The Fix
Upgrade to version 0.13.0 or later.
Based on closed encode/httpx issue #557 · 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%.
@@ -227,11 +227,10 @@ def semaphore(self) -> typing.Optional[asyncio.BoundedSemaphore]:
return self._semaphore
- async def acquire(self) -> None:
+ async def acquire(self, timeout: float = None) -> None:
if self.semaphore is None:
repro.py
from quart import Quart, request
app = Quart(__name__)
@app.route("/", methods=["POST"])
async def post():
data = await request.get_data()
return {"size": len(data)}
if __name__ == "__main__":
app.run(host="0.0.0.0", port=5000)
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 the timeout behavior needs to remain tied to PoolLimits.\n\n
Why This Fix Works in Production
- Trigger: await asyncio.wait_for(self.semaphore.acquire(), timeout)
- Mechanism: Timeout handling in the connection pool was incorrectly managed through PoolLimits instead of TimeoutConfig
- Why the fix works: Changes the timeout handling in the connection pool to ensure that pool timeouts are managed through the TimeoutConfig instead of PoolLimits, addressing the timeout issue when exceeding max connections. (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).
- Timeout handling in the connection pool was incorrectly managed through PoolLimits instead of TimeoutConfig
- Surfaces as: Traceback (most recent call last):
Proof / Evidence
- GitHub issue: #557
- Fix PR: https://github.com/encode/httpx/pull/563
- 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: 0.44
Discussion
High-signal excerpts from the issue thread (symptoms, repros, edge-cases).
“Right, yup”
Failure Signature (Search String)
- await asyncio.wait_for(self.semaphore.acquire(), timeout)
Error Message
Stack trace
error.txt
Error Message
-------------
Traceback (most recent call last):
File "/Users/yeray/code/personal/_forks/httpx/httpx/concurrency/asyncio.py", line 236, in acquire
await asyncio.wait_for(self.semaphore.acquire(), timeout)
File "/Users/yeray/.pyenv/versions/3.7.4/lib/python3.7/asyncio/tasks.py", line 449, in wait_for
raise futures.TimeoutError()
concurrent.futures._base.TimeoutError
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "__dev__/concurrent_writes.py", line 48, in <module>
run()
File "/Users/yeray/.pyenv/versions/httpx/lib/python3.7/site-packages/click/core.py", line 764, in __call__
return self.main(*args, **kwargs)
File "/Users/yeray/.pyenv/versions/httpx/lib/python3.7/site-packages/click/core.py", line 717, in main
rv = self.invoke(ctx)
File "/Users/yeray/.pyenv/versions/httpx/lib/python3.7/site-packages/click/core.py", line 956, in invoke
return ctx.invoke(self.callback, **ctx.params)
File "/Users/yeray/.pyenv/versions/httpx/lib/python3.7/site-packages/click/core.py", line 555, in invoke
return callback(*args, **kwargs)
File "__dev__/concurrent_writes.py", line 44, in run
asyncio.run(send_requests(config))
File "/Users/yeray/.pyenv/versions/3.7.4/lib/python3.7/asyncio/runners.py", line 43, in run
return loop.run_until_complete(main)
File "/Users/yeray/.pyenv/versions/3.7.4/lib/pyt
... (truncated) ...
Minimal Reproduction
repro.py
from quart import Quart, request
app = Quart(__name__)
@app.route("/", methods=["POST"])
async def post():
data = await request.get_data()
return {"size": len(data)}
if __name__ == "__main__":
app.run(host="0.0.0.0", port=5000)
Environment
- Python: 3.7
What Broke
High number of concurrent requests resulted in TimeoutError, causing request failures.
Why It Broke
Timeout handling in the connection pool was incorrectly managed through PoolLimits instead of TimeoutConfig
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 the timeout behavior needs to remain tied to PoolLimits.
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/563
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 the timeout behavior needs to remain tied to PoolLimits.
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.
- Track RSS + object counts after deployments; alert on monotonic growth and GC pressure.
- Add a long-running test that repeats the failing call path and asserts stable memory.
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.