# FastMCP (Python) (fastmcp) - AgentBreakWatch Reference

> TL;DR Verdict: 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 last 90 days. FastMCP v4.x introduces OpenAPI/MCP composition mounts, OAuth proxy providers, and strict output_schema serialization.

## Live Telemetry Metadata
- Canonical Page: https://agentbreakwatch.vercel.app/p/fastmcp-python
- Category Hub: https://agentbreakwatch.vercel.app/c/agent-sdks (AI Agent SDKs & Frameworks Breaking-Change Index)
- Package Name: `fastmcp`
- Registry: PYPI (https://pypi.org/project/fastmcp/)
- GitHub Repository: https://github.com/jlowin/fastmcp
- Vendor: Prefect / Jlowin
- Latest Live Version: v4.1.3
- Published Date: 2026-10-20T22:44:33.334457Z
- 90-Day Release Count: 23
- Direct Dependencies: 9
- License: Apache-2.0
- Stability Score: 74/100 (Grade C+)
- Schema Drift Risk: HIGH
- Recommended Exact Pin Command: `pip install "fastmcp==4.1.3"`
- Watched Symbols / MCP Tools: FastMCP, Context, Client, mount
- Snapshot Timestamp: 2026-10-24T12:11:27.697Z

## Breaking Change & Schema Drift Summary
FastMCP v4.x introduces OpenAPI/MCP composition mounts, OAuth proxy providers, and strict output_schema serialization.

## Migration Diff: Before (Deprecated / Unpinned Pattern)
```
mcp.mount("sub", sub_server)
```

## Migration Diff: After (Verified v4.1.3 Pattern)
```
mcp.mount(sub_server, prefix="sub")
```

## Recent Verified Releases
- v3.4.11 (published 2026-10-20) [LATEST]
- v4.1.3 (published 2026-10-20)
- v3.4.10 (published 2026-10-20)
- v4.1.2 (published 2026-10-20)
- v4.1.1 (published 2026-10-18)
- v3.4.9 (published 2026-10-16)