httpx Drop NPN support (Fix)

The Fix

Upgrade to version 0.7.3 or later.

Based on closed encode/httpx issue #280 · 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

@@ -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 if the application requires NPN for legacy compatibility.\n\n

Why This Fix Works in Production

Trigger: Clients may experience connection issues due to reliance on deprecated NPN support.
Mechanism: NPN support was deprecated and replaced by ALPN for HTTP protocol negotiation
Why the fix works: Drops NPN support in favor of ALPN-only for HTTP protocol negotiation. (first fixed release: 0.7.3).

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

NPN support was deprecated and replaced by ALPN for HTTP protocol negotiation
Production symptom (often without a traceback): Clients may experience connection issues due to reliance on deprecated NPN support.

Proof / Evidence

GitHub issue: #280
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: 0.75
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

“You can add macOS to the list as well. The new Network module only supports ALPN.”

@sethmlarson · 2019-08-27 · source

“Fair enough, that sounds like a nice little improvement.”

@lovelydinosaur · 2019-08-27 · source

Failure Signature (Search String)

Clients may experience connection issues due to reliance on deprecated NPN support.

Copy-friendly signature

signature.txt

Failure Signature
-----------------
Clients may experience connection issues due to reliance on deprecated NPN support.

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
Clients may experience connection issues due to reliance on deprecated NPN support.

What Broke

Clients may experience connection issues due to reliance on deprecated NPN support.

Why It Broke

NPN support was deprecated and replaced by ALPN for HTTP protocol negotiation

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 if the application requires NPN for legacy compatibility.

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 if the application requires NPN for legacy compatibility.

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 TLS smoke test that performs a real handshake in CI (include CA bundle validation and hostname checks).
Alert on handshake failures by error string and endpoint to catch cert/CA changes quickly.

Version Compatibility Table

Version	Status
0.7.3	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.