Pydantic `mypy` throws `Too few arguments for "__call_

The Fix

pip install pydantic==1.10.18

Based on closed pydantic/pydantic issue #9870 · PR/commit linked

Jump to Verify Open PR/Commit

@@ -262,10 +262,9 @@ def __call__(
     _V2BeforeAfterOrPlainValidatorType = TypeVar(
         '_V2BeforeAfterOrPlainValidatorType',
-        _V2Validator,
-        _PartialClsOrStaticMethod,
+        bound=Union[_V2Validator, _PartialClsOrStaticMethod],

repro.py

from pydantic import BaseModel, field_validator


class SomeModel(BaseModel):
    field: int

    @field_validator("field")
    @classmethod
    def validate_field(cls, v: int) -> int:
        return v


class OtherModel(BaseModel):
    field: int

    @field_validator("field")
    @classmethod
    def validate_field(cls, v: int) -> int:
        return SomeModel.validate_field(v)


OtherModel(field=1)

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==1.10.18\nWhen NOT to use: This fix should not be applied if the typing issue is unrelated to field_validator usage.\n\n

Why This Fix Works in Production

Trigger: `mypy` throws `Too few arguments for "__call__"` error on `field_validator`
Mechanism: The field_validator decorator has a typing issue causing mypy to report incorrect argument count errors
Why the fix works: Fix typing issue with field_validator-decorated methods, addressing a mypy error related to argument count. (first fixed release: 1.10.18).

Why This Breaks in Prod

Shows up under Python 3.12 in real deployments (not just unit tests).
The field_validator decorator has a typing issue causing mypy to report incorrect argument count errors
Production symptom (often without a traceback): `mypy` throws `Too few arguments for "__call__"` error on `field_validator`

Proof / Evidence

GitHub issue: #9870
Fix PR: https://github.com/pydantic/pydantic/pull/9914
First fixed release: 1.10.18
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.60

Discussion

High-signal excerpts from the issue thread (symptoms, repros, edge-cases).

Jump to Sources Open on GitHub

“I'll note this is not related to the mypy plugin.”

@dmontagu · 2024-07-17 · source

“I believe the issue is that: Here, we have annotated the decorator as returning a _V2BeforeAfterOrPlainValidatorType”

@dmontagu · 2024-07-17 · source

Failure Signature (Search String)

`mypy` throws `Too few arguments for "__call__"` error on `field_validator`
Please see the below code snippet. `mypy` will throw a `call-arg` error on `field_validator`:

Copy-friendly signature

signature.txt

Failure Signature
-----------------
`mypy` throws `Too few arguments for "__call__"` error on `field_validator`
Please see the below code snippet. `mypy` will throw a `call-arg` error on `field_validator`:

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
`mypy` throws `Too few arguments for "__call__"` error on `field_validator`
Please see the below code snippet. `mypy` will throw a `call-arg` error on `field_validator`:

Minimal Reproduction

repro.py

from pydantic import BaseModel, field_validator


class SomeModel(BaseModel):
    field: int

    @field_validator("field")
    @classmethod
    def validate_field(cls, v: int) -> int:
        return v


class OtherModel(BaseModel):
    field: int

    @field_validator("field")
    @classmethod
    def validate_field(cls, v: int) -> int:
        return SomeModel.validate_field(v)


OtherModel(field=1)

Environment

Python: 3.12
Pydantic: 2

What Broke

Developers encounter mypy errors when using field_validator, leading to confusion and potential delays.

Why It Broke

The field_validator decorator has a typing issue causing mypy to report incorrect argument count errors

Fix Options (Details)

Option A — Upgrade to fixed release Safe default (recommended)

pip install pydantic==1.10.18

When NOT to use: This fix should not be applied if the typing issue is unrelated to field_validator usage.

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/9914

First fixed release: 1.10.18

Last verified: 2026-02-09. Validate in your environment.

When NOT to Use This Fix

This fix should not be applied if the typing issue is unrelated to field_validator usage.

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
1.10.18	Fixed

Related Issues

No related fixes found.

Cluster: pydantic:webhook Pydantic hub Pydantic best practices All hubs All clusters

Related clusters: Configuration error Data consistency OpenAPI schema

Sources

We don’t republish the full GitHub discussion text. Use the links above for context.

Pydantic `mypy` throws `Too few arguments for "__call__"` error on `field_validator` (Fix)