The Fix
pip install pydantic==1.10.18
Based on closed pydantic/pydantic issue #10078 · PR/commit linked
@@ -864,39 +864,40 @@ except PydanticUserError as exc_info:
```py test="skip" lint="skip" upgrade="skip"
-from pydantic import field_serializer
+from pydantic import FieldSerializationInfo, SerializerFunctionWrapHandler, field_serializer
fix.md
Option A — Upgrade to fixed release\npip install pydantic==1.10.18\nWhen NOT to use: This fix is not applicable if the project does not utilize `pyright` for type checking.\n\n
Why This Fix Works in Production
- Trigger: - Document that we support `mypy` and `pyright` as type checkers. It might be that some features we use are unsupported by `mypy`, we do our best on that end.
- Mechanism: The `pyright` test suite lacked comprehensive type checking tests and needed refactoring
- Why the fix works: Improves the `pyright` test suite by adding type checking tests and refactoring model decorators' signatures. (first fixed release: 1.10.18).
Why This Breaks in Prod
- The `pyright` test suite lacked comprehensive type checking tests and needed refactoring
- Production symptom (often without a traceback): - Document that we support `mypy` and `pyright` as type checkers. It might be that some features we use are unsupported by `mypy`, we do our best on that end.
Proof / Evidence
- GitHub issue: #10078
- Fix PR: https://github.com/pydantic/pydantic/pull/10092
- First fixed release: 1.10.18
- Reproduced locally: No (not executed)
- Last verified: 2026-02-09
- Confidence: 0.95
- Did this fix it?: Yes (upstream fix exists)
- Own content ratio: 0.72
Discussion
High-signal excerpts from the issue thread (symptoms, repros, edge-cases).
“@Viicos, We have a few - https://github.com/pydantic/pydantic/tree/main/tests/pyright”
“Ah nice! Will continue working on this then”
Failure Signature (Search String)
- - Document that we support `mypy` and `pyright` as type checkers. It might be that some features we use are unsupported by `mypy`, we do our best on that end.
- - [ ] [Compatibility between releases](https://docs.pydantic.dev/changelog/)
Copy-friendly signature
signature.txt
Failure Signature
-----------------
- Document that we support `mypy` and `pyright` as type checkers. It might be that some features we use are unsupported by `mypy`, we do our best on that end.
- [ ] [Compatibility between releases](https://docs.pydantic.dev/changelog/)
Error Message
Signature-only (no traceback captured)
error.txt
Error Message
-------------
- Document that we support `mypy` and `pyright` as type checkers. It might be that some features we use are unsupported by `mypy`, we do our best on that end.
- [ ] [Compatibility between releases](https://docs.pydantic.dev/changelog/)
What Broke
Inadequate type checking could lead to runtime errors in production.
Why It Broke
The `pyright` test suite lacked comprehensive type checking tests and needed refactoring
Fix Options (Details)
Option A — Upgrade to fixed release Safe default (recommended)
pip install pydantic==1.10.18
When NOT to use: This fix is not applicable if the project does not utilize `pyright` for type checking.
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/10092
First fixed release: 1.10.18
Last verified: 2026-02-09. Validate in your environment.
When NOT to Use This Fix
- This fix is not applicable if the project does not utilize `pyright` for type checking.
- 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 |
|---|---|
| 1.10.18 | Fixed |
Related Issues
No related fixes found.
Sources
We don’t republish the full GitHub discussion text. Use the links above for context.