httpx Support httpcore>=0.16.0 (Fix)

The Fix

Upgrade to version 0.23.1 or later.

Based on closed encode/httpx issue #2443 · 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%.

Open PR/Commit

@@ -4,6 +4,25 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 
+## 0.23.1
+
+### Added

fix.md

Option A — Upgrade to fixed release\nUpgrade to version 0.23.1 or later.\nWhen NOT to use: Do not use if it changes public behavior or if the failure cannot be reproduced.\n\n

Why This Fix Works in Production

Trigger: I have a project that uses `fastapi` and `uvicorn` which demands `h11>=0.8`. Latest version of `fastapi` changed their test suite from `requests` to `httpx`…
Mechanism: The dependency constraint for httpcore was too restrictive, preventing compatibility with h11 versions
Why the fix works: Updates the dependency constraint for httpcore to allow versions >=0.16.0, resolving the issue with h11 compatibility. (first fixed release: 0.23.1).

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

The dependency constraint for httpcore was too restrictive, preventing compatibility with h11 versions
Production symptom (often without a traceback): I have a project that uses `fastapi` and `uvicorn` which demands `h11>=0.8`. Latest version of `fastapi` changed their test suite from `requests` to `httpx`. So I had to add `httpx` to my tests requirements.

Proof / Evidence

GitHub issue: #2443
Fix PR: https://github.com/encode/httpx/pull/2442
First fixed release: 0.23.1
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.75

Discussion

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

Jump to Sources Open on GitHub

“Hi, I have a project that uses fastapi and uvicorn which demands h11>=0.8. Latest version of fastapi changed their test suite from requests to httpx. So I had to add httpx to my tests requirements. Now the problem is, that httpx requires ht”

Issue thread · issue description · source

Failure Signature (Search String)

I have a project that uses `fastapi` and `uvicorn` which demands `h11>=0.8`. Latest version of `fastapi` changed their test suite from `requests` to `httpx`. So I had to add

Copy-friendly signature

signature.txt

Failure Signature
-----------------
I have a project that uses `fastapi` and `uvicorn` which demands `h11>=0.8`. Latest version of `fastapi` changed their test suite from `requests` to `httpx`. So I had to add `httpx` to my tests requirements.

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
I have a project that uses `fastapi` and `uvicorn` which demands `h11>=0.8`. Latest version of `fastapi` changed their test suite from `requests` to `httpx`. So I had to add `httpx` to my tests requirements.

What Broke

Users experienced compatibility issues when upgrading h11 due to httpcore's version constraints.

Why It Broke

The dependency constraint for httpcore was too restrictive, preventing compatibility with h11 versions

Fix Options (Details)

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

Upgrade to version 0.23.1 or later.

When NOT to use: Do not use if it changes public behavior or if the failure cannot be reproduced.

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/2442

First fixed release: 0.23.1

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

When NOT to Use This Fix

Do not use if it changes public behavior or if the failure cannot be reproduced.

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
0.23.1	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.