Max Isbey
fee8c52acc
The stdio driver derived a connection's protocol era from whichever request finished first, so it could never host a request that does not return: subscriptions/listen was hard-refused with -32601 even though the same connection advertised list-changed capabilities, and a legacy handshake arriving during a slow 2026 request was accepted and locked the connection out from under it. Decide the era from the client's first request instead, once, before any handler runs: a request carrying the 2026-07-28 per-request envelope opens a 2026 connection, anything else (the initialize handshake) opens a 2025 one, and the deciding frame is replayed into the chosen serving loop. A conflicting later claim is refused in one place (initialize on a 2026 connection gets -32022 naming the served versions; an enveloped request on a handshake connection gets -32600). The listen refusal is deleted; the handler was always transport-agnostic.
MCP Python SDK
Python implementation of the Model Context Protocol (MCP)
Caution
This README documents v2 of the MCP Python SDK — a pre-release (alpha/beta) line under active development. Do not use v2 in production. Pre-releases are published to PyPI as
2.0.0aN/2.0.0bN, and each pre-release may contain breaking changes from the previous one. Pin an exact version and expect to update your code when you bump the pin.v1.x is the only stable release line and remains recommended for production. It lives on the
v1.xbranch and continues to receive critical bug fixes and security patches; see the v1.x README for its documentation.pipanduvdon't select a pre-release unless you explicitly request one, so existing installs are unaffected. If your package depends onmcp, add a<2upper bound to your version constraint (for examplemcp>=1.27,<2) before the stable release lands.v2 is a major rework of the SDK, both to support the 2026-07-28 MCP specification release and to fix long-standing architectural issues. See What's new in v2 for the tour of what changed, and the migration guide for every breaking change. Stable v2 is targeted for 2026-07-27, alongside the spec release. Try the pre-releases and tell us what breaks, or discuss in #python-sdk-dev on the MCP Contributors Discord.
Documentation
The documentation lives at https://py.sdk.modelcontextprotocol.io/v2/.
It has a Get started guide, What's new in v2, the API reference, and the migration guide.
What is MCP?
The Model Context Protocol lets you build servers that expose data and functionality to LLM applications in a secure, standardized way. Think of it like a web API, but designed for LLM interactions. With this SDK you can:
- Build MCP servers that expose tools, resources, and prompts to any MCP host
- Build MCP clients that connect to any MCP server
- Speak every standard transport: stdio, Streamable HTTP, and SSE
Requirements
Python 3.10+.
Installation
uv add "mcp[cli]==2.0.0b1" # or: pip install "mcp[cli]==2.0.0b1"
The pin matters while v2 is in pre-release: an unpinned install resolves to the latest stable v1.x, which this README does not describe. Check PyPI for the newest pre-release, and use uv run --with "mcp==2.0.0b1" for one-off commands.
A server in 15 lines
Create a server.py:
from mcp.server import MCPServer
mcp = MCPServer("Demo")
@mcp.tool()
def add(a: int, b: int) -> int:
"""Add two numbers."""
return a + b
@mcp.resource("greeting://{name}")
def greeting(name: str) -> str:
"""Greet someone by name."""
return f"Hello, {name}!"
Full example: docs_src/index/tutorial001.py
That's a complete MCP server: one tool, one templated resource. Open it in the MCP Inspector:
uv run mcp dev server.py
Call add with a=1, b=2 and you get 3 back.
Notice what you did not write: no JSON Schema (a: int, b: int is the schema), no request parsing, no validation code, no protocol handling. Two type-hinted Python functions and a docstring.
Get started takes it from here.
A client in 10 lines
The same package is a full MCP client. Client connects to a URL, a stdio subprocess, a custom transport, or (for tests) straight to a server object in memory with no transport at all:
import asyncio
from mcp import Client
from server import mcp
async def main() -> None:
async with Client(mcp) as client:
result = await client.call_tool("add", {"a": 1, "b": 2})
print(result.structured_content) # {'result': 3}
asyncio.run(main())
Swap mcp for "http://localhost:8000/mcp" and the exact same code talks to a remote server.
Contributing
We are passionate about supporting contributors of all levels of experience and would love to see you get involved in the project. See the contributing guide to get started.
License
This project is licensed under the MIT License. See the LICENSE file for details.