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from __future__ import annotations
import abc
import asyncio
from contextlib import AbstractAsyncContextManager, AsyncExitStack
from datetime import timedelta
from pathlib import Path
from typing import Any, Literal
from anyio.streams.memory import MemoryObjectReceiveStream, MemoryObjectSendStream
from mcp import ClientSession, StdioServerParameters, Tool as MCPTool, stdio_client
from mcp.client.sse import sse_client
from mcp.client.streamable_http import GetSessionIdCallback, streamablehttp_client
from mcp.shared.message import SessionMessage
from mcp.types import CallToolResult, InitializeResult
from typing_extensions import NotRequired, TypedDict
from ..exceptions import UserError
from ..logger import logger
class MCPServer(abc.ABC):
"""Base class for Model Context Protocol servers."""
@abc.abstractmethod
async def connect(self):
"""Connect to the server. For example, this might mean spawning a subprocess or
opening a network connection. The server is expected to remain connected until
`cleanup()` is called.
"""
pass
@property
@abc.abstractmethod
def name(self) -> str:
"""A readable name for the server."""
pass
@abc.abstractmethod
async def cleanup(self):
"""Cleanup the server. For example, this might mean closing a subprocess or
closing a network connection.
"""
pass
@abc.abstractmethod
async def list_tools(self) -> list[MCPTool]:
"""List the tools available on the server."""
pass
@abc.abstractmethod
async def call_tool(self, tool_name: str, arguments: dict[str, Any] | None) -> CallToolResult:
"""Invoke a tool on the server."""
pass
class _MCPServerWithClientSession(MCPServer, abc.ABC):
"""Base class for MCP servers that use a `ClientSession` to communicate with the server."""
def __init__(self, cache_tools_list: bool, client_session_timeout_seconds: float | None):
"""
Args:
cache_tools_list: Whether to cache the tools list. If `True`, the tools list will be
cached and only fetched from the server once. If `False`, the tools list will be
fetched from the server on each call to `list_tools()`. The cache can be invalidated
by calling `invalidate_tools_cache()`. You should set this to `True` if you know the
server will not change its tools list, because it can drastically improve latency
(by avoiding a round-trip to the server every time).
client_session_timeout_seconds: the read timeout passed to the MCP ClientSession.
"""
self.session: ClientSession | None = None
self.exit_stack: AsyncExitStack = AsyncExitStack()
self._cleanup_lock: asyncio.Lock = asyncio.Lock()
self.cache_tools_list = cache_tools_list
self.server_initialize_result: InitializeResult | None = None
self.client_session_timeout_seconds = client_session_timeout_seconds
# The cache is always dirty at startup, so that we fetch tools at least once
self._cache_dirty = True
self._tools_list: list[MCPTool] | None = None
@abc.abstractmethod
def create_streams(
self,
) -> AbstractAsyncContextManager[
tuple[
MemoryObjectReceiveStream[SessionMessage | Exception],
MemoryObjectSendStream[SessionMessage],
GetSessionIdCallback | None,
]
]:
"""Create the streams for the server."""
pass
async def __aenter__(self):
await self.connect()
return self
async def __aexit__(self, exc_type, exc_value, traceback):
await self.cleanup()
def invalidate_tools_cache(self):
"""Invalidate the tools cache."""
self._cache_dirty = True
async def connect(self):
"""Connect to the server."""
try:
transport = await self.exit_stack.enter_async_context(self.create_streams())
# streamablehttp_client returns (read, write, get_session_id)
# sse_client returns (read, write)
read, write, *_ = transport
session = await self.exit_stack.enter_async_context(
ClientSession(
read,
write,
timedelta(seconds=self.client_session_timeout_seconds)
if self.client_session_timeout_seconds
else None,
)
)
server_result = await session.initialize()
self.server_initialize_result = server_result
self.session = session
except Exception as e:
logger.error(f"Error initializing MCP server: {e}")
await self.cleanup()
raise
async def list_tools(self) -> list[MCPTool]:
"""List the tools available on the server."""
if not self.session:
raise UserError("Server not initialized. Make sure you call `connect()` first.")
