The Fix
Upgrade to version 0.7.3 or later.
Based on closed encode/httpx issue #299 · 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%.
@@ -20,6 +20,7 @@ matrix:
- python: 3.8-dev
env: NOX_SESSION=test-3.8
+ dist: bionic # Required to get OpenSSL 1.1.1+
install:
Option A — Upgrade to fixed release\nUpgrade to version 0.7.3 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: Add support for SSLKEYLOGFILE for Python 3.8b4+
- Mechanism: Adds support for the SSLKEYLOGFILE environment variable in Python 3.8 and OpenSSL 1.1.1, allowing for TLS key logging for debugging purposes.
- Why the fix works: Adds support for the SSLKEYLOGFILE environment variable in Python 3.8 and OpenSSL 1.1.1, allowing for TLS key logging for debugging purposes. (first fixed release: 0.7.3).
- 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
- Shows up under Python 3.8 in real deployments (not just unit tests).
- Production symptom (often without a traceback): Add support for SSLKEYLOGFILE for Python 3.8b4+
Proof / Evidence
- GitHub issue: #299
- Fix PR: https://github.com/encode/httpx/pull/301
- 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: 0.75
Discussion
High-signal excerpts from the issue thread (symptoms, repros, edge-cases).
“Python 3.8b4 (with OpenSSL 1.1.1) added support for the SSLKEYLOGFILE environment variable which will be very useful for using Wireshark to do analysis of HTTP/2 requests for example. This should be a small change within SSLConfig._create_d”
Failure Signature (Search String)
- Add support for SSLKEYLOGFILE for Python 3.8b4+
Copy-friendly signature
Failure Signature
-----------------
Add support for SSLKEYLOGFILE for Python 3.8b4+
Python 3.8b4 (with OpenSSL 1.1.1) added support for the `SSLKEYLOGFILE` environment variable which will be very useful for using Wireshark to do analysis of HTTP/2 requests for example.
Error Message
Signature-only (no traceback captured)
Error Message
-------------
Add support for SSLKEYLOGFILE for Python 3.8b4+
Python 3.8b4 (with OpenSSL 1.1.1) added support for the `SSLKEYLOGFILE` environment variable which will be very useful for using Wireshark to do analysis of HTTP/2 requests for example.
Environment
- Python: 3.8
What Broke
Lack of support for SSLKEYLOGFILE prevents TLS key logging for debugging HTTP/2 requests.
Fix Options (Details)
Option A — Upgrade to fixed release Safe default (recommended)
Upgrade to version 0.7.3 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/301
First fixed release: 0.7.3
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.
- 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.
Sources
We don’t republish the full GitHub discussion text. Use the links above for context.