FastAPI Bug with async callable class dependency in

The Fix

pip install fastapi==0.128.4

Based on closed fastapi/fastapi issue #14466 · PR/commit linked

Production note: Most teams hit this during upgrades or environment changes. Roll out with a canary and smoke critical endpoints (health, OpenAPI/docs) before 100%.

Jump to Verify Open PR/Commit

@@ -110,6 +110,8 @@ def is_gen_callable(self) -> bool:
         ) or inspect.isgeneratorfunction(_unwrapped_call(self.call)):
             return True
+        if inspect.isclass(_unwrapped_call(self.call)):
+            return False
         dunder_call = getattr(_impartial(self.call), "__call__", None)  # noqa: B004

repro.py

from typing import Annotated

from fastapi import Depends, FastAPI
from fastapi.dependencies.models import Dependant
from uvicorn import run


class Dependency:
    def __init__(self):
        print("init")

    async def __call__(self):
        print("call")


print(Dependant(Dependency).is_coroutine_callable)
# `True` with fastapi>=0.123.6
# `False` with fastapi<0.123.6

app = FastAPI()


@app.get("/")
async def route(*, dependency: Annotated[Dependency, Depends()]):
    await dependency()


run(app)

# then `curl localhost:8000`

verify

Re-run the minimal reproduction on your broken version, then apply the fix and re-run.

fix.md

Option A — Upgrade to fixed release\npip install fastapi==0.128.4\nWhen NOT to use: Do not use this fix if the dependency is not a class with a __call__ method.\n\n

Why This Fix Works in Production

Trigger: solved = await call(**solved_result.values)
Mechanism: FastAPI incorrectly identifies class dependencies with a __call__ method as coroutine callables
Why the fix works: Fixed a regression in FastAPI 0.123.6 where class dependencies with a __call__ method were incorrectly identified as coroutine callables, causing a TypeError when awaited. (first fixed release: 0.128.4).

Production impact:

If left unfixed, the same config can fail only in production (env differences), causing startup failures or partial feature outages.

Why This Breaks in Prod

Shows up under Python 3.14 in real deployments (not just unit tests).
FastAPI incorrectly identifies class dependencies with a __call__ method as coroutine callables
Surfaces as: File ".venv/lib/python3.14/site-packages/fastapi/dependencies/utils.py", line 647, in solve_dependencies

Proof / Evidence

GitHub issue: #14466
Fix PR: https://github.com/fastapi/fastapi/pull/14458
First fixed release: 0.128.4
Reproduced locally: No (not executed)
Last verified: 2026-02-08
Confidence: 0.95
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.54

Discussion

High-signal excerpts from the issue thread (symptoms, repros, edge-cases).

Jump to Sources Open on GitHub

“Fixed by https://github.com/fastapi/fastapi/pull/14458”

@tiangolo · 2025-12-05 · source

Failure Signature (Search String)

solved = await call(**solved_result.values)

Error Message

Stack trace

error.txt

Error Message
-------------
File ".venv/lib/python3.14/site-packages/fastapi/dependencies/utils.py", line 647, in solve_dependencies
    solved = await call(**solved_result.values)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: 'Dependency' object can't be awaited

Minimal Reproduction

repro.py

from typing import Annotated

from fastapi import Depends, FastAPI
from fastapi.dependencies.models import Dependant
from uvicorn import run


class Dependency:
    def __init__(self):
        print("init")

    async def __call__(self):
        print("call")


print(Dependant(Dependency).is_coroutine_callable)
# `True` with fastapi>=0.123.6
# `False` with fastapi<0.123.6

app = FastAPI()


@app.get("/")
async def route(*, dependency: Annotated[Dependency, Depends()]):
    await dependency()


run(app)

# then `curl localhost:8000`

Environment

Python: 3.14

What Broke

TypeError occurs when trying to await an instance of a class dependency in FastAPI routes.

Why It Broke

FastAPI incorrectly identifies class dependencies with a __call__ method as coroutine callables

Fix Options (Details)

Option A — Upgrade to fixed release Safe default (recommended)

pip install fastapi==0.128.4

When NOT to use: Do not use this fix if the dependency is not a class with a __call__ method.

Use when you can deploy the upstream fix. It is usually lower-risk than long-lived workarounds.

Fix reference: https://github.com/fastapi/fastapi/pull/14458

First fixed release: 0.128.4

Last verified: 2026-02-08. Validate in your environment.

When NOT to Use This Fix

Do not use this fix if the dependency is not a class with a __call__ method.

Verify Fix

verify

Re-run the minimal reproduction on your broken version, then apply the fix and re-run.

Did This Fix Work in Your Case?

Quick signal helps us prioritize which fixes to verify and improve.

Prevention

Capture the exact failing error string in logs and tests so you can reproduce via a minimal script.
Pin production dependencies and upgrade only with a reproducible test that hits the failing path.

Version Compatibility Table

Version	Status
0.128.4	Fixed

Related Issues

No related fixes found.

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Related clusters: Data consistency Rate limit Race condition

Sources

We don’t republish the full GitHub discussion text. Use the links above for context.

FastAPI Bug with async callable class dependency in fastapi>=0.123.6 (Fix)