Build a Custom MCP Server in Python (MCPServer, formerly FastMCP)

uv init my-mcp && cd my-mcp
uv add "mcp[cli]>=2"

from mcp.server.mcpserver import MCPServer  # mcp>=2; on mcp<2: from mcp.server.fastmcp import FastMCP

mcp = MCPServer("notes")
NOTES: dict[str, str] = {}

@mcp.tool()
def add_note(title: str, body: str) -> str:
    """Save a note under a title."""
    NOTES[title] = body
    return f"Saved '{title}'"

@mcp.tool()
def search_notes(query: str) -> list[str]:
    """Return titles of notes containing the query."""
    return [t for t, b in NOTES.items() if query.lower() in (t + b).lower()]

@mcp.resource("notes://{title}")
def read_note(title: str) -> str:
    return NOTES.get(title, "Not found")

if __name__ == "__main__":
    mcp.run()  # stdio transport by default

uv run mcp dev server.py      # opens the MCP Inspector

{"mcpServers": {"notes": {"command": "uv", "args": ["--directory", "/abs/path/my-mcp", "run", "server.py"]}}}