# Return from cache if caching is enabled, we have tools, and the cache is not dirty
if self.cache_tools_list and not self._cache_dirty and self._tools_list:
return self._tools_list
# Reset the cache dirty to False
self._cache_dirty = False
# Fetch the tools from the server
self._tools_list = (await self.session.list_tools()).tools
return self._tools_list
async def call_tool(self, tool_name: str, arguments: dict[str, Any] | None) -> CallToolResult:
"""Invoke a tool on the server."""
if not self.session:
raise UserError("Server not initialized. Make sure you call `connect()` first.")
return await self.session.call_tool(tool_name, arguments)
async def cleanup(self):
"""Cleanup the server."""
async with self._cleanup_lock:
try:
await self.exit_stack.aclose()
except Exception as e:
logger.error(f"Error cleaning up server: {e}")
finally:
self.session = None
class MCPServerStdioParams(TypedDict):
"""Mirrors `mcp.client.stdio.StdioServerParameters`, but lets you pass params without another
import.
"""
command: str
"""The executable to run to start the server. For example, `python` or `node`."""
args: NotRequired[list[str]]
"""Command line args to pass to the `command` executable. For example, `['foo.py']` or
`['server.js', '--port', '8080']`."""
env: NotRequired[dict[str, str]]
"""The environment variables to set for the server. ."""
cwd: NotRequired[str | Path]
"""The working directory to use when spawning the process."""
encoding: NotRequired[str]
"""The text encoding used when sending/receiving messages to the server. Defaults to `utf-8`."""
encoding_error_handler: NotRequired[Literal["strict", "ignore", "replace"]]
"""The text encoding error handler. Defaults to `strict`.
See https://docs.python.org/3/library/codecs.html#codec-base-classes for
explanations of possible values.
"""
class MCPServerStdio(_MCPServerWithClientSession):
"""MCP server implementation that uses the stdio transport. See the [spec]
(https://spec.modelcontextprotocol.io/specification/2024-11-05/basic/transports/#stdio) for
details.
"""
def __init__(
self,
params: MCPServerStdioParams,
cache_tools_list: bool = False,
name: str | None = None,
client_session_timeout_seconds: float | None = 5,
):
"""Create a new MCP server based on the stdio transport.
Args:
params: The params that configure the server. This includes the command to run to
start the server, the args to pass to the command, the environment variables to
set for the server, the working directory to use when spawning the process, and
the text encoding used when sending/receiving messages to the server.
cache_tools_list: Whether to cache the tools list. If `True`, the tools list will be
cached and only fetched from the server once. If `False`, the tools list will be
fetched from the server on each call to `list_tools()`. The cache can be
invalidated by calling `invalidate_tools_cache()`. You should set this to `True`
if you know the server will not change its tools list, because it can drastically
improve latency (by avoiding a round-trip to the server every time).
name: A readable name for the server. If not provided, we'll create one from the
command.
client_session_timeout_seconds: the read timeout passed to the MCP ClientSession.
"""
super().__init__(cache_tools_list, client_session_timeout_seconds)
self.params = StdioServerParameters(
command=params["command"],
args=params.get("args", []),
env=params.get("env"),
cwd=params.get("cwd"),
encoding=params.get("encoding", "utf-8"),
encoding_error_handler=params.get("encoding_error_handler", "strict"),
)
self._name = name or f"stdio: {self.params.command}"
def create_streams(
self,
) -> AbstractAsyncContextManager[
tuple[
MemoryObjectReceiveStream[SessionMessage | Exception],
MemoryObjectSendStream[SessionMessage],
GetSessionIdCallback | None,
]
]:
"""Create the streams for the server."""
return stdio_client(self.params)
@property
def name(self) -> str:
"""A readable name for the server."""
return self._name
class MCPServerSseParams(TypedDict):
"""Mirrors the params in`mcp.client.sse.sse_client`."""
url: str
"""The URL of the server."""
headers: NotRequired[dict[str, str]]
"""The headers to send to the server."""
timeout: NotRequired[float]
"""The timeout for the HTTP request. Defaults to 5 seconds."""
sse_read_timeout: NotRequired[float]
"""The timeout for the SSE connection, in seconds. Defaults to 5 minutes."""
class MCPServerSse(_MCPServerWithClientSession):
"""MCP server implementation that uses the HTTP with SSE transport. See the [spec]
(https://spec.modelcontextprotocol.io/specification/2024-11-05/basic/transports/#http-with-sse)
for details.
