    ) -> InitializationOptions:
        """Create initialization options from this server instance."""

        def pkg_version(package: str) -> str:
            try:
                from importlib.metadata import version

                return version(package)
            except Exception:
                pass

            return "unknown"

        return InitializationOptions(
            server_name=self.name,
            server_version=self.version if self.version else pkg_version("mcp"),
            capabilities=self.get_capabilities(
                notification_options or NotificationOptions(),
                experimental_capabilities or {},
            ),
            instructions=self.instructions,
        )

    def get_capabilities(
        self,
        notification_options: NotificationOptions,
        experimental_capabilities: dict[str, dict[str, Any]],
    ) -> types.ServerCapabilities:
        """Convert existing handlers to a ServerCapabilities object."""
        prompts_capability = None
        resources_capability = None
        tools_capability = None
        logging_capability = None

        # Set prompt capabilities if handler exists
        if types.ListPromptsRequest in self.request_handlers:
            prompts_capability = types.PromptsCapability(
                listChanged=notification_options.prompts_changed
            )

        # Set resource capabilities if handler exists
        if types.ListResourcesRequest in self.request_handlers:
            resources_capability = types.ResourcesCapability(
                subscribe=False, listChanged=notification_options.resources_changed
            )

        # Set tool capabilities if handler exists
        if types.ListToolsRequest in self.request_handlers:
            tools_capability = types.ToolsCapability(
                listChanged=notification_options.tools_changed
            )

        # Set logging capabilities if handler exists
        if types.SetLevelRequest in self.request_handlers:
            logging_capability = types.LoggingCapability()

        return types.ServerCapabilities(
            prompts=prompts_capability,
            resources=resources_capability,
            tools=tools_capability,
            logging=logging_capability,
            experimental=experimental_capabilities,
        )

    @property
    def request_context(
        self,
    ) -> RequestContext[ServerSession, LifespanResultT, RequestT]:
        """If called outside of a request context, this will raise a LookupError."""
        return request_ctx.get()

    def list_prompts(self):
        def decorator(func: Callable[[], Awaitable[list[types.Prompt]]]):
            logger.debug("Registering handler for PromptListRequest")

            async def handler(_: Any):
                prompts = await func()
                return types.ServerResult(types.ListPromptsResult(prompts=prompts))

            self.request_handlers[types.ListPromptsRequest] = handler
            return func

        return decorator

    def get_prompt(self):
        def decorator(
            func: Callable[
                [str, dict[str, str] | None], Awaitable[types.GetPromptResult]
            ],
        ):
            logger.debug("Registering handler for GetPromptRequest")

            async def handler(req: types.GetPromptRequest):
                prompt_get = await func(req.params.name, req.params.arguments)
                return types.ServerResult(prompt_get)

            self.request_handlers[types.GetPromptRequest] = handler
            return func

        return decorator

    def list_resources(self):
        def decorator(func: Callable[[], Awaitable[list[types.Resource]]]):
            logger.debug("Registering handler for ListResourcesRequest")

            async def handler(_: Any):
                resources = await func()
                return types.ServerResult(
                    types.ListResourcesResult(resources=resources)
                )

            self.request_handlers[types.ListResourcesRequest] = handler
            return func

        return decorator

    def list_resource_templates(self):
        def decorator(func: Callable[[], Awaitable[list[types.ResourceTemplate]]]):
            logger.debug("Registering handler for ListResourceTemplatesRequest")

            async def handler(_: Any):
                templates = await func()
                return types.ServerResult(
                    types.ListResourceTemplatesResult(resourceTemplates=templates)
                )

            self.request_handlers[types.ListResourceTemplatesRequest] = handler
            return func

        return decorator

    def read_resource(self):
        def decorator(
            func: Callable[
                [AnyUrl], Awaitable[str | bytes | Iterable[ReadResourceContents]]
            ],
        ):
            logger.debug("Registering handler for ReadResourceRequest")

            async def handler(req: types.ReadResourceRequest):
                result = await func(req.params.uri)

                def create_content(data: str | bytes, mime_type: str | None):
                    match data:
                        case str() as data:
                            return types.TextResourceContents(
                                uri=req.params.uri,
                                text=data,
                                mimeType=mime_type or "text/plain",
                            )
                        case bytes() as data:
                            import base64

