httpx Should AsyncClient accept a callable for

The Fix

Upgrade to version 0.23.1 or later.

Based on closed encode/httpx issue #2246 · 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%.

Open PR/Commit

@@ -1363,7 +1363,7 @@ def __init__(
         app: typing.Optional[typing.Callable] = None,
         trust_env: bool = True,
-        default_encoding: str = "utf-8",
+        default_encoding: typing.Union[str, typing.Callable[[bytes], str]] = "utf-8",
     ):

fix.md

Option A — Upgrade to fixed release\nUpgrade to version 0.23.1 or later.\nWhen NOT to use: This fix is not applicable if the default_encoding should strictly remain a string.\n\n

Why This Fix Works in Production

Trigger: Should the AsyncClient accept a callable like `httpx.Client` does? If so, I'm happy to make the change and submit a PR.
Mechanism: The AsyncClient's default_encoding parameter did not accept a callable as intended
Why the fix works: Updates the type annotation for the default_encoding parameter of AsyncClient's initializer to accept a callable of the form (bytes) -> str, addressing issue #2246. (first fixed release: 0.23.1).

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

The AsyncClient's default_encoding parameter did not accept a callable as intended
Production symptom (often without a traceback): Should the AsyncClient accept a callable like `httpx.Client` does? If so, I'm happy to make the change and submit a PR.

Proof / Evidence

GitHub issue: #2246
Fix PR: https://github.com/encode/httpx/pull/2247
First fixed release: 0.23.1
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.78

Discussion

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

Jump to Sources Open on GitHub

“https://github.com/encode/httpx/blob/89cdd903b2f339c6ae319773c31b8c9352bd9cd8/httpx/_client.py#L1366 According to the online docs, the AsyncClient should also accept a callable in the form of (bytes) -> str for the default_encoding keyword”

Issue thread · issue description · source

Failure Signature (Search String)

Should the AsyncClient accept a callable like `httpx.Client` does? If so, I'm happy to make the change and submit a PR.

Copy-friendly signature

signature.txt

Failure Signature
-----------------
Should the AsyncClient accept a callable like `httpx.Client` does? If so, I'm happy to make the change and submit a PR.

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
Should the AsyncClient accept a callable like `httpx.Client` does? If so, I'm happy to make the change and submit a PR.

What Broke

Users were unable to pass a callable for encoding, leading to confusion and limitations.

Why It Broke

The AsyncClient's default_encoding parameter did not accept a callable as intended

Fix Options (Details)

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

Upgrade to version 0.23.1 or later.

When NOT to use: This fix is not applicable if the default_encoding should strictly remain a string.

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.

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/encode/httpx/pull/2247

First fixed release: 0.23.1

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

When NOT to Use This Fix

This fix is not applicable if the default_encoding should strictly remain a string.
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
0.23.1	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.

httpx Should AsyncClient accept a callable for default_encoding? (Fix)