httpx Unexpected AssertionError (httpx 0.9.5) (Fix)

The Fix

Upgrade to version 0.13.0 or later.

Based on closed encode/httpx issue #689 · PR/commit linked

Production note: This tends to surface only under concurrency. Reproduce with load tests and watch for lock contention/cancellation paths.

Jump to Verify Open PR/Commit

@@ -50,26 +50,3 @@ async def __aexit__(
     ) -> None:
         await self.close()
-
-
-class OpenConnection:

repro.py

class GNS3Project:
    def __init__(self, ...):
          self.http_client = httpx.AsyncClient()

   async def provision_router(self, sema, ...):
         async with sema:
             # several sequential HTTP operations: put/post/get, e.g.:
             # await self.http_client.post(url)

async def main():
    gns3_project = GNS3Project(...)
    sema = asyncio.BoundedSemaphore(25)
    tasks = [
        asyncio.create_task(
            gns3_project.provision_router(
                devices[i], template=router_config_template, sema=sema
            )
        )
        for i in range(500, 600)
    ]
    await asyncio.gather(*tasks)

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 suitable for scenarios requiring the `OpenConnection` abstraction.\n\n

Why This Fix Works in Production

Trigger: return await self.request(
Mechanism: Dropped the `OpenConnection` abstraction to resolve an AssertionError issue related to connection handling
Why the fix works: Dropped the `OpenConnection` abstraction to resolve an AssertionError issue related to connection handling. (first fixed release: 0.13.0).

Production impact:

If left unfixed, failures can be intermittent under concurrency (hard to reproduce; shows up as sporadic 5xx/timeouts).

Why This Breaks in Prod

Shows up under Python 3.8 in real deployments (not just unit tests).
Dropped the `OpenConnection` abstraction to resolve an AssertionError issue related to connection handling
Surfaces as: File "/opt/venv/lib/python3.8/site-packages/httpx/client.py", line 679, in get

Proof / Evidence

GitHub issue: #689
Fix PR: https://github.com/encode/httpx/pull/700
First fixed release: 0.13.0
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.75
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.39

Discussion

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

Jump to Sources Open on GitHub

“@florimondmanca I keep forgetting about the environmental variables! I didn't see anything obvious with a quick test. Let me see if I can whip up…”

@iwoloschin · 2019-12-28 · repro detail · source

“No luck on making a minimal test here, but I did figure out that it's not only happening on the first connection, sometimes it happens…”

@iwoloschin · 2019-12-28 · repro detail · source

“Hi @iwoloschin! A minimal reproduction example might be useful, indeed! At first glance it seems like a race condition”

@florimondmanca · 2019-12-27 · repro detail · source

“The issue was worse at 0.9.5, a failed request would hang/kill the entire task”

@iwoloschin · 2019-12-30 · source

Failure Signature (Search String)

return await self.request(

Error Message

Stack trace

error.txt

Error Message
-------------
File "/opt/venv/lib/python3.8/site-packages/httpx/client.py", line 679, in get
    return await self.request(
  File "/opt/venv/lib/python3.8/site-packages/httpx/client.py", line 259, in request
    response = await self.send(
  File "/opt/venv/lib/python3.8/site-packages/httpx/client.py", line 403, in send
    response = await self.send_handling_redirects(
  File "/opt/venv/lib/python3.8/site-packages/httpx/client.py", line 465, in send_handling_redirects
    response = await self.send_handling_auth(
  File "/opt/venv/lib/python3.8/site-packages/httpx/client.py", line 591, in send_handling_auth
    response = await self.send_single_request(request, timeout, verify, cert)
  File "/opt/venv/lib/python3.8/site-packages/httpx/client.py", line 618, in send_single_request
    response = await dispatcher.send(
  File "/opt/venv/lib/python3.8/site-packages/httpx/dispatch/connection_pool.py", line 147, in send
    connection = await self.acquire_connection(
  File "/opt/venv/lib/python3.8/site-packages/httpx/dispatch/connection_pool.py", line 165, in acquire_connection
    connection = self.pop_connection(origin)
  File "/opt/venv/lib/python3.8/site-packages/httpx/dispatch/connection_pool.py", line 215, in pop_connection
    connection = self.active_connections.pop_by_origin(origin, http2_only=True)
  File "/opt/venv/lib/python3.8/site-packages/httpx/dispatch/connection_pool.py", line
... (truncated) ...

Minimal Reproduction

repro.py

class GNS3Project:
    def __init__(self, ...):
          self.http_client = httpx.AsyncClient()

   async def provision_router(self, sema, ...):
         async with sema:
             # several sequential HTTP operations: put/post/get, e.g.:
             # await self.http_client.post(url)

async def main():
    gns3_project = GNS3Project(...)
    sema = asyncio.BoundedSemaphore(25)
    tasks = [
        asyncio.create_task(
            gns3_project.provision_router(
                devices[i], template=router_config_template, sema=sema
            )
        )
        for i in range(500, 600)
    ]
    await asyncio.gather(*tasks)

Environment

Python: 3.8
httpx: 0.9.5

What Broke

Unexpected AssertionError during concurrent requests leading to potential task hangs.

Why It Broke

Dropped the `OpenConnection` abstraction to resolve an AssertionError issue related to connection handling

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 suitable for scenarios requiring the `OpenConnection` abstraction.

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/700

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 suitable for scenarios requiring the `OpenConnection` abstraction.

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 stress test that runs high-concurrency workloads and fails on thread dumps / blocked locks.
Enable watchdog dumps in prod (faulthandler, thread dump endpoint) to capture deadlocks quickly.

Version Compatibility Table

Version	Status
0.13.0	Fixed

Related Issues

No related fixes found.

Cluster: httpx:race-condition All hubs All clusters

Related clusters: Configuration error Data consistency Other

Sources

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