httpx 0.27.0: replacing `app=app` with

The Fix

Upgrade to version 0.27.1 or later.

Based on closed encode/httpx issue #3111 · 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

@@ -6,6 +6,10 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 ## Unreleased
 
+### Fixed
+
+* Fix `app` type signature in `ASGITransport`. (#3109)

repro.py

@pytest_asyncio.fixture
 async def deprecated_client(deprecated_db_manager: MainDbManager) -> AsyncGenerator[AsyncClient, None]:
     async with _make_mocked_test_app(db_manager=deprecated_db_manager, authorization=False) as app:
-        async with AsyncClient(app=app, base_url=URL, headers=USER_AGENT) as ac:
+        async with AsyncClient(transport=ASGITransport(app), base_url=URL, headers=USER_AGENT) as ac:
             yield ac

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\nUpgrade to version 0.27.1 or later.\nWhen NOT to use: Do not use this fix if your application relies on the previous type signature.\n\n

Why This Fix Works in Production

Trigger: 0.27.0: replacing `app=app` with `transport=ASGITransport(app)` doesn't type check
Mechanism: The type signature of the `app` parameter in `ASGITransport` was incompatible with FastAPI
Why the fix works: Modifies the type signature of the `app` parameter in `ASGITransport` to use more permissive types, resolving type checking issues with FastAPI. (first fixed release: 0.27.1).

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

The type signature of the `app` parameter in `ASGITransport` was incompatible with FastAPI
Production symptom (often without a traceback): 0.27.0: replacing `app=app` with `transport=ASGITransport(app)` doesn't type check

Proof / Evidence

GitHub issue: #3111
Fix PR: https://github.com/encode/httpx/pull/3109
First fixed release: 0.27.1
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.61

Discussion

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

Jump to Sources Open on GitHub

“% poetry run mypy . tests/conftest.py:64: error: Argument 1 to "ASGITransport" has incompatible type "FastAPI"; expected "Callable[[dict[str, Any], Callable[[], Awaitable[dict[str, Any]]], Callable[[dict[str, Any]], Coroutine[None, None, No”

Issue thread · issue description · source

Failure Signature (Search String)

0.27.0: replacing `app=app` with `transport=ASGITransport(app)` doesn't type check
Found 1 error in 1 file (checked 472 source files)

Copy-friendly signature

signature.txt

Failure Signature
-----------------
0.27.0: replacing `app=app` with `transport=ASGITransport(app)` doesn't type check
Found 1 error in 1 file (checked 472 source files)

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
0.27.0: replacing `app=app` with `transport=ASGITransport(app)` doesn't type check
Found 1 error in 1 file (checked 472 source files)

Minimal Reproduction

repro.py

@pytest_asyncio.fixture
 async def deprecated_client(deprecated_db_manager: MainDbManager) -> AsyncGenerator[AsyncClient, None]:
     async with _make_mocked_test_app(db_manager=deprecated_db_manager, authorization=False) as app:
-        async with AsyncClient(app=app, base_url=URL, headers=USER_AGENT) as ac:
+        async with AsyncClient(transport=ASGITransport(app), base_url=URL, headers=USER_AGENT) as ac:
             yield ac

What Broke

Type checking errors occurred when using FastAPI with the AsyncClient.

Why It Broke

The type signature of the `app` parameter in `ASGITransport` was incompatible with FastAPI

Fix Options (Details)

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

Upgrade to version 0.27.1 or later.

When NOT to use: Do not use this fix if your application relies on the previous type signature.

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

Fix reference: https://github.com/encode/httpx/pull/3109

First fixed release: 0.27.1

Last verified: 2026-02-09. Validate in your environment.

When NOT to Use This Fix

Do not use this fix if your application relies on the previous type signature.

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

Add a CI check that diffs key outputs after upgrades (OpenAPI schema snapshots, JSON payload shapes, CLI output).
Upgrade behind a canary and run integration tests against the canary before 100% rollout.

Version Compatibility Table

Version	Status
0.27.1	Fixed

Related Issues

No related fixes found.

Cluster: httpx:configuration-error All hubs All clusters

Related clusters: Data consistency Other Timeout

Sources

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

httpx 0.27.0: replacing `app=app` with `transport=ASGITransport(app)` doesn't type check (Fix)