httpx Concurrency backends vs AnyIO (Fix)

The Fix

Upgrade to version 0.7.3 or later.

Based on closed encode/httpx issue #296 · PR/commit linked

Jump to Verify Open PR/Commit

@@ -81,7 +81,7 @@ def __init__(
                 dispatch = WSGIDispatch(app=app)
             else:
-                dispatch = ASGIDispatch(app=app)
+                dispatch = ASGIDispatch(app=app, backend=backend)

repro.py

import anyio


async def sometask(num):
    print('Task', num, 'running')
    await anyio.sleep(1)
    print('Task', num, 'finished')


async def main():
    async with anyio.create_task_group() as group:
        for num in range(5):
            await group.spawn(sometask, num)

    print('All tasks finished!')


anyio.run(main)

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 0.7.3 or later.\nWhen NOT to use: This fix should not be used if the application does not require trio support.\n\n

Why This Fix Works in Production

Trigger: It'd cost us a bunch of work, and it doesn't get us any closer to 1.0, so putting time into it probably ought to be contingent on an interested party deciding…
Mechanism: Added a trio concurrency backend to support async operations
Why the fix works: Added a trio concurrency backend to support async operations. (first fixed release: 0.7.3).

Why This Breaks in Prod

Added a trio concurrency backend to support async operations
Production symptom (often without a traceback): It'd cost us a bunch of work, and it doesn't get us any closer to 1.0, so putting time into it probably ought to be contingent on an interested party deciding to take a crack at it, right?

Proof / Evidence

GitHub issue: #296
Fix PR: https://github.com/encode/httpx/pull/276
First fixed release: 0.7.3
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.59

Discussion

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

Jump to Sources Open on GitHub

“From a glance over the docs it *looks* like it's got everything we'd need, yup. (But that's about as much depth as I'd gone into)”

@lovelydinosaur · 2019-09-03 · source

“> should we consider replacing our ConcurrencyBackend API with AnyIO? Given where we are now, I don't think I see a clear enough benefit for…”

@lovelydinosaur · 2019-09-02 · source

“Agreed with your position @tomchristie, thanks for your input!”

@florimondmanca · 2019-09-02 · source

“I think there are clear benefits, we're basically re-implementing a subset of work that's already been proven and we end up getting Trio support along…”

@sethmlarson · 2019-09-02 · source

Failure Signature (Search String)

It'd cost us a bunch of work, and it doesn't get us any closer to 1.0, so putting time into it probably ought to be contingent on an interested party deciding to take a crack at

Copy-friendly signature

signature.txt

Failure Signature
-----------------
It'd cost us a bunch of work, and it doesn't get us any closer to 1.0, so putting time into it probably ought to be contingent on an interested party deciding to take a crack at it, right?

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
It'd cost us a bunch of work, and it doesn't get us any closer to 1.0, so putting time into it probably ought to be contingent on an interested party deciding to take a crack at it, right?

Minimal Reproduction

repro.py

import anyio


async def sometask(num):
    print('Task', num, 'running')
    await anyio.sleep(1)
    print('Task', num, 'finished')


async def main():
    async with anyio.create_task_group() as group:
        for num in range(5):
            await group.spawn(sometask, num)

    print('All tasks finished!')


anyio.run(main)

What Broke

Concurrency issues leading to potential timeouts and degraded performance.

Why It Broke

Added a trio concurrency backend to support async operations

Fix Options (Details)

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

Upgrade to version 0.7.3 or later.

When NOT to use: This fix should not be used if the application does not require trio support.

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/276

First fixed release: 0.7.3

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

When NOT to Use This Fix

This fix should not be used if the application does not require trio support.

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
0.7.3	Fixed

Related Issues

No related fixes found.

Cluster: httpx:other All hubs All clusters

Related clusters: Configuration error Data consistency Timeout

Sources

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