The Fix
pip install urllib3==1.25
Based on closed urllib3/urllib3 issue #1447 · 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%.
@@ -47,6 +47,7 @@
'key_assert_hostname', # bool or string
'key_assert_fingerprint', # str
+ 'key_server_hostname', #str
)
Option A — Upgrade to fixed release\npip install urllib3==1.25\nWhen NOT to use: This fix should not be used if custom SNI is not required.\n\n
Why This Fix Works in Production
- Trigger: PoolManager does not allow custom server_hostname
- Mechanism: The PoolManager did not include the key_server_hostname variable, preventing custom SNI usage
- Why the fix works: Adds the missing key_server_hostname variable to the PoolManager, allowing custom SNI to be specified. (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 did not include the key_server_hostname variable, preventing custom SNI usage
- Production symptom (often without a traceback): PoolManager does not allow custom server_hostname
Proof / Evidence
- GitHub issue: #1447
- Fix PR: https://github.com/urllib3/urllib3/pull/1449
- 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: 0.69
Discussion
High-signal excerpts from the issue thread (symptoms, repros, edge-cases).
“PR here: https://github.com/urllib3/urllib3/pull/1449”
Failure Signature (Search String)
- PoolManager does not allow custom server_hostname
- I am trying to pass a custom SNI string on the Python requests module, but it doesn't look like I can create a PoolManager with a custom server_hostname via **pool_kw because it's
Copy-friendly signature
Failure Signature
-----------------
PoolManager does not allow custom server_hostname
I am trying to pass a custom SNI string on the Python requests module, but it doesn't look like I can create a PoolManager with a custom server_hostname via **pool_kw because it's not defined inside _key_fields in PoolManager.
Error Message
Signature-only (no traceback captured)
Error Message
-------------
PoolManager does not allow custom server_hostname
I am trying to pass a custom SNI string on the Python requests module, but it doesn't look like I can create a PoolManager with a custom server_hostname via **pool_kw because it's not defined inside _key_fields in PoolManager.
What Broke
Users cannot specify custom SNI, leading to potential connection issues.
Why It Broke
The PoolManager did not include the key_server_hostname variable, preventing custom SNI usage
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/1449
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 used if custom SNI is not 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.