"""
def __init__(
self,
params: MCPServerSseParams,
cache_tools_list: bool = False,
name: str | None = None,
client_session_timeout_seconds: float | None = 5,
):
"""Create a new MCP server based on the HTTP with SSE transport.
Args:
params: The params that configure the server. This includes the URL of the server,
the headers to send to the server, the timeout for the HTTP request, and the
timeout for the SSE connection.
cache_tools_list: Whether to cache the tools list. If `True`, the tools list will be
cached and only fetched from the server once. If `False`, the tools list will be
fetched from the server on each call to `list_tools()`. The cache can be
invalidated by calling `invalidate_tools_cache()`. You should set this to `True`
if you know the server will not change its tools list, because it can drastically
improve latency (by avoiding a round-trip to the server every time).
name: A readable name for the server. If not provided, we'll create one from the
URL.
client_session_timeout_seconds: the read timeout passed to the MCP ClientSession.
"""
super().__init__(cache_tools_list, client_session_timeout_seconds)
self.params = params
self._name = name or f"sse: {self.params['url']}"
def create_streams(
self,
) -> AbstractAsyncContextManager[
tuple[
MemoryObjectReceiveStream[SessionMessage | Exception],
MemoryObjectSendStream[SessionMessage],
GetSessionIdCallback | None,
]
]:
"""Create the streams for the server."""
return sse_client(
url=self.params["url"],
headers=self.params.get("headers", None),
timeout=self.params.get("timeout", 5),
sse_read_timeout=self.params.get("sse_read_timeout", 60 * 5),
)
@property
def name(self) -> str:
"""A readable name for the server."""
return self._name
class MCPServerStreamableHttpParams(TypedDict):
"""Mirrors the params in`mcp.client.streamable_http.streamablehttp_client`."""
url: str
"""The URL of the server."""
headers: NotRequired[dict[str, str]]
"""The headers to send to the server."""
timeout: NotRequired[timedelta | float]
"""The timeout for the HTTP request. Defaults to 5 seconds."""
sse_read_timeout: NotRequired[timedelta | float]
"""The timeout for the SSE connection, in seconds. Defaults to 5 minutes."""
terminate_on_close: NotRequired[bool]
"""Terminate on close"""
class MCPServerStreamableHttp(_MCPServerWithClientSession):
"""MCP server implementation that uses the Streamable HTTP transport. See the [spec]
(https://modelcontextprotocol.io/specification/2025-03-26/basic/transports#streamable-http)
for details.
"""
def __init__(
self,
params: MCPServerStreamableHttpParams,
cache_tools_list: bool = False,
name: str | None = None,
client_session_timeout_seconds: float | None = 5,
):
"""Create a new MCP server based on the Streamable HTTP transport.
Args:
params: The params that configure the server. This includes the URL of the server,
the headers to send to the server, the timeout for the HTTP request, and the
timeout for the Streamable HTTP connection and whether we need to
terminate on close.
cache_tools_list: Whether to cache the tools list. If `True`, the tools list will be
cached and only fetched from the server once. If `False`, the tools list will be
fetched from the server on each call to `list_tools()`. The cache can be
invalidated by calling `invalidate_tools_cache()`. You should set this to `True`
if you know the server will not change its tools list, because it can drastically
improve latency (by avoiding a round-trip to the server every time).
name: A readable name for the server. If not provided, we'll create one from the
URL.
client_session_timeout_seconds: the read timeout passed to the MCP ClientSession.
"""
super().__init__(cache_tools_list, client_session_timeout_seconds)
self.params = params
self._name = name or f"streamable_http: {self.params['url']}"
def create_streams(
self,
) -> AbstractAsyncContextManager[
tuple[
MemoryObjectReceiveStream[SessionMessage | Exception],
MemoryObjectSendStream[SessionMessage],
GetSessionIdCallback | None,
]
]:
"""Create the streams for the server."""
return streamablehttp_client(
url=self.params["url"],
headers=self.params.get("headers", None),
timeout=self.params.get("timeout", 5),
sse_read_timeout=self.params.get("sse_read_timeout", 60 * 5),
terminate_on_close=self.params.get("terminate_on_close", True),
)
@property
def name(self) -> str:
"""A readable name for the server."""
return self._name
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