The Fix
pip install pydantic==1.10.16
Based on closed pydantic/pydantic issue #9463 · 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%.
@@ -514,12 +514,11 @@ def inspect_validator(validator: Callable[..., Any], mode: FieldValidatorModes)
try:
sig = signature(validator)
- except ValueError:
- # builtins and some C extensions don't have signatures
- # assume that they don't take an info argument and only take a single argument
repro.py
from typing import Annotated
from pydantic import BaseModel, PlainSerializer
def my_callable(value, unrelated_arg = None, other_unrelated_arg = 1, even_more = "a"): ...
class Model(BaseModel):
foo: Annotated[int, PlainSerializer(my_callable)]
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 pydantic==1.10.16\nWhen NOT to use: This fix should not be used if backward compatibility with existing serializers is a concern.\n\n
Why This Fix Works in Production
- Trigger: - [ ] [Compatibility between releases](https://docs.pydantic.dev/changelog/)
- Mechanism: The `PlainSerializer` did not accept callables with default arguments due to positional parameter count limitations
- Why the fix works: Allows validator and serializer functions to have default values, addressing the issue where callables with default arguments were not accepted in `PlainSerializer`. (first fixed release: 1.10.16).
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 `PlainSerializer` did not accept callables with default arguments due to positional parameter count limitations
- Production symptom (often without a traceback): - [ ] [Compatibility between releases](https://docs.pydantic.dev/changelog/)
Proof / Evidence
- GitHub issue: #9463
- Fix PR: https://github.com/pydantic/pydantic/pull/9478
- First fixed release: 1.10.16
- Reproduced locally: No (not executed)
- Last verified: 2026-02-09
- Confidence: 0.85
- Did this fix it?: Yes (upstream fix exists)
- Own content ratio: 0.66
Discussion
High-signal excerpts from the issue thread (symptoms, repros, edge-cases).
“### Initial Checks - [X] I have searched Google & GitHub for similar requests and couldn't find anything - [X] I have read and followed the docs and still think this feature is missing ### Description As noticed in https://github.com/pydant”
Failure Signature (Search String)
- - [ ] [Compatibility between releases](https://docs.pydantic.dev/changelog/)
- - [ ] [Data validation/parsing](https://docs.pydantic.dev/concepts/models/#basic-model-usage)
Copy-friendly signature
signature.txt
Failure Signature
-----------------
- [ ] [Compatibility between releases](https://docs.pydantic.dev/changelog/)
- [ ] [Data validation/parsing](https://docs.pydantic.dev/concepts/models/#basic-model-usage)
Error Message
Signature-only (no traceback captured)
error.txt
Error Message
-------------
- [ ] [Compatibility between releases](https://docs.pydantic.dev/changelog/)
- [ ] [Data validation/parsing](https://docs.pydantic.dev/concepts/models/#basic-model-usage)
Minimal Reproduction
repro.py
from typing import Annotated
from pydantic import BaseModel, PlainSerializer
def my_callable(value, unrelated_arg = None, other_unrelated_arg = 1, even_more = "a"): ...
class Model(BaseModel):
foo: Annotated[int, PlainSerializer(my_callable)]
What Broke
Users experienced failures when using callables with default arguments in serializers.
Why It Broke
The `PlainSerializer` did not accept callables with default arguments due to positional parameter count limitations
Fix Options (Details)
Option A — Upgrade to fixed release Safe default (recommended)
pip install pydantic==1.10.16
When NOT to use: This fix should not be used if backward compatibility with existing serializers is a concern.
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)
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/pydantic/pydantic/pull/9478
First fixed release: 1.10.16
Last verified: 2026-02-09. Validate in your environment.
When NOT to Use This Fix
- This fix should not be used if backward compatibility with existing serializers is a concern.
- 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.
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 |
|---|---|
| 1.10.16 | Fixed |
Related Issues
No related fixes found.
Sources
We don’t republish the full GitHub discussion text. Use the links above for context.