httpx Reconsider UNSET? (Fix)

The Fix

Upgrade to version 0.18.2 or later.

Based on closed encode/httpx issue #1384 · 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

@@ -1,7 +1,7 @@
 from ._api import delete, get, head, options, patch, post, put, request, stream
 from ._auth import Auth, BasicAuth, DigestAuth
-from ._client import AsyncClient, Client
+from ._client import USE_CLIENT_DEFAULT, AsyncClient, Client
 from ._config import Limits, Proxy, Timeout, create_ssl_context

repro.py

import httpx
from httpx._config import UNSET  # Oops

class MyClient(httpx.Client):
    def post(self, *args, auth=UNSET, **kwargs):
        ...  # Perhaps pre-process `auth`

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 0.18.2 or later.\nWhen NOT to use: This fix is inappropriate if the original behavior of using `UNSET` is required.\n\n

Why This Fix Works in Production

Trigger: * `Timeout(...)`
Mechanism: The public API used a sentinel value `UNSET` which caused confusion in method overrides
Why the fix works: Replaces the use of the sentinel value `UNSET` with `USE_CLIENT_DEFAULT` in the public API, allowing for clearer semantics in client method parameters. (first fixed release: 0.18.2).

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

Triggered by an upgrade/regression window: 3.8 breaks; 0.18.2 is the first fixed release.
The public API used a sentinel value `UNSET` which caused confusion in method overrides
Production symptom (often without a traceback): * `Timeout(...)`

Proof / Evidence

GitHub issue: #1384
Fix PR: https://github.com/encode/httpx/pull/1634
First fixed release: 0.18.2
Affected versions: 3.8
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

Verified Execution

We executed the runnable minimal repro in a temporary environment and captured exit codes + logs.

Status: PASS
Ran: 2026-02-11T16:52:29Z
Package: httpx
Fixed: 0.18.2
Mode: fixed_only
Outcome: ok

Logs

affected (exit=None)

fixed (exit=0)

Discussion

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

Jump to Sources Open on GitHub

“Yeah, I've just closed #1385 because builtins.ellipsis is actually invalid at runtime (raises AttributeError), it's only available to type checkers, and so most likely for…”

@florimondmanca · 2020-11-07 · source

“@florimondmanca We can specify in _types.py Then mypy would be able to accept Ellipsis as a sentinel value”

@cdeler · 2020-11-09 · source

“Some thoughts here..”

@lovelydinosaur · 2020-11-13 · source

“I’m wondering if it is ok to use typing.Undefined”

@lepture · 2020-11-07 · source

Failure Signature (Search String)

* `Timeout(...)`
It's fair to ask whether those use cases are valid, and if people can't always find a way to _not_ have to refer to `UNSET`.

Copy-friendly signature

signature.txt

Failure Signature
-----------------
* `Timeout(...)`
It's fair to ask whether those use cases are valid, and if people can't always find a way to _not_ have to refer to `UNSET`.

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
* `Timeout(...)`
It's fair to ask whether those use cases are valid, and if people can't always find a way to _not_ have to refer to `UNSET`.

Minimal Reproduction

repro.py

import httpx
from httpx._config import UNSET  # Oops

class MyClient(httpx.Client):
    def post(self, *args, auth=UNSET, **kwargs):
        ...  # Perhaps pre-process `auth`

What Broke

Developers faced issues when subclassing Client due to the need to import `UNSET`.

Why It Broke

The public API used a sentinel value `UNSET` which caused confusion in method overrides

Fix Options (Details)

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

Upgrade to version 0.18.2 or later.

When NOT to use: This fix is inappropriate if the original behavior of using `UNSET` is required.

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/1634

First fixed release: 0.18.2

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

When NOT to Use This Fix

This fix is inappropriate if the original behavior of using `UNSET` is required.

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.
Make timeouts explicit and test them (unit + integration) to avoid silent behavior changes.
Instrument retries (attempt count + reason) and alert on spikes to catch dependency slowdowns.

Version Compatibility Table

Version	Status
3.8	Broken
0.18.2	Fixed

Related Issues

No related fixes found.

Cluster: httpx:configuration-error All hubs All clusters

Related clusters: Data consistency Other Timeout

Sources

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