Build a minimal MCP server with tools using the official Python SDK (v2) and run it over stdio

Documented steps

1. Install with the CLI extra: uv add "mcp[cli]" or pip install "mcp[cli]" (Python 3.10+). The cli extra provides the mcp command (mcp dev / mcp run / mcp install); plain mcp is enough for a deployed server. Docs: https://raw.githubusercontent.com/modelcontextprotocol/python-sdk/main/README.md
2. Import the v2 server class: from mcp.server import MCPServer, then mcp = MCPServer("YourServerName").
3. Define a tool with the decorator: @mcp.tool() on a plain function, e.g. def add(a: int, b: int) -> int with a docstring — tool name comes from the function name, description from the docstring, JSON Schema from the type hints; no manual schema.
4. Add if __name__ == "__main__": mcp.run() — run() is synchronous, blocks for the server's life, and defaults to the stdio transport with no arguments.
5. Test interactively: uv run mcp dev server.py launches MCP Inspector (needs npx on PATH) with a Tools tab; uv run mcp run server.py runs it directly.
6. For fast in-memory tests without a subprocess: from mcp import Client; async with Client(mcp) as client: await client.call_tool("add", {"a": 1, "b": 2}) — the SDK's own test pattern.