The Fix
pip install pydantic==2.10.6
Based on closed pydantic/pydantic issue #10707 · 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%.
@@ -95,6 +95,7 @@
'pydantic.deprecated.class_validators.root_validator',
}
+IMPLICIT_CLASSMETHOD_DECORATOR_FULLNAMES = DECORATOR_FULLNAMES - {'pydantic.functional_serializers.model_serializer'}
repro.py
from typing import Literal
from pydantic import BaseModel, model_serializer
class TemperatureModel(BaseModel):
unit: Literal["C", "F"]
value: int
def _calculate_is_hot(self, value: int) -> bool:
if self.unit == "F":
return value > 100
return value > 38
@model_serializer(when_used="json")
def serialize_model(self) -> dict[str, int | str | bool]:
var: int = self.value
if self.unit == "F":
return {
"unit": "C",
"value": int((self.value - 32) / 1.8),
"is_hot": self._calculate_is_hot(var),
}
return {
"unit": self.unit,
"value": self.value,
"is_hot": self._calculate_is_hot(var),
}
temperature = TemperatureModel(unit="F", value=212)
print(temperature.model_dump_json())
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==2.10.6\nWhen NOT to use: This fix should not be applied if the mypy plugin is not used.\n\n
Why This Fix Works in Production
- Trigger: mypy incorrectly identifies argument of method within a model_serializer as incorrect
- Mechanism: Mypy plugin incorrectly transforms model serializer functions as class methods
- Why the fix works: Addresses an issue where mypy incorrectly identifies arguments of methods within a model_serializer as incorrect. (first fixed release: 2.10.6).
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
- Shows up under Python 3.12 in real deployments (not just unit tests).
- Mypy plugin incorrectly transforms model serializer functions as class methods
- Production symptom (often without a traceback): mypy incorrectly identifies argument of method within a model_serializer as incorrect
Proof / Evidence
- GitHub issue: #10707
- Fix PR: https://github.com/pydantic/pydantic/pull/11298
- First fixed release: 2.10.6
- Reproduced locally: No (not executed)
- Last verified: 2026-02-09
- Confidence: 0.95
- Did this fix it?: Yes (upstream fix exists)
- Own content ratio: 0.51
Verified Execution
We executed the runnable minimal repro in a temporary environment and captured exit codes + logs.
- Status: PASS
- Ran: 2026-02-11T16:52:29Z
- Package: pydantic
- Fixed: 2.10.6
- Mode: fixed_only
- Outcome: ok
Logs
affected (exit=None)
fixed (exit=0)
{"unit":"C","value":100,"is_hot":true}
Discussion
High-signal excerpts from the issue thread (symptoms, repros, edge-cases).
“I have noticed the following: And then you do: With the plugin enabled you get: FYI @Viicos”
Failure Signature (Search String)
- mypy incorrectly identifies argument of method within a model_serializer as incorrect
- demo.py:21: error: Missing positional argument "value" in call to "_calculate_is_hot" of "TemperatureModel" [call-arg]
Copy-friendly signature
signature.txt
Failure Signature
-----------------
mypy incorrectly identifies argument of method within a model_serializer as incorrect
demo.py:21: error: Missing positional argument "value" in call to "_calculate_is_hot" of "TemperatureModel" [call-arg]
Error Message
Signature-only (no traceback captured)
error.txt
Error Message
-------------
mypy incorrectly identifies argument of method within a model_serializer as incorrect
demo.py:21: error: Missing positional argument "value" in call to "_calculate_is_hot" of "TemperatureModel" [call-arg]
Minimal Reproduction
repro.py
from typing import Literal
from pydantic import BaseModel, model_serializer
class TemperatureModel(BaseModel):
unit: Literal["C", "F"]
value: int
def _calculate_is_hot(self, value: int) -> bool:
if self.unit == "F":
return value > 100
return value > 38
@model_serializer(when_used="json")
def serialize_model(self) -> dict[str, int | str | bool]:
var: int = self.value
if self.unit == "F":
return {
"unit": "C",
"value": int((self.value - 32) / 1.8),
"is_hot": self._calculate_is_hot(var),
}
return {
"unit": self.unit,
"value": self.value,
"is_hot": self._calculate_is_hot(var),
}
temperature = TemperatureModel(unit="F", value=212)
print(temperature.model_dump_json())
Environment
- Python: 3.12
- Pydantic: 2
What Broke
Mypy reports incorrect type errors when using model_serializer decorator.
Why It Broke
Mypy plugin incorrectly transforms model serializer functions as class methods
Fix Options (Details)
Option A — Upgrade to fixed release Safe default (recommended)
pip install pydantic==2.10.6
When NOT to use: This fix should not be applied if the mypy plugin is not used.
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/11298
First fixed release: 2.10.6
Last verified: 2026-02-09. Validate in your environment.
When NOT to Use This Fix
- This fix should not be applied if the mypy plugin is not used.
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 |
|---|---|
| 2.10.6 | Fixed |
Related Issues
No related fixes found.
Sources
We don’t republish the full GitHub discussion text. Use the links above for context.