Max Isbey
98bd125ddd
The 2026-07-28 protocol did not work over stdio: subscriptions/listen was hard-refused, a legacy initialize arriving during an in-flight modern request was accepted and re-locked the connection, and a peer cancel produced a trailing "Request cancelled" frame. All three share one root cause: the connection's era was derived from which requests had completed instead of being decided once, in wire order, from how the client opened the connection. Replace serve_dual_era_loop and serve_loop with a single serve_stream driver that decides the era synchronously in the dispatcher's read loop, before the request body is spawned. initialize (or any envelope-less request) opens the legacy era, an enveloped request opens the modern era, server/discover is answered without pinning, and a stray leading notification opens nothing. A conflicting era claim on a committed connection is refused (-32022 or -32600) rather than silently switching. Cancel silence is structural: each request answers through a one-shot channel whose write target becomes a powerless void on a peer cancel, so there is no cancelled-check at any write site. The generic JSON-RPC dispatcher loses the code-0 cancel frame, the code-0 str(exc) catch-all, and the inline_methods knob, and documents that handlers are invoked synchronously in receive order with the returned awaitable as the body. Add a Posture enum (DUAL default, LEGACY_ONLY, MODERN_ONLY) on the Server and MCPServer constructors, honoured by the stream driver and the streamable-HTTP manager alike. Server.run(read, write) now stands alone, Server.lifespan() is a bound context manager, and serve_listener / newline_json_transport / close_subscriptions() give a straightforward path for custom transports. See docs/migration.md for the full list of observable changes.
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.