Bug: When integrating with FastAPI, Depends cannot be overridden

[Horrih]

Describe the bug
Hello,
First of all thank you for your work on faststream, its integration works really well despite the project's young age.

This ticket looks like a duplicate of #2372 but I don't have the rights to reopen it.
I saw this comment #2372 (comment) but could not make it work

Fastapi dependencies work fine, but cannot be overridden.

This is most probably a skill issue on my part, I'd be really grateful for any tip about this issue.

How to reproduce
Here is a minimalist example I tried, I also tried using FastDepends overriding mechanisms but could not make it work

import pytest
import typing
import fastapi
import faststream
from pydantic import BaseModel
from faststream.confluent.fastapi import KafkaRouter
from faststream.confluent import TestKafkaBroker


class ProcessDataCommand(BaseModel):
    user_id: int


router = KafkaRouter("localhost:9092")
app = fastapi.FastAPI()


def my_dependency():
    return "PROD_VERSION"


@router.subscriber("process-data-topic")
async def process_data_handler(
    msg: ProcessDataCommand,
    value: typing.Annotated[
        str,
        fastapi.Depends(my_dependency), # Is not overridden
        faststream.Depends(my_dependency),  # Could not override it either
    ],
):
    print(f"Received message for user {msg.user_id} with my_dependency={value}")


app.include_router(router)


@pytest.mark.asyncio
async def test_process_data_handler_with_mocked_db():
    app.dependency_overrides[my_dependency] = lambda: "TEST_VERSION"
    async with TestKafkaBroker(router.broker) as tester:
        await tester.publish({"user_id": 42}, topic="process-data-topic")

1. ...

Expected behavior
I expect to be able to override a dependency for testing, either through fastapi's app.dependency_overrides, or FastDepends provider.scope() or equivalent.

Observed behavior
The above snippet always shows the prod message string, not the overridden test one

Screenshots
If applicable, attach screenshots to help illustrate the problem.

Environment
Linux RHEL9 , using uv with "faststream[confluent]>=0.7.1",

[Lancetnik]

Hi @Horrih, thanks for the kind words and the clear repro! 🙏

This is a usage issue rather than a bug — and a subtle one, so no shame at all.

The key detail: app.dependency_overrides only takes effect once the FastAPI app's lifespan runs. FastStream doesn't read the overrides directly from the app — it reads them from a provider that gets wired to your app inside the app's lifespan startup. In your snippet only the broker is started (async with TestKafkaBroker(router.broker)), but the FastAPI app is never started, so the override is never wired up. That's why injection works but the override is silently ignored.

The fix is to also enter the app via TestClient, which runs the lifespan:

from fastapi.testclient import TestClient

@pytest.mark.asyncio
async def test_process_data_handler_with_mocked_db():
    app.dependency_overrides[my_dependency] = lambda: "TEST_VERSION"
    async with TestKafkaBroker(router.broker) as tester:
        with TestClient(app):  # <-- runs the app lifespan, wires up dependency_overrides
            await tester.publish({"user_id": 42}, topic="process-data-topic")

With that, value resolves to "TEST_VERSION". I verified this against faststream[confluent] with your exact example.

Two side notes:

• In Annotated[str, fastapi.Depends(...), faststream.Depends(...)], the faststream.Depends is ignored in FastAPI mode — the fastapi.Depends wins. So you only need to override the fastapi.Depends one (which the snippet above does).
• If you'd rather not enter TestClient, you can instead wire the provider at construction time: KafkaRouter("localhost:9092", dependency_overrides_provider=app). Then app.dependency_overrides works without starting the app lifespan.

Hope that unblocks you!

[Horrih]

Thanks a lot, this works flawlessly.
You can fly away captain !
Cheers