Max Isbey
ecafdc78d6
The goal is that migrated v1 code runs on v2 on its legacy paths, not that it adopts v2 idioms. Applying that bar: - Leave e.error.code / .message / .data chains alone: v2's MCPError keeps a typed .error ErrorData, so the v1 spelling runs and type-checks unchanged. The except-binding tracking goes with it. - Rewrite one-argument McpError(...) calls to MCPError.from_error_data(...) instead of flattening the inline ErrorData: the user's expression is kept as written and the non-inline form no longer needs a marker. - Convert v1 positional arguments on the lowlevel Server constructor to keywords (v2 is keyword-only after name but kept v1's names and order), pinned against the installed signature by a new ratchet test. - Reword every marker message that pointed at replaced internals or at the successor of the removed experimental tasks API; state removals plainly instead of steering users onto new surfaces. - Teach the batch harness that a reportArgumentType error naming a detonating argument type (timedelta, AnyUrl) is a real break, never v2 strictness drift, and ignore stale work/ directories.
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 the migration guide for what's changed;
uvx mcp-codemod v1-to-v2 ./srcautomates the mechanical half of it and marks the rest with# mcp-codemod:comments. 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 the full tutorial, 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.
The tutorial 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.