FastMCP 4.0.3 fixes a bug that sent images twice

FastMCP released version 4.0.3 on September 5, and the fix list says more about how the framework was misbehaving than the version bump does. Tools that return an unconstrained sequence, a common shape for image output, were sending that content twice on the wire. Anyone running one of those tools was paying double the bandwidth on every call, with downstream code getting two images where it expected one.

The bug traces to schema inference. When a tool returned an unconstrained sequence, FastMCP tried to infer an output schema for it, and that inference caused the duplication. PR #4999, detailed in the 4.0.3 release notes, stops FastMCP from inferring a schema in that case, which stops the duplicate images.

Three other fixes are in the same release. Multi-server clients talking to legacy-only backends retried startup when they shouldn't have. FastMCP serialized a task timing field in a way strict clients rejected. Monty callbacks were also left unfinished at the end of execution, though the notes don't say what those callbacks do.

This is a PrefectHQ repo now, not jlowin's original. If you've got FastMCP pinned to an old fork path or cached the wrong remote, that's a separate problem 4.0.3 won't fix for you.