FastMCP compatibility

The dependency is declared as fastmcp>=2.12.2,<5. The ceiling moves up once a new major version passes the test suite.

FastMCP	Status
2.12.2	Tested in CI on every push, on SQLite and PostgreSQL
2.14.3	Tested in CI on every push
3.0.2, 3.3.1, 3.4.7	Tested before release; CI runs the newest 3.x (>=3.3,<4)
4.0.10	Tested before release; CI runs the newest 4.x, on SQLite and PostgreSQL
5.x	Not supported yet. A non-blocking CI job runs the suite against the newest FastMCP with no ceiling, to show what breaks before users find out.

CI also runs weekly, so a new FastMCP release inside the range is tested even when nothing is pushed.

Python 3.11 or newer is required. CI runs Python 3.13; the examples on this site were run on Python 3.12 with FastMCP 4.0.10.

Differences between FastMCP versions

The package hides these, but they explain some choices:

• Composition. import_server() became async in FastMCP 2.14 and was removed in 4. Use mount(server, "prefix"), which works on every supported version.
• HTTP client. FastMCP 2 and 3 depend on httpx, FastMCP 4 on httpx2. OpenAIEmbedder uses whichever is installed.
• MCP errors. From FastMCP 3.4, a result marked isError keeps the flag on the tool result rather than in its payload; the classifier sees it either way.
• Sessions. Clients on MCP protocol 2026-07-28, including FastMCP 4’s own Client, open no session over HTTP. See How correlation works.