                            return types.BlobResourceContents(
                                uri=req.params.uri,
                                blob=base64.b64encode(data).decode(),
                                mimeType=mime_type or "application/octet-stream",
                            )

                match result:
                    case str() | bytes() as data:
                        warnings.warn(
                            "Returning str or bytes from read_resource is deprecated. "
                            "Use Iterable[ReadResourceContents] instead.",
                            DeprecationWarning,
                            stacklevel=2,
                        )
                        content = create_content(data, None)
                    case Iterable() as contents:
                        contents_list = [
                            create_content(content_item.content, content_item.mime_type)
                            for content_item in contents
                        ]
                        return types.ServerResult(
                            types.ReadResourceResult(
                                contents=contents_list,
                            )
                        )
                    case _:
                        raise ValueError(
                            f"Unexpected return type from read_resource: {type(result)}"
                        )

                return types.ServerResult(
                    types.ReadResourceResult(
                        contents=[content],
                    )
                )

            self.request_handlers[types.ReadResourceRequest] = handler
            return func

        return decorator

    def set_logging_level(self):
        def decorator(func: Callable[[types.LoggingLevel], Awaitable[None]]):
            logger.debug("Registering handler for SetLevelRequest")

            async def handler(req: types.SetLevelRequest):
                await func(req.params.level)
                return types.ServerResult(types.EmptyResult())

            self.request_handlers[types.SetLevelRequest] = handler
            return func

        return decorator

    def subscribe_resource(self):
        def decorator(func: Callable[[AnyUrl], Awaitable[None]]):
            logger.debug("Registering handler for SubscribeRequest")

            async def handler(req: types.SubscribeRequest):
                await func(req.params.uri)
                return types.ServerResult(types.EmptyResult())

            self.request_handlers[types.SubscribeRequest] = handler
            return func

        return decorator

    def unsubscribe_resource(self):
        def decorator(func: Callable[[AnyUrl], Awaitable[None]]):
            logger.debug("Registering handler for UnsubscribeRequest")

            async def handler(req: types.UnsubscribeRequest):
                await func(req.params.uri)
                return types.ServerResult(types.EmptyResult())

            self.request_handlers[types.UnsubscribeRequest] = handler
            return func

        return decorator

    def list_tools(self):
        def decorator(func: Callable[[], Awaitable[list[types.Tool]]]):
            logger.debug("Registering handler for ListToolsRequest")

            async def handler(_: Any):
                tools = await func()
                return types.ServerResult(types.ListToolsResult(tools=tools))

            self.request_handlers[types.ListToolsRequest] = handler
            return func

        return decorator

    def call_tool(self):
        def decorator(
            func: Callable[
                ...,
                Awaitable[
                    Iterable[
                        types.TextContent | types.ImageContent | types.EmbeddedResource
                    ]
                ],
            ],
        ):
            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:
                    return types.ServerResult(
                        types.CallToolResult(
                            content=[types.TextContent(type="text", text=str(e))],
                            isError=True,
                        )
                    )

            self.request_handlers[types.CallToolRequest] = handler
            return func

        return decorator

    def progress_notification(self):
        def decorator(
            func: Callable[
                [str | int, float, float | None, str | None], Awaitable[None]
            ],
        ):
            logger.debug("Registering handler for ProgressNotification")

            async def handler(req: types.ProgressNotification):
                await func(
                    req.params.progressToken,
                    req.params.progress,
                    req.params.total,
                    req.params.message,
                )

            self.notification_handlers[types.ProgressNotification] = handler
            return func

        return decorator

    def completion(self):
        """Provides completions for prompts and resource templates"""

        def decorator(
            func: Callable[
                [
                    types.PromptReference | types.ResourceReference,
                    types.CompletionArgument,
                ],
                Awaitable[types.Completion | None],
            ],
        ):
            logger.debug("Registering handler for CompleteRequest")

            async def handler(req: types.CompleteRequest):
                completion = await func(req.params.ref, req.params.argument)
                return types.ServerResult(
                    types.CompleteResult(
                        completion=completion
                        if completion is not None
                        else types.Completion(values=[], total=None, hasMore=None),
                    )
                )

