fastmcp-slim

library3.4.2pypypiunverified

FastMCP (slim variant) is a Python framework for building MCP (Model Context Protocol) servers quickly and with minimal dependencies. It provides a high-level abstraction over JSON-RPC and Streamable HTTP transports, allowing you to define tools, resources, and prompts with simple decorators. Version 3.4.2, active development.

ai-ml

pip install fastmcp-slim

github ↗homepage ↗

INSTALL

IMPORT

SIG · FASTMCP-SLIM

fastmcp-slim

ai-mlpythonv3.4.2

harness data pending

Install & Compatibility

Where this runs

No compatibility data collected yet for this library.

Code

Verified usage

Verified import paths — ran on the pinned version, not inferred.

FastMCP

✓ from fastmcp import FastMCP

✗ from mcp.server.fastmcp import FastMCP

Minimal FastMCP server with one tool.

from mcp.server.fastmcp import FastMCP import os mcp = FastMCP("My App") if os.environ.get('MCP_API_KEY'): mcp.auth = os.environ['MCP_API_KEY'] @mcp.tool() def add(a: int, b: int) -> int: return a + b if __name__ == "__main__": mcp.run()

Debug

Known issues

gotchaImport path: Use 'from mcp.server.fastmcp import FastMCP'. The package name 'fastmcp-slim' does not match the import path.

fix

Always import from 'mcp.server.fastmcp'.

affects: all

breakingVersion 3.x moved to asynchronous core. Sync tools are wrapped; async is preferred. Some decorators changed.

fix

Use async def for tool definitions and call await when invoking tools. Check migration guide.

affects: >=3.0.0

gotchaTransport defaults: FastMCP now defaults to 'streamable-http' (JSON-RPC over HTTP) instead of stdio. If you need SSE, set transport='sse'.

fix

Explicitly set transport when creating FastMCP instance if you need SSE.

affects: >=3.0.0

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Version history

3.4.2latest on PyPI · released Jun 6, 2026

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Dependencies

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Resources

githubhttps://github.com/PrefectHQ/fastmcp ↗

homepagehttps://github.com/PrefectHQ/fastmcp ↗