httpx Auto headers inserted into `headers.

The Fix

Upgrade to version 0.14.2 or later.

Based on closed encode/httpx issue #1203 · PR/commit linked

Jump to Verify Open PR/Commit

@@ -411,9 +411,8 @@ def encoding(self, value: str) -> None:
         """
         Returns a list of the raw header items, as byte pairs.
-        May be mutated in-place.
         """
-        return self._list

repro.py

>> import httpx
>> client = httpx.Client()
>> request = client.build_request("GET", "https://google.com")
>> requests.headers
Headers({'host': 'google.com', 'user-agent': 'python-httpx/0.14.1', 'accept': '*/*', 'accept-encoding': 'gzip, deflate, br', 'connection': 'keep-alive'})
>> "host" in request.headers
False
>> request.headers.pop("host")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "D:\programs\anaconda3\lib\_collections_abc.py", line 801, in pop
    del self[key]
  File "E:\projects\pycharm\httpx\httpx\_models.py", line 536, in __delitem__
    del self._dict[del_key]
KeyError: b'host'

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\nUpgrade to version 0.14.2 or later.\nWhen NOT to use: Do not use this fix if the header behavior is expected to remain unchanged.\n\n

Why This Fix Works in Production

Trigger: >> import httpx
Mechanism: Fixes the issue where auto headers were inserted into headers._list but not into headers._dict.
Why the fix works: Fixes the issue where auto headers were inserted into headers._list but not into headers._dict. (first fixed release: 0.14.2).

Production impact:

If left unfixed, this can cause silent data inconsistencies that propagate (bad cache entries, incorrect downstream decisions).

Why This Breaks in Prod

Surfaces as: >> import httpx

Proof / Evidence

GitHub issue: #1203
Fix PR: https://github.com/encode/httpx/pull/1205
First fixed release: 0.14.2
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.37

Discussion

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

Jump to Sources Open on GitHub

“Introduced by #1089. Resolved and test case'd in #1205.”

@lovelydinosaur · 2020-08-20 · confirmation · source

Failure Signature (Search String)

>> import httpx

Error Message

Stack trace

error.txt

Error Message
-------------
>> import httpx
>> client = httpx.Client()
>> request = client.build_request("GET", "https://google.com")
>> requests.headers
Headers({'host': 'google.com', 'user-agent': 'python-httpx/0.14.1', 'accept': '*/*', 'accept-encoding': 'gzip, deflate, br', 'connection': 'keep-alive'})
>> "host" in request.headers
False
>> request.headers.pop("host")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "D:\programs\anaconda3\lib\_collections_abc.py", line 801, in pop
    del self[key]
  File "E:\projects\pycharm\httpx\httpx\_models.py", line 536, in __delitem__
    del self._dict[del_key]
KeyError: b'host'

Minimal Reproduction

repro.py

>> import httpx
>> client = httpx.Client()
>> request = client.build_request("GET", "https://google.com")
>> requests.headers
Headers({'host': 'google.com', 'user-agent': 'python-httpx/0.14.1', 'accept': '*/*', 'accept-encoding': 'gzip, deflate, br', 'connection': 'keep-alive'})
>> "host" in request.headers
False
>> request.headers.pop("host")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "D:\programs\anaconda3\lib\_collections_abc.py", line 801, in pop
    del self[key]
  File "E:\projects\pycharm\httpx\httpx\_models.py", line 536, in __delitem__
    del self._dict[del_key]
KeyError: b'host'

What Broke

Headers were not accessible as expected, leading to KeyError on header access.

Fix Options (Details)

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

Upgrade to version 0.14.2 or later.

When NOT to use: Do not use this fix if the header behavior is expected to remain unchanged.

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

Fix reference: https://github.com/encode/httpx/pull/1205

First fixed release: 0.14.2

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

When NOT to Use This Fix

Do not use this fix if the header behavior is expected to remain unchanged.

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
0.14.2	Fixed

Related Issues

No related fixes found.

Cluster: httpx:data-consistency All hubs All clusters

Related clusters: Configuration error Other Timeout

Sources

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

httpx Auto headers inserted into `headers._list` while not into `headers._dict` (Fix)