fastmcp

FastMCP (Python) by Prefect / Jlowin

The fast, Pythonic way to build MCP servers and clients.

Live Registry Versionv4.1.3Previous: v4.1.3Published 2026-10-20

Stability Grade

C+ (74/100)

90-Day Velocity

23 releases

High release cadence

Dependencies

9 direct

PYPI manifest

License

Apache-2.0

Verified on PYPI

Recommended Exact Pin: pip install "fastmcp==4.1.3"

PYPI Registry ->GitHub Repo ->

TL;DR Verdict & Quick Citation Summary (2026-10-24)Source: AgentBreakWatch Live PYPI Index

FastMCP (Python) (fastmcp) is currently at v4.1.3 on PYPI (published 2026-10-20), with a 90-day stability score of 74/100 (C+) and HIGH schema drift risk across 23 releases in the past 90 days. FastMCP v4.x introduces OpenAPI/MCP composition mounts, OAuth proxy providers, and strict output_schema serialization. Production teams should pin pip install "fastmcp==4.1.3" rather than using unpinned @latest tags.

Breaking Changes & Tool Schema Drift Analysis

FastMCP v4.x introduces OpenAPI/MCP composition mounts, OAuth proxy providers, and strict output_schema serialization.

Watched Exports / MCP Tool Signatures:

FastMCPContextClientmount

Before (Deprecated / Unpinned Pattern)

mcp.mount("sub", sub_server)

After (Verified v4.1.3 Pattern)

mcp.mount(sub_server, prefix="sub")

Frequently Asked Questions: fastmcp

What is the latest version of fastmcp on PYPI?

As of 2026-10-24, the latest stable version of fastmcp (FastMCP (Python) by Prefect / Jlowin) on PYPI is v4.1.3, published on 2026-10-20. It has shipped 23 releases in the last 90 days.

Does fastmcp v4.1.3 have breaking changes or MCP schema drift risk?

fastmcp currently carries a HIGH schema drift risk rating with a Stability Score of 74/100 (Grade C+). FastMCP v4.x introduces OpenAPI/MCP composition mounts, OAuth proxy providers, and strict output_schema serialization.

How do I safely pin fastmcp in production?

Avoid unpinned @latest tags in production agent configs. Pin the exact verified version using `pip install "fastmcp==4.1.3"` and monitor FastMCP, Context, Client, mount for parameter or schema changes.

30-Min Registry & Schema Poller