The last successful Prefect test run used FastAPI

0.137.1

. Why I’m confident: - Prefect’s CI installs dependencies with

uv sync --locked

- The repo’s

uv.lock

pins

fastapi

to

0.137.1

- The latest successful run was on a commit where that lockfile already contained

0.137.1

Relevant repo files: - `uv.lock` - `pyproject.toml` Important nuance: -

pyproject.toml

allows a range like

fastapi>=0.111.0,<1.0.0

- But the actual test run version comes from the lockfile, which resolves that to

0.137.1

If you want, I can also help identify which workflow/run that was and point to the exact GitHub Actions run page.

Yes — there were recent commits directly related to FastAPI

0.137.0/0.137.1

. Direct compatibility fixes 1.

254498fac2cc324bb152c9c6ba3f82b1c44e257b

- First fix attempt for FastAPI

0.137.0

compatibility - Adjusted Prefect’s router-handling logic in

src/prefect/server/api/server.py

- This addressed a breaking change in FastAPI’s

include_router

behavior 2.

336bb4900a3bbca169ca3582aeeff9d58976a51c

- Follow-up / refined fix, associated with PR

#22277

- Replaced a more fragile feature-detection approach with explicit FastAPI version comparison - Also in

src/prefect/server/api/server.py

- This looks like the final compatibility fix for

0.137.0+

Dependency bump 3.

b0ae1da3bf55b4131e41eb0fa30df115dfb09452

- Associated with PR

#22281

- Bumped FastAPI from

0.136.3

to

0.137.1

- This was primarily a dependency update, not the original compatibility fix - It also included some test changes, but those appear to be broader test maintenance rather than the core FastAPI compatibility logic Bottom line - Yes, there were explicit compatibility commits for FastAPI

0.137.0

. - Then Prefect bumped to

0.137.1

after those fixes were in place. - So the sequence was basically: 1. detect breakage with

0.137.0

2. add compatibility fix 3. refine the fix 4. update dependency to

0.137.1

If you want, I can also summarize exactly what changed in

server.py

and why FastAPI

0.137

broke the previous Prefect behavior.

Yes — but only in a dev release on PyPI right now, not the latest stable release. First Prefect release containing both fixes -

3.7.5.dev4

That dev release includes: - the FastAPI

0.137.0+

compatibility fix:

336bb4900a3bbca169ca3582aeeff9d58976a51c

- the FastAPI bump to `0.137.1`:

b0ae1da3bf55b4131e41eb0fa30df115dfb09452

PyPI status -

prefect==3.7.5.dev4

→ available on PyPI -

prefect==3.7.4

→ latest stable at the time of this check, but does not include those commits - stable

3.7.5

→ not yet published at the time of this check So if you want those fixes now, you need either: 1. The dev release on PyPI

bash
pip install prefect==3.7.5.dev4

2. A Git checkout / Git install at or after: -

b0ae1da3bf55b4131e41eb0fa30df115dfb09452

For example:

bash
pip install git+<https://github.com/PrefectHQ/prefect.git@b0ae1da3bf55b4131e41eb0fa30df115dfb09452>

Or just use `main`:

bash
pip install git+<https://github.com/PrefectHQ/prefect.git@main>

Recommendation - For testing the fix quickly: use

3.7.5.dev4

- For production: I’d generally wait for the stable

3.7.5

release unless you specifically need the FastAPI compatibility fix immediately Useful links: - Prefect 3.7.5.dev4 on PyPI - Compatibility fix commit - FastAPI 0.137.1 bump commit If you want, I can also tell you the smallest possible git ref/tag to pin to, or help confirm whether a newer stable release has gone out since this check.