urllib3 urllib3 2.2 explicitly casts all headers to

The Fix

pip install urllib3==2.2.1

Based on closed urllib3/urllib3 issue #3343 · 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%.

Jump to Verify Open PR/Commit

@@ -0,0 +1 @@
@@ -0,0 +1 @@
+Fixed ``HTTPConnectionPool.urlopen`` to stop automatically casting non-proxy headers to ``HTTPHeaderDict``. This change was premature as it did not apply to proxy headers and ``HTTPHeaderDict`` does not handle byte header values correctly yet.
diff --git a/src/urllib3/connectionpool.py b/src/urllib3/connectionpool.py
index 1036f0d718..bd58ff14dd 100644

repro.py

from urllib3._collections import HTTPHeaderDict

hdict = HTTPHeaderDict({'x-my-header': b'my-binary-value'})
# Any access that invokes `.get()` will fail (print, get, getall, etc.)
print(hdict)

verify

Re-run the minimal reproduction on your broken version, then apply the fix and re-run.

fix.md

Option A — Upgrade to fixed release\npip install urllib3==2.2.1\nWhen NOT to use: This fix should not be used if the application relies on the previous behavior of casting headers.\n\n

Why This Fix Works in Production

Trigger: >> print(hdict)
Mechanism: Stops automatically casting non-proxy headers to HTTPHeaderDict, addressing issues with byte header values.
Why the fix works: Stops automatically casting non-proxy headers to HTTPHeaderDict, addressing issues with byte header values. (first fixed release: 2.2.1).

Production impact:

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 3.11 in real deployments (not just unit tests).
Surfaces as: >> print(hdict)

Proof / Evidence

GitHub issue: #3343
Fix PR: https://github.com/urllib3/urllib3/pull/3344
First fixed release: 2.2.1
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.46

Discussion

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

Jump to Sources Open on GitHub

“There is already an existing ticket + PR to accept bytes values in the HTTPHeaderDict: https://github.com/urllib3/urllib3/issues/3072 https://github.com/urllib3/urllib3/pull/3279 Would be interesting to see if that PR…”

@bblommers · 2024-02-14 · source

“Thanks for the investigation! We need to review and merge #3279, but we should also consider if we really want to cast headers to HTTPHeaderDict.”

@pquentin · 2024-02-15 · source

“Thanks for pointing those out, @bblommers! Yeah, I agree those seem to be inline with this issue”

@nateprewitt · 2024-02-14 · source

“Adding some more context after further digging”

@nateprewitt · 2024-02-14 · source

Failure Signature (Search String)

>> print(hdict)

Error Message

Stack trace

error.txt

Error Message
-------------
>> print(hdict)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/private/tmp/.venv/lib/python3.11/site-packages/urllib3/_collections.py", line 423, in __repr__
    return f"{type(self).__name__}({dict(self.itermerged())})"
                                    ^^^^^^^^^^^^^^^^^^^^^^^
  File "/private/tmp/.venv/lib/python3.11/site-packages/urllib3/_collections.py", line 446, in itermerged
    yield val[0], ", ".join(val[1:])
                  ^^^^^^^^^^^^^^^^^^

Minimal Reproduction

repro.py

from urllib3._collections import HTTPHeaderDict

hdict = HTTPHeaderDict({'x-my-header': b'my-binary-value'})
# Any access that invokes `.get()` will fail (print, get, getall, etc.)
print(hdict)

Environment

Python: 3.11
urllib3: 2.2

What Broke

Users experienced TypeErrors when accessing headers defined as bytes in HTTPHeaderDict.

Fix Options (Details)

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

pip install urllib3==2.2.1

When NOT to use: This fix should not be used if the application relies on the previous behavior of casting headers.

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/3344

First fixed release: 2.2.1

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

When NOT to Use This Fix

This fix should not be used if the application relies on the previous behavior of casting headers.

Verify Fix

verify

Re-run the minimal reproduction on your broken version, then apply the fix and re-run.

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
2.2.1	Fixed

Related Issues

No related fixes found.

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Related clusters: Data consistency SSL handshake Other

Sources

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

urllib3 urllib3 2.2 explicitly casts all headers to HTTPHeaderDict (Fix)