The Fix
Upgrade to version 0.13.0 or later.
Based on closed encode/httpx issue #879 · 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%.
@@ -38,6 +38,13 @@
:members: headers cookies params request get head options post put patch delete build_request send close
+## `AsyncClient`
+
+::: httpx.AsyncClient
repro.py
import httpx
from fastapi import FastAPI
app = FastAPI()
@app.get("/users/{user_id}")
async def read_user(user_id: str):
return {'user_id': user_id}
with httpx.Client(app=app) as client:
response = client.get("http://www.example.com/users/foo")
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 0.13.0 or later.\nWhen NOT to use: This fix is not applicable if you intend to use an ASGI app with the sync client.\n\n
Why This Fix Works in Production
- Trigger: response = client.get("http://www.example.com/users/foo")
- Mechanism: The sync client incorrectly documented support for ASGI apps instead of WSGI apps
- Why the fix works: Updated the Client docstring to clarify that it takes a WSGI app instead of an ASGI app, and added the AsyncClient API to the documentation. (first fixed release: 0.13.0).
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 3.7 in real deployments (not just unit tests).
- The sync client incorrectly documented support for ASGI apps instead of WSGI apps
- Surfaces as: Traceback (most recent call last):
Proof / Evidence
- GitHub issue: #879
- Fix PR: https://github.com/encode/httpx/pull/883
- First fixed release: 0.13.0
- Reproduced locally: No (not executed)
- Last verified: 2026-02-09
- Confidence: 0.85
- Did this fix it?: Yes (upstream fix exists)
- Own content ratio: 0.45
Discussion
High-signal excerpts from the issue thread (symptoms, repros, edge-cases).
“I went ahead and updated the issue description”
“Hi Ed! This is actually expected behavior”
“Thanks for the quick response on this”
Failure Signature (Search String)
- response = client.get("http://www.example.com/users/foo")
Error Message
Stack trace
error.txt
Error Message
-------------
Traceback (most recent call last):
File "temp.py", line 12, in <module>
response = client.get("http://www.example.com/users/foo")
File "/Users/singletoned/.envs/testino/lib/python3.7/site-packages/httpx/_client.py", line 714, in get
timeout=timeout,
File "/Users/singletoned/.envs/testino/lib/python3.7/site-packages/httpx/_client.py", line 571, in request
request, auth=auth, allow_redirects=allow_redirects, timeout=timeout,
File "/Users/singletoned/.envs/testino/lib/python3.7/site-packages/httpx/_client.py", line 591, in send
request, auth=auth, timeout=timeout, allow_redirects=allow_redirects,
File "/Users/singletoned/.envs/testino/lib/python3.7/site-packages/httpx/_client.py", line 618, in send_handling_redirects
request, auth=auth, timeout=timeout, history=history
File "/Users/singletoned/.envs/testino/lib/python3.7/site-packages/httpx/_client.py", line 654, in send_handling_auth
response = self.send_single_request(request, timeout)
File "/Users/singletoned/.envs/testino/lib/python3.7/site-packages/httpx/_client.py", line 678, in send_single_request
response = dispatcher.send(request, timeout=timeout)
File "/Users/singletoned/.envs/testino/lib/python3.7/site-packages/httpx/_dispatch/wsgi.py", line 90, in send
result = self.app(environ, start_response)
TypeError: __call__() missing 1 required positional argument: 'send'
Minimal Reproduction
repro.py
import httpx
from fastapi import FastAPI
app = FastAPI()
@app.get("/users/{user_id}")
async def read_user(user_id: str):
return {'user_id': user_id}
with httpx.Client(app=app) as client:
response = client.get("http://www.example.com/users/foo")
Environment
- Python: 3.7
What Broke
Using an ASGI app with the sync client results in a TypeError during request handling.
Why It Broke
The sync client incorrectly documented support for ASGI apps instead of WSGI apps
Fix Options (Details)
Option A — Upgrade to fixed release Safe default (recommended)
Upgrade to version 0.13.0 or later.
When NOT to use: This fix is not applicable if you intend to use an ASGI app with the sync client.
Use when you can deploy the upstream fix. It is usually lower-risk than long-lived workarounds.
Option D — Guard side-effects with OnceOnly Guardrail for side-effects
Mitigate duplicate external side-effects under retries/timeouts/agent loops by gating the operation before calling external systems.
- Place OnceOnly between your code/agent and real side-effects (Stripe, emails, CRM, APIs).
- Use a stable key per side-effect (e.g., customer_id + action + idempotency_key).
- Fail-safe: configure fail-open vs fail-closed based on blast radius and spend risk.
Show example snippet (optional)
onceonly.py
from onceonly import OnceOnly
import os
once = OnceOnly(api_key=os.environ["ONCEONLY_API_KEY"], fail_open=True)
# Stable idempotency key per real side-effect.
# Use a request id / job id / webhook delivery id / Stripe event id, etc.
event_id = "evt_..." # replace
key = f"stripe:webhook:{event_id}"
res = once.check_lock(key=key, ttl=3600)
if res.duplicate:
return {"status": "already_processed"}
# Safe to execute the side-effect exactly once.
handle_event(event_id)
When NOT to use: Do not use this to hide logic bugs or data corruption. Use it to block duplicate external side-effects and enforce tool permissions/spend caps.
Fix reference: https://github.com/encode/httpx/pull/883
First fixed release: 0.13.0
Last verified: 2026-02-09. Validate in your environment.
When NOT to Use This Fix
- This fix is not applicable if you intend to use an ASGI app with the sync client.
- Do not use this to hide logic bugs or data corruption. Use it to block duplicate external side-effects and enforce tool permissions/spend caps.
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.
- Make timeouts explicit and test them (unit + integration) to avoid silent behavior changes.
- Instrument retries (attempt count + reason) and alert on spikes to catch dependency slowdowns.
Version Compatibility Table
| Version | Status |
|---|---|
| 0.13.0 | Fixed |
Related Issues
No related fixes found.
Sources
We don’t republish the full GitHub discussion text. Use the links above for context.