httpx Add support for a Trio ConcurrencyBackend (Fix)

The Fix

Upgrade to version 0.7.3 or later.

Based on closed encode/httpx issue #120 · PR/commit linked

Open PR/Commit

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

fix.md

Option A — Upgrade to fixed release\nUpgrade to version 0.7.3 or later.\nWhen NOT to use: This fix is not suitable for projects that do not use the Trio library.\n\n

Why This Fix Works in Production

Trigger: Incompatibility with Trio caused failures in concurrent HTTP requests.
Mechanism: The connection pool relied on asyncio, which required refactoring to support Trio
Why the fix works: Adds support for a Trio concurrency backend to httpx, addressing issue #120. (first fixed release: 0.7.3).

Why This Breaks in Prod

The connection pool relied on asyncio, which required refactoring to support Trio
Production symptom (often without a traceback): Incompatibility with Trio caused failures in concurrent HTTP requests.

Proof / Evidence

GitHub issue: #120
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: 0.79

Discussion

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

Jump to Sources Open on GitHub

“Indeed, yes”

@lovelydinosaur · 2019-07-14 · source

“Regarding whether alternative loops/IO libraries should be in third party packages, I think it makes sense in terms of relying on the community to support…”

@florimondmanca · 2019-08-15 · source

Failure Signature (Search String)

Incompatibility with Trio caused failures in concurrent HTTP requests.

Copy-friendly signature

signature.txt

Failure Signature
-----------------
Incompatibility with Trio caused failures in concurrent HTTP requests.

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
Incompatibility with Trio caused failures in concurrent HTTP requests.

What Broke

Incompatibility with Trio caused failures in concurrent HTTP requests.

Why It Broke

The connection pool relied on asyncio, which required refactoring to support Trio

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 is not suitable for projects that do not use the Trio library.

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 is not suitable for projects that do not use the Trio library.

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.