The Fix
Upgrade to version 0.7.2 or later.
Based on closed encode/httpx issue #247 · 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%.
@@ -48,13 +48,13 @@ def __init__(
root_path: str = "",
client: typing.Tuple[str, int] = ("127.0.0.1", 123),
+ backend: ConcurrencyBackend = None,
) -> None:
self.app = app
Option A — Upgrade to fixed release\nUpgrade to version 0.7.2 or later.\nWhen NOT to use: This fix should not be used if the application is strictly bound to asyncio.\n\n
Why This Fix Works in Production
- Trigger: *Note: this issue breaks apart steps originally taken in #217.*
- Mechanism: The ASGI dispatcher relied on asyncio-specific APIs, limiting support for alternative concurrency backends
- Why the fix works: Added a backend parameter to the ASGIDispatcher to allow for alternative concurrency backends, addressing issue #247. (first fixed release: 0.7.2).
- 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
- The ASGI dispatcher relied on asyncio-specific APIs, limiting support for alternative concurrency backends
- Production symptom (often without a traceback): *Note: this issue breaks apart steps originally taken in #217.*
Proof / Evidence
- GitHub issue: #247
- Fix PR: https://github.com/encode/httpx/pull/270
- First fixed release: 0.7.2
- Reproduced locally: No (not executed)
- Last verified: 2026-02-09
- Confidence: 0.75
- Did this fix it?: Yes (upstream fix exists)
- Own content ratio: 0.68
Discussion
High-signal excerpts from the issue thread (symptoms, repros, edge-cases).
“Going to see if I can take a crack at dropping the asyncio.FIRST_COMPLETED bit of implementation, since that'd make this issue somewhat narrower in scope.”
“Yeah it'll tie in. We'll want to get this side of things all nice and clean and sorted first, then we'll be able to take…”
“A thought here: we could probably forgo needing to implement a wait_first_completed() by rejigging the implementation to use queues for signalling completion from the two…”
“I’ll update the issue description soon taking into account the simplified concurrency in the ASGI dispatcher. :+1:”
Failure Signature (Search String)
- *Note: this issue breaks apart steps originally taken in #217.*
- I'd probably suggest "to be used *by* the BodyIterator" rather than "to be used instead of BodyIterator". That'd likely be a smaller change footprint. (Tho we can thrash that
Copy-friendly signature
Failure Signature
-----------------
*Note: this issue breaks apart steps originally taken in #217.*
I'd probably suggest "to be used *by* the BodyIterator" rather than "to be used instead of BodyIterator". That'd likely be a smaller change footprint. (Tho we can thrash that level of detail out against the pull request once we're there)
Error Message
Signature-only (no traceback captured)
Error Message
-------------
*Note: this issue breaks apart steps originally taken in #217.*
I'd probably suggest "to be used *by* the BodyIterator" rather than "to be used instead of BodyIterator". That'd likely be a smaller change footprint. (Tho we can thrash that level of detail out against the pull request once we're there)
What Broke
The application could not support alternative concurrency models, leading to limited scalability.
Why It Broke
The ASGI dispatcher relied on asyncio-specific APIs, limiting support for alternative concurrency backends
Fix Options (Details)
Option A — Upgrade to fixed release Safe default (recommended)
Upgrade to version 0.7.2 or later.
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/270
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 used if the application is strictly bound to asyncio.
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 |
|---|---|
| 0.7.2 | Fixed |
Related Issues
No related fixes found.
Sources
We don’t republish the full GitHub discussion text. Use the links above for context.