The Fix
pip install pydantic==2.6.1
Based on closed pydantic/pydantic issue #8649 · 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%.
@@ -479,14 +479,18 @@ def __get_pydantic_core_schema__(
) -> core_schema.CoreSchema:
import_email_validator()
+
return core_schema.no_info_after_validator_function(
cls._validate,
import pydantic
class Model(BaseModel):
v: NameEmail
Model.model_validate_json('{"v":{"name":"foo bar","email":"[email protected]"}}')
Model.model_validate_json('{"v":"foo bar <[email protected]>"}')
Re-run the minimal reproduction on your broken version, then apply the fix and re-run.
Option A — Upgrade to fixed release\npip install pydantic==2.6.1\nWhen NOT to use: Do not use this fix if you require backward compatibility with previous versions of `NameEmail`.\n\n
Why This Fix Works in Production
- Trigger: NotImplementedError: Cannot check isinstance when validating from json, use a JsonOrPython validator instead.
- Mechanism: The `NameEmail` type does not support JSON deserialization, causing validation errors
- Why the fix works: Fixes json validation for `NameEmail` type, allowing it to parse from JSON input. (first fixed release: 2.6.1).
- 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 `NameEmail` type does not support JSON deserialization, causing validation errors
- Surfaces as: NotImplementedError: Cannot check isinstance when validating from json, use a JsonOrPython validator instead.
Proof / Evidence
- GitHub issue: #8649
- Fix PR: https://github.com/pydantic/pydantic/pull/8650
- First fixed release: 2.6.1
- Reproduced locally: No (not executed)
- Last verified: 2026-02-09
- Confidence: 0.85
- Did this fix it?: Yes (upstream fix exists)
- Own content ratio: 0.67
Discussion
High-signal excerpts from the issue thread (symptoms, repros, edge-cases).
“@Holi0317, Ah yes, thanks for reporting this. Thanks a bunch for contributing a fix. I'll review your PR later today :). Great work!”
Failure Signature (Search String)
- NotImplementedError: Cannot check isinstance when validating from json, use a JsonOrPython validator instead.
Error Message
Stack trace
Error Message
-------------
NotImplementedError: Cannot check isinstance when validating from json, use a JsonOrPython validator instead.
Minimal Reproduction
import pydantic
class Model(BaseModel):
v: NameEmail
Model.model_validate_json('{"v":{"name":"foo bar","email":"[email protected]"}}')
Model.model_validate_json('{"v":"foo bar <[email protected]>"}')
Environment
- Pydantic: 2
What Broke
Validating JSON input with `NameEmail` raises a NotImplementedError, leading to failed requests.
Why It Broke
The `NameEmail` type does not support JSON deserialization, causing validation errors
Fix Options (Details)
Option A — Upgrade to fixed release Safe default (recommended)
pip install pydantic==2.6.1
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/8650
First fixed release: 2.6.1
Last verified: 2026-02-09. Validate in your environment.
When NOT to Use This Fix
- Do not use this fix if you require backward compatibility with previous versions of `NameEmail`.
Verify Fix
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 |
|---|---|
| 2.6.1 | Fixed |
Related Issues
No related fixes found.
Sources
We don’t republish the full GitHub discussion text. Use the links above for context.