The Fix
pip install pydantic==1.10.18
Based on closed pydantic/pydantic issue #9988 · 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%.
@@ -205,7 +205,7 @@ jobs:
- name: install example plugin
- run: pdm add ./tests/plugin
+ run: pdm add ./tests/plugin -v
Option A — Upgrade to fixed release\npip install pydantic==1.10.18\nWhen NOT to use: Do not use this fix if backward compatibility with existing annotations is required.\n\n
Why This Fix Works in Production
- Trigger: With recent changes to the core schema, we're being slightly more lax with our annotation checks. See…
- Mechanism: Recent changes to the core schema made annotation checks less strict
- Why the fix works: Addresses issue #9988 by moving annotation compatibility errors to the validation phase, ensuring stricter checks on annotations. (first fixed release: 1.10.18).
- 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
- Recent changes to the core schema made annotation checks less strict
- Production symptom (often without a traceback): With recent changes to the core schema, we're being slightly more lax with our annotation checks. See https://github.com/pydantic/pydantic/pull/9977#discussion_r1691883538 for an example of a newly failing test.
Proof / Evidence
- GitHub issue: #9988
- Fix PR: https://github.com/pydantic/pydantic/pull/9999
- First fixed release: 1.10.18
- Reproduced locally: No (not executed)
- Last verified: 2026-02-09
- Confidence: 0.75
- Did this fix it?: Yes (upstream fix exists)
- Own content ratio: 0.77
Discussion
High-signal excerpts from the issue thread (symptoms, repros, edge-cases).
“With recent changes to the core schema, we're being slightly more lax with our annotation checks. See https://github.com/pydantic/pydantic/pull/9977#discussion_r1691883538 for an example of a newly failing test. We should close this before”
Failure Signature (Search String)
- With recent changes to the core schema, we're being slightly more lax with our annotation checks. See https://github.com/pydantic/pydantic/pull/9977#discussion_r1691883538 for an
Copy-friendly signature
Failure Signature
-----------------
With recent changes to the core schema, we're being slightly more lax with our annotation checks. See https://github.com/pydantic/pydantic/pull/9977#discussion_r1691883538 for an example of a newly failing test.
Error Message
Signature-only (no traceback captured)
Error Message
-------------
With recent changes to the core schema, we're being slightly more lax with our annotation checks. See https://github.com/pydantic/pydantic/pull/9977#discussion_r1691883538 for an example of a newly failing test.
What Broke
Tests began failing due to lax annotation checks, impacting release readiness.
Why It Broke
Recent changes to the core schema made annotation checks less strict
Fix Options (Details)
Option A — Upgrade to fixed release Safe default (recommended)
pip install pydantic==1.10.18
Use when you can deploy the upstream fix. It is usually lower-risk than long-lived workarounds.
Fix reference: https://github.com/pydantic/pydantic/pull/9999
First fixed release: 1.10.18
Last verified: 2026-02-09. Validate in your environment.
When NOT to Use This Fix
- Do not use this fix if backward compatibility with existing annotations is required.
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.