urllib3 NodeJS + pyodide support (Fix)

The Fix

pip install urllib3==2.3.0

Based on closed urllib3/urllib3 issue #3400 · PR/commit linked

Production note: This usually shows up under retries/timeouts. Treat it as a side-effect risk until you can verify behavior with a canary + real traffic.

Open PR/Commit

@@ -75,6 +75,10 @@ jobs:
             experimental: false
             nox-session: test-3.9
+          - python-version: "3.12"
+            os: ubuntu-latest
+            nox-session: emscripten(node)

fix.md

Option A — Upgrade to fixed release\npip install urllib3==2.3.0\nWhen NOT to use: This fix is not suitable for environments that do not support JavaScript Promise Integration.\n\n

Why This Fix Works in Production

Trigger: NodeJS + pyodide support
Mechanism: Added support for JavaScript Promise Integration in the emscripten/pyodide backend
Why the fix works: Adds support to the emscripten/pyodide backend for JavaScript Promise Integration, allowing urllib3 to work in Node.js and simplifying its use in browsers. (first fixed release: 2.3.0).

Production impact:

If left unfixed, retries/timeouts can trigger duplicate external side-effects (double charges, duplicate emails, repeated writes).

Why This Breaks in Prod

Added support for JavaScript Promise Integration in the emscripten/pyodide backend

Proof / Evidence

GitHub issue: #3400
Fix PR: https://github.com/urllib3/urllib3/pull/3427
First fixed release: 2.3.0
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.84

Discussion

High-signal excerpts from the issue thread (symptoms, repros, edge-cases).

Jump to Sources Open on GitHub

“Done in https://github.com/urllib3/urllib3/pull/3427, closing, thank you!”

@pquentin · 2024-12-22 · confirmation · source

“Hey @joemarshall, nice to see you again! :) Happy to accept patches which makes urllib3 work better in those environments”

@sethmlarson · 2024-06-05 · source

Failure Signature (Search String)

NodeJS + pyodide support

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
NodeJS + pyodide support

Why It Broke

Added support for JavaScript Promise Integration in the emscripten/pyodide backend

Fix Options (Details)

Option A — Upgrade to fixed release Safe default (recommended)

pip install urllib3==2.3.0

When NOT to use: This fix is not suitable for environments that do not support JavaScript Promise Integration.

Use when you can deploy the upstream fix. It is usually lower-risk than long-lived workarounds.

Option D — Guard side-effects with OnceOnly Guardrail for side-effects

Mitigate duplicate external side-effects under retries/timeouts/agent loops by gating the operation before calling external systems.

Place OnceOnly between your code/agent and real side-effects (Stripe, emails, CRM, APIs).
Use a stable key per side-effect (e.g., customer_id + action + idempotency_key).
Fail-safe: configure fail-open vs fail-closed based on blast radius and spend risk.
This is most useful when retries/timeouts can re-trigger the same external call.

Show example snippet (optional)

onceonly.py

from onceonly import OnceOnly
import os

once = OnceOnly(api_key=os.environ["ONCEONLY_API_KEY"], fail_open=True)

# Stable idempotency key per real side-effect.
# Use a request id / job id / webhook delivery id / Stripe event id, etc.
event_id = "evt_..."  # replace
key = f"stripe:webhook:{event_id}"

res = once.check_lock(key=key, ttl=3600)
if res.duplicate:
    return {"status": "already_processed"}

# Safe to execute the side-effect exactly once.
handle_event(event_id)

See OnceOnly SDK

When NOT to use: Do not use this to hide logic bugs or data corruption. Use it to block duplicate external side-effects and enforce tool permissions/spend caps.

Fix reference: https://github.com/urllib3/urllib3/pull/3427

First fixed release: 2.3.0

Last verified: 2026-02-09. Validate in your environment.

When NOT to Use This Fix

This fix is not suitable for environments that do not support JavaScript Promise Integration.
Do not use this to hide logic bugs or data corruption. Use it to block duplicate external side-effects and enforce tool permissions/spend caps.

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
2.3.0	Fixed

Related Issues

No related fixes found.

Cluster: urllib3:duplicates urllib3 hub urllib3 best practices All hubs All clusters

Related clusters: Configuration error Data consistency SSL handshake

Sources

We don’t republish the full GitHub discussion text. Use the links above for context.