The Fix
pip install urllib3==1.25
Based on closed urllib3/urllib3 issue #833 · 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%.
@@ -13,6 +13,8 @@ dev (master)
(Issue #858)
+* Normalize the scheme and host in the URL parser (Issue #833)
+
* ... [Short description of non-trivial change.] (Issue #)
Option A — Upgrade to fixed release\npip install urllib3==1.25\nWhen NOT to use: This fix should not be applied if case sensitivity for schemes and hosts is required.\n\n
Why This Fix Works in Production
- Trigger: I'm not entirely sure that's enough, but to be fair I'm not sat down with the code right now so I can't be sure.
- Mechanism: The PoolManager and ProxyManager do not consistently handle case insensitivity for schemes and hosts
- Why the fix works: Normalizes the scheme and host in the URL parser to ensure case insensitivity, addressing issue #833. (first fixed release: 1.25).
- 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 PoolManager and ProxyManager do not consistently handle case insensitivity for schemes and hosts
- Production symptom (often without a traceback): I'm not entirely sure that's enough, but to be fair I'm not sat down with the code right now so I can't be sure.
Proof / Evidence
- GitHub issue: #833
- Fix PR: https://github.com/urllib3/urllib3/pull/911
- First fixed release: 1.25
- Reproduced locally: No (not executed)
- Last verified: 2026-02-09
- Confidence: 0.85
- Did this fix it?: Yes (upstream fix exists)
- Own content ratio: 0.79
Discussion
High-signal excerpts from the issue thread (symptoms, repros, edge-cases).
“+1, we should build this behaviour into our new PoolKey namedtuples somehow”
“I'm not entirely sure that's enough, but to be fair I'm not sat down with the code right now so I can't be sure.”
Failure Signature (Search String)
- I'm not entirely sure that's enough, but to be fair I'm not sat down with the code right now so I can't be sure.
Copy-friendly signature
Failure Signature
-----------------
I'm not entirely sure that's enough, but to be fair I'm not sat down with the code right now so I can't be sure.
Error Message
Signature-only (no traceback captured)
Error Message
-------------
I'm not entirely sure that's enough, but to be fair I'm not sat down with the code right now so I can't be sure.
What Broke
HTTPS connections with capital letters in the scheme are incorrectly treated as HTTP connections.
Why It Broke
The PoolManager and ProxyManager do not consistently handle case insensitivity for schemes and hosts
Fix Options (Details)
Option A — Upgrade to fixed release Safe default (recommended)
pip install urllib3==1.25
Use when you can deploy the upstream fix. It is usually lower-risk than long-lived workarounds.
Fix reference: https://github.com/urllib3/urllib3/pull/911
First fixed release: 1.25
Last verified: 2026-02-09. Validate in your environment.
When NOT to Use This Fix
- This fix should not be applied if case sensitivity for schemes and hosts is required.
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 |
|---|---|
| 1.25 | Fixed |
Related Issues
No related fixes found.
Sources
We don’t republish the full GitHub discussion text. Use the links above for context.