DM-55493: cap fastapi below 0.140 (faststream incompatibility) - #299
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FastAPI 0.140 made Dependant a slotted dataclass, which breaks FastStream's FastAPI plugin (it monkey-patches model, custom_fields and flat_params onto the instance). Upstream bug ag2ai/faststream#2959 is still open, so cap fastapi until it is fixed. The lockfile already resolved fastapi 0.139.2; relocking with plain 'uv lock' records the constraint without moving any package.
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Cap FastAPI at
<0.140in[project] dependenciesso a futuremake updatecannot silently bake a broken FastAPI into the lockfile.Why
FastAPI 0.140.0 (2026-07-24, fastapi/fastapi#16049 "Reduce memory usage in dependencies") changed
Dependantto@dataclass(slots=True). FastStream's FastAPI plugin monkey-patchesdependant.model,.custom_fieldsand.flat_paramsonto that instance (faststream/_internal/fastapi/get_dependant.py:131,135,136), which now raises:Upstream bug: ag2ai/faststream#2959 — open and unfixed on main (note the repo moved from
airtai/toag2ai/). faststream 0.7.2 is the latest release and is still broken; downgrading does not help either, since 0.6.7 carries the same patch.Ook depends on FastStream both directly (
faststream[kafka]) and viasafir[db,kafka].Impact today
The committed
uv.lockstill pins a pre-break FastAPI (0.139.2), so production and per-PR CI are green. Only the scheduled Periodic CI job — which resolves dependencies unpinned — is red. This PR is preventative: without the cap, the nextmake updatewould resolve to 0.140.x and ship a broken image.Changes
pyproject.toml:"fastapi"->"fastapi<0.140", with a# TODO: remove when ag2ai/faststream#2959 is fixedcomment.uv.lock: relocked with plainuv lock(deliberately not--upgrade). The only change is the constraint metadata:The resolved fastapi version is unchanged at 0.139.2. No other package moved.
Changelog fragment under
changelog.d/.Testing
nox -s typing testpasses locally:typing: mypy clean — "Success: no issues found in 202 source files".test: 451 passed, 1 skipped in 464.61s.Note this run exercises the locked FastAPI 0.139.2, so it confirms the cap breaks nothing — it does not itself reproduce the 0.140 failure.
Follow-up
Remove the cap once ag2ai/faststream#2959 ships a fix.