httpx BoundedSemaphore error when using proxies with async

The Fix

Upgrade to version 0.7.2 or later.

Based on closed encode/httpx issue #922 · 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

@@ -1,4 +1,3 @@
@@ -1,4 +1,3 @@
-import collections.abc
 import typing

repro.py

import asyncio
import httpx

proxies = {"http": "http://127.0.0.1:8080/"}


async def request():
    async with httpx.AsyncClient(proxies=proxies) as client:
        response = await client.get("http://example.org")
        assert response.status_code == 200


asyncio.run(request())

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 0.7.2 or later.\nWhen NOT to use: This fix should not be applied if using a non-async environment.\n\n

Why This Fix Works in Production

Trigger: ✗ HTTPX_LOG_LEVEL=debug python .dev/bounded_semaphore_error.py
Mechanism: A BoundedSemaphore error occurs during the cleanup phase when using proxies with the async client
Why the fix works: Fixed a BoundedSemaphore error when using proxies with the async client in httpx. (first fixed release: 0.7.2).

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 3.8 in real deployments (not just unit tests).
A BoundedSemaphore error occurs during the cleanup phase when using proxies with the async client
Surfaces as: ✗ HTTPX_LOG_LEVEL=debug python .dev/bounded_semaphore_error.py

Proof / Evidence

GitHub issue: #922
Fix PR: https://github.com/encode/httpcore/pull/70
First fixed release: 0.7.2
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.42

Discussion

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

Jump to Sources Open on GitHub

“I found the root cause in httpcore opening a PR there.”

@yeraydiazdiaz · 2020-05-02 · source

Failure Signature (Search String)

✗ HTTPX_LOG_LEVEL=debug python .dev/bounded_semaphore_error.py

Error Message

Stack trace

error.txt

Error Message
-------------
✗ HTTPX_LOG_LEVEL=debug python .dev/bounded_semaphore_error.py
DEBUG [2020-05-02 11:30:17] httpx._client - HTTP Request: GET http://example.org "HTTP/1.1 200 OK"
Traceback (most recent call last):
  File ".dev/bounded_semaphore_error.py", line 13, in <module>
    asyncio.run(request())
  File "/Users/yeray/.pyenv/versions/3.8.1/lib/python3.8/asyncio/runners.py", line 43, in run
    return loop.run_until_complete(main)
  File "/Users/yeray/.pyenv/versions/3.8.1/lib/python3.8/asyncio/base_events.py", line 612, in run_until_complete
    return future.result()
  File ".dev/bounded_semaphore_error.py", line 10, in request
    assert response.status_code == 200
  File "/Users/yeray/code/personal/_forks/httpx/httpx/_client.py", line 1449, in __aexit__
    await self.aclose()
  File "/Users/yeray/code/personal/_forks/httpx/httpx/_client.py", line 1438, in aclose
    await proxy.aclose()
  File "/Users/yeray/.pyenv/versions/httpcore/lib/python3.8/site-packages/httpcore/_async/connection_pool.py", line 274, in aclose
    await self._remove_from_pool(connection)
  File "/Users/yeray/.pyenv/versions/httpcore/lib/python3.8/site-packages/httpcore/_async/connection_pool.py", line 257, in _remove_from_pool
    self._connection_semaphore.release()
  File "/Users/yeray/.pyenv/versions/httpcore/lib/python3.8/site-packages/httpcore/_backends/asyncio.py", line 208, in release
    self.semaphore.rel
... (truncated) ...

Minimal Reproduction

repro.py

import asyncio
import httpx

proxies = {"http": "http://127.0.0.1:8080/"}


async def request():
    async with httpx.AsyncClient(proxies=proxies) as client:
        response = await client.get("http://example.org")
        assert response.status_code == 200


asyncio.run(request())

Environment

Python: 3.8

What Broke

Requests complete successfully, but an exception is raised during resource cleanup.

Why It Broke

A BoundedSemaphore error occurs during the cleanup phase when using proxies with the async client

Fix Options (Details)

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

Upgrade to version 0.7.2 or later.

When NOT to use: This fix should not be applied if using a non-async environment.

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

Fix reference: https://github.com/encode/httpcore/pull/70

First fixed release: 0.7.2

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

When NOT to Use This Fix

This fix should not be applied if using a non-async environment.

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.
Add a TLS smoke test that performs a real handshake in CI (include CA bundle validation and hostname checks).
Alert on handshake failures by error string and endpoint to catch cert/CA changes quickly.

Version Compatibility Table

Version	Status
0.7.2	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 BoundedSemaphore error when using proxies with async client (Fix)