The Fix
Added support for `validation_alias` in the mypy plugin to resolve issues with missing named arguments.
Based on closed pydantic/pydantic issue #10991 · 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%.
@@ -1040,15 +1040,17 @@ def get_alias_info(stmt: AssignmentStmt) -> tuple[str | None, bool]:
return None, False
- for i, arg_name in enumerate(expr.arg_names):
- if arg_name != 'alias':
- continue
import json
from typing import ClassVar, Generic, Self, Set, TypeVar, Union
from pydantic import BaseModel, ConfigDict, Field, field_validator
_ResponseT = TypeVar('_ResponseT', bound=Union[BaseModel, dict])
class APIResponse(BaseModel, Generic[_ResponseT]):
model_config = ConfigDict(populate_by_name=True) # This does not matter
ok: bool = Field(validation_alias='is_success')
status_code: int
headers: dict
body: _ResponseT = Field(validation_alias='content') # using `alias` works fine
@field_validator('body', mode='before')
@classmethod
def convert_to_dict(cls, v: Union[str, bytes]) -> dict:
return json.loads(v)
APIResponse[dict](
ok=True,
status_code=200,
headers=headers or {},
content=b'{}',
) # Missing named argument "body" for "APIResponse" [call-arg]
Re-run the minimal reproduction on your broken version, then apply the fix and re-run.
Option A — Apply the official fix\nAdded support for `validation_alias` in the mypy plugin to resolve issues with missing named arguments.\nWhen NOT to use: This fix should not be used if the mypy plugin is not being utilized.\n\n
Why This Fix Works in Production
- Trigger: Pydantic-Mypy works with `alias`, but not with `validation_alias`
- Mechanism: The mypy plugin did not support `validation_alias`, causing missing named argument errors
- 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).
- The mypy plugin did not support `validation_alias`, causing missing named argument errors
- Production symptom (often without a traceback): Pydantic-Mypy works with `alias`, but not with `validation_alias`
Proof / Evidence
- GitHub issue: #10991
- Fix PR: https://github.com/pydantic/pydantic/pull/11295
- Reproduced locally: No (not executed)
- Last verified: 2026-02-11
- Confidence: 0.80
- Did this fix it?: Yes (upstream fix exists)
- Own content ratio: 0.50
Discussion
High-signal excerpts from the issue thread (symptoms, repros, edge-cases).
“### Initial Checks - [X] I confirm that I'm using Pydantic V2 ### Description So, when I use pydantic-mypy plugin, and define my fields and use model I get mypy error. The thing is, my field is using validation_alias (instead of alias). Run”
Failure Signature (Search String)
- Pydantic-Mypy works with `alias`, but not with `validation_alias`
- model_config = ConfigDict(populate_by_name=True) # This does not matter
Copy-friendly signature
Failure Signature
-----------------
Pydantic-Mypy works with `alias`, but not with `validation_alias`
model_config = ConfigDict(populate_by_name=True) # This does not matter
Error Message
Signature-only (no traceback captured)
Error Message
-------------
Pydantic-Mypy works with `alias`, but not with `validation_alias`
model_config = ConfigDict(populate_by_name=True) # This does not matter
Minimal Reproduction
import json
from typing import ClassVar, Generic, Self, Set, TypeVar, Union
from pydantic import BaseModel, ConfigDict, Field, field_validator
_ResponseT = TypeVar('_ResponseT', bound=Union[BaseModel, dict])
class APIResponse(BaseModel, Generic[_ResponseT]):
model_config = ConfigDict(populate_by_name=True) # This does not matter
ok: bool = Field(validation_alias='is_success')
status_code: int
headers: dict
body: _ResponseT = Field(validation_alias='content') # using `alias` works fine
@field_validator('body', mode='before')
@classmethod
def convert_to_dict(cls, v: Union[str, bytes]) -> dict:
return json.loads(v)
APIResponse[dict](
ok=True,
status_code=200,
headers=headers or {},
content=b'{}',
) # Missing named argument "body" for "APIResponse" [call-arg]
Environment
- Python: 3.12
- Pydantic: 2
What Broke
Users experienced mypy errors related to missing named arguments when using `validation_alias`.
Why It Broke
The mypy plugin did not support `validation_alias`, causing missing named argument errors
Fix Options (Details)
Option A — Apply the official fix
Added support for `validation_alias` in the mypy plugin to resolve issues with missing named arguments.
Fix reference: https://github.com/pydantic/pydantic/pull/11295
Last verified: 2026-02-11. Validate in your environment.
When NOT to Use This Fix
- This fix should not be used if the mypy plugin is not being utilized.
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.
Related Issues
No related fixes found.
Sources
We don’t republish the full GitHub discussion text. Use the links above for context.