The Fix
Upgrade to version 0.25.0 or later.
Based on closed encode/httpx issue #2794 · 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%.
@@ -12,6 +12,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
### Added
+* Change the type of `Extensions` from `Mapping[Str, Any]` to `MutableMapping[Str, Any]`. (#2803)
* Add `socket_options` argument to `httpx.HTTPTransport` and `httpx.AsyncHTTPTransport` classes. (#2716)
* The `Response.raise_for_status()` method now returns the response instance. For example: `data = httpx.get('...').raise_for_status().json()`. (#2776)
Option A — Upgrade to fixed release\nUpgrade to version 0.25.0 or later.\nWhen NOT to use: This fix should not be applied if the application relies on the original `Mapping` behavior.\n\n
Why This Fix Works in Production
- Trigger: error: Unsupported target for indexed assignment ("Mapping[Str, Any]") [index]
- Mechanism: The type of `Extensions` was incorrectly defined as `Mapping`, preventing mutable operations
- Why the fix works: Changed the type of `Extensions` from `Mapping` to `MutableMapping` to allow adding custom extensions after response creation. (first fixed release: 0.25.0).
- 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 type of `Extensions` was incorrectly defined as `Mapping`, preventing mutable operations
- Surfaces as: error: Unsupported target for indexed assignment ("Mapping[Str, Any]") [index]
Proof / Evidence
- GitHub issue: #2794
- Fix PR: https://github.com/encode/httpx/pull/2803
- First fixed release: 0.25.0
- Reproduced locally: No (not executed)
- Last verified: 2026-02-09
- Confidence: 0.85
- Did this fix it?: Yes (upstream fix exists)
- Own content ratio: 0.66
Discussion
High-signal excerpts from the issue thread (symptoms, repros, edge-cases).
“### Discussed in https://github.com/encode/httpx/discussions/2793 <div type='discussions-op-text'> <sup>Originally posted by **karosis88** July 28, 2023</sup> I'm working on a library that implements HTTP Caching for httpx and httpcore (it”
Failure Signature (Search String)
- error: Unsupported target for indexed assignment ("Mapping[Str, Any]") [index]
Error Message
Stack trace
Error Message
-------------
error: Unsupported target for indexed assignment ("Mapping[Str, Any]") [index]
What Broke
Users encountered type errors when trying to add custom extensions to responses.
Why It Broke
The type of `Extensions` was incorrectly defined as `Mapping`, preventing mutable operations
Fix Options (Details)
Option A — Upgrade to fixed release Safe default (recommended)
Upgrade to version 0.25.0 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/2803
First fixed release: 0.25.0
Last verified: 2026-02-09. Validate in your environment.
When NOT to Use This Fix
- This fix should not be applied if the application relies on the original `Mapping` behavior.
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 |
|---|---|
| 0.25.0 | Fixed |
Related Issues
No related fixes found.
Sources
We don’t republish the full GitHub discussion text. Use the links above for context.