            self.request_handlers[types.CompleteRequest] = handler
            return func

        return decorator

    async def run(
        self,
        read_stream: MemoryObjectReceiveStream[SessionMessage | Exception],
        write_stream: MemoryObjectSendStream[SessionMessage],
        initialization_options: InitializationOptions,
        # When False, exceptions are returned as messages to the client.
        # When True, exceptions are raised, which will cause the server to shut down
        # but also make tracing exceptions much easier during testing and when using
        # in-process servers.
        raise_exceptions: bool = False,
        # When True, the server is stateless and
        # clients can perform initialization with any node. The client must still follow
        # the initialization lifecycle, but can do so with any available node
        # rather than requiring initialization for each connection.
        stateless: bool = False,
    ):
        async with AsyncExitStack() as stack:
            lifespan_context = await stack.enter_async_context(self.lifespan(self))
            session = await stack.enter_async_context(
                ServerSession(
                    read_stream,
                    write_stream,
                    initialization_options,
                    stateless=stateless,
                )
            )

            async with anyio.create_task_group() as tg:
                async for message in session.incoming_messages:
                    logger.debug(f"Received message: {message}")

                    tg.start_soon(
                        self._handle_message,
                        message,
                        session,
                        lifespan_context,
                        raise_exceptions,
                    )

    async def _handle_message(
        self,
        message: RequestResponder[types.ClientRequest, types.ServerResult]
        | types.ClientNotification
        | Exception,
        session: ServerSession,
        lifespan_context: LifespanResultT,
        raise_exceptions: bool = False,
    ):
        with warnings.catch_warnings(record=True) as w:
            # TODO(Marcelo): We should be checking if message is Exception here.
            match message:  # type: ignore[reportMatchNotExhaustive]
                case (
                    RequestResponder(request=types.ClientRequest(root=req)) as responder
                ):
                    with responder:
                        await self._handle_request(
                            message, req, session, lifespan_context, raise_exceptions
                        )
                case types.ClientNotification(root=notify):
                    await self._handle_notification(notify)

            for warning in w:
                logger.info(f"Warning: {warning.category.__name__}: {warning.message}")

    async def _handle_request(
        self,
        message: RequestResponder[types.ClientRequest, types.ServerResult],
        req: Any,
        session: ServerSession,
        lifespan_context: LifespanResultT,
        raise_exceptions: bool,
    ):
        logger.info(f"Processing request of type {type(req).__name__}")
        if type(req) in self.request_handlers:
            handler = self.request_handlers[type(req)]
            logger.debug(f"Dispatching request of type {type(req).__name__}")

            token = None
            try:
                # Extract request context from message metadata
                request_data = None
                if message.message_metadata is not None and isinstance(
                    message.message_metadata, ServerMessageMetadata
                ):
                    request_data = message.message_metadata.request_context

                # Set our global state that can be retrieved via
                # app.get_request_context()
                token = request_ctx.set(
                    RequestContext(
                        message.request_id,
                        message.request_meta,
                        session,
                        lifespan_context,
                        request=request_data,
                    )
                )
                response = await handler(req)
            except McpError as err:
                response = err.error
            except Exception as err:
                if raise_exceptions:
                    raise err
                response = types.ErrorData(code=0, message=str(err), data=None)
            finally:
                # Reset the global state after we are done
                if token is not None:
                    request_ctx.reset(token)

            await message.respond(response)
        else:
            await message.respond(
                types.ErrorData(
                    code=types.METHOD_NOT_FOUND,
                    message="Method not found",
                )
            )

        logger.debug("Response sent")

    async def _handle_notification(self, notify: Any):
        if type(notify) in self.notification_handlers:
            assert type(notify) in self.notification_handlers

            handler = self.notification_handlers[type(notify)]
            logger.debug(f"Dispatching notification of type {type(notify).__name__}")

            try:
                await handler(notify)
            except Exception as err:
                logger.error(f"Uncaught exception in notification handler: {err}")


async def _ping_handler(request: types.PingRequest) -> types.ServerResult:
    return types.ServerResult(types.EmptyResult())