The Fix
pip install urllib3==2.2.0
Based on closed urllib3/urllib3 issue #2951 · PR/commit linked
@@ -0,0 +1,7 @@
@@ -0,0 +1,7 @@
+env:
+ es2020 : true
+ worker: true
Option A — Upgrade to fixed release\npip install urllib3==2.2.0\nWhen NOT to use: This fix is not suitable for environments without cross-origin isolation or where streaming is not required.\n\n
Why This Fix Works in Production
- Trigger: Synchronous HTTP requests failed in Pyodide/PyScript environments, causing application errors.
- Mechanism: This PR adds native support for Emscripten in urllib3, enabling streaming HTTP requests in cross-origin isolated browser environments
- Why the fix works: Adds native support for Emscripten in urllib3, enabling streaming HTTP requests in cross-origin isolated browser environments. (first fixed release: 2.2.0).
Why This Breaks in Prod
- This PR adds native support for Emscripten in urllib3, enabling streaming HTTP requests in cross-origin isolated browser environments
- Production symptom (often without a traceback): Synchronous HTTP requests failed in Pyodide/PyScript environments, causing application errors.
Proof / Evidence
- GitHub issue: #2951
- Fix PR: https://github.com/urllib3/urllib3/pull/3195
- First fixed release: 2.2.0
- Reproduced locally: No (not executed)
- Last verified: 2026-02-09
- Confidence: 0.85
- Did this fix it?: Yes (upstream fix exists)
- Own content ratio: 0.90
Discussion
High-signal excerpts from the issue thread (symptoms, repros, edge-cases).
“Created this project on pyscript.com which shows the current state of affairs: https://b15ad9a9-0744-48ba-8e37-eedddba9292c.pyscriptapps.com/7b5d346e-e490-43f0-b43f-cdeae7f24665/latest/”
“This is awesome! This means I can add an urllib3 version check in pyodide-http and skip the patching, right?”
“@koenvo - yes. I'm gonna put a PR into pyodide to include requests and urllib3 as a core package when I have the time.”
“@joemarshall Thanks for doing that! Could you include me on those PRs if only just to see what is happening over there? :)”
Failure Signature (Search String)
- Synchronous HTTP requests failed in Pyodide/PyScript environments, causing application errors.
Copy-friendly signature
Failure Signature
-----------------
Synchronous HTTP requests failed in Pyodide/PyScript environments, causing application errors.
Error Message
Signature-only (no traceback captured)
Error Message
-------------
Synchronous HTTP requests failed in Pyodide/PyScript environments, causing application errors.
What Broke
Synchronous HTTP requests failed in Pyodide/PyScript environments, causing application errors.
Why It Broke
This PR adds native support for Emscripten in urllib3, enabling streaming HTTP requests in cross-origin isolated browser environments
Fix Options (Details)
Option A — Upgrade to fixed release Safe default (recommended)
pip install urllib3==2.2.0
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/3195
First fixed release: 2.2.0
Last verified: 2026-02-09. Validate in your environment.
When NOT to Use This Fix
- This fix is not suitable for environments without cross-origin isolation or where streaming 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.
- 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 |
|---|---|
| 2.2.0 | Fixed |
Related Issues
No related fixes found.
Sources
We don’t republish the full GitHub discussion text. Use the links above for context.