Pydantic Pydantic v2.7+ fails to validate MultiValueEnum

The Fix

pip install pydantic==1.10.1

Based on closed pydantic/pydantic issue #9572 · 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%.

Jump to Verify Open PR/Commit

@@ -34,7 +34,7 @@ A few notes:
 
 Type coercion like this can be extremely helpful but also confusing or not desired,
-see [below](#coercion-and-stictness) for a discussion of `validate_arguments`'s limitations in this regard.
+see [below](#coercion-and-strictness) for a discussion of `validate_arguments`'s limitations in this regard.

repro.py

from aenum import MultiValueEnum
from fastapi import Depends, FastAPI, Query, Request
from fastapi.responses import JSONResponse
from pydantic import BaseModel


class BooleanField(MultiValueEnum):
    TRUE = "1", "True", "true", "Yes", 1
    FALSE = "0", "False", "false", "No", 0


class Params(BaseModel):
    field_1: BooleanField | None = None

    @classmethod
    def from_request(cls, field_1: str | None = Query(None)):
        return cls(
            field_1=BooleanField(field_1) if field_1 is not None else None
        )


app = FastAPI()


@app.exception_handler(ValueError)
async def valuer_error_exception_handler(
    request: Request, exc: Exception
) -> JSONResponse:
    return JSONResponse(status_code=422, content={"message": f"{exc}"})


@app.get("/sample")
async def sample(q: Params = Depends(Params.from_request)) -> Params:
    return q

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==1.10.1\nWhen NOT to use: Do not use if it changes public behavior or if the failure cannot be reproduced.\n\nOption C — Workaround\nfor this this issue?\nWhen NOT to use: Do not use if it changes public behavior or if the failure cannot be reproduced.\n\n

Why This Fix Works in Production

Trigger: print(Sample(**{"field_1": 2, "field_2": "true", "field_3": "Yes", "field_4": "1"}))
Mechanism: Addresses the validation issue with MultiValueEnum fields in Pydantic v2.7+ by ensuring proper handling of enum values.
Why the fix works: Addresses the validation issue with MultiValueEnum fields in Pydantic v2.7+ by ensuring proper handling of enum values. (first fixed release: 1.10.1).

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

Triggered by an upgrade/regression window: 2.7 breaks; 1.10.1 is the first fixed release.
Shows up under Python 3.12 in real deployments (not just unit tests).
Surfaces as: print(Sample(**{"field_1": 2, "field_2": "true", "field_3": "Yes", "field_4": "1"}))

Proof / Evidence

GitHub issue: #9572
Fix PR: https://github.com/pydantic/pydantic-core/pull/1456
First fixed release: 1.10.1
Affected versions: 2.7
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.34

Discussion

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

Jump to Sources Open on GitHub

“It'll be because before v2.7 we just called the enum with the value so MultiValueEnum took care of validating that the value was allowed”

@samuelcolvin · 2024-06-06 · source

“Not at the moment, but I'll try to redo the enum validator in rust for v2.10 to support things like this. Thanks for following up!”

@sydney-runkle · 2024-08-27 · source

“@gbatagian, Yeah likely on our end, we made some major changes to enum handling in 2.7. Will look into a fix for this!”

@sydney-runkle · 2024-06-05 · source

“There is sort of a workaround within the Pydantic scope by manually casting the values into a multivalue enum through a field_validator: The value validation…”

@gbatagian · 2024-08-27 · source

Failure Signature (Search String)

print(Sample(**{"field_1": 2, "field_2": "true", "field_3": "Yes", "field_4": "1"}))

Error Message

Stack trace

error.txt

Error Message
-------------
print(Sample(**{"field_1": 2, "field_2": "true", "field_3": "Yes", "field_4": "1"}))


---------------------------------------------------------------------------
ValidationError                           Traceback (most recent call last)
Cell In[2], line 1
----> 1 print(Sample(**{"field_1": 2, "field_2": "true", "field_3": "Yes", "field_4": "1"}))

File /Library/Frameworks/Python.framework/Versions/3.10/lib/python3.10/site-packages/pydantic/main.py:192, in BaseModel.__init__(self, **data)
    190 # `__tracebackhide__` tells pytest and some other tools to omit this function from tracebacks
    191 __tracebackhide__ = True
--> 192 self.__pydantic_validator__.validate_python(data, self_instance=self)

ValidationError: 1 validation error for Sample
field_1
  Value error, 2 is not a valid BooleanField [type=value_error, input_value=2, input_type=int]
    For further information visit https://errors.pydantic.dev/2.8/v/value_error

Minimal Reproduction

repro.py

from aenum import MultiValueEnum
from fastapi import Depends, FastAPI, Query, Request
from fastapi.responses import JSONResponse
from pydantic import BaseModel


class BooleanField(MultiValueEnum):
    TRUE = "1", "True", "true", "Yes", 1
    FALSE = "0", "False", "false", "No", 0


class Params(BaseModel):
    field_1: BooleanField | None = None

    @classmethod
    def from_request(cls, field_1: str | None = Query(None)):
        return cls(
            field_1=BooleanField(field_1) if field_1 is not None else None
        )


app = FastAPI()


@app.exception_handler(ValueError)
async def valuer_error_exception_handler(
    request: Request, exc: Exception
) -> JSONResponse:
    return JSONResponse(status_code=422, content={"message": f"{exc}"})


@app.get("/sample")
async def sample(q: Params = Depends(Params.from_request)) -> Params:
    return q

Environment

Python: 3.12
Pydantic: 2.7

What Broke

Validation errors occur when using MultiValueEnum fields in Pydantic v2.7+.

Fix Options (Details)

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

pip install pydantic==1.10.1

When NOT to use: Do not use if it changes public behavior or if the failure cannot be reproduced.

Use when you can deploy the upstream fix. It is usually lower-risk than long-lived workarounds.

Option C — Workaround Temporary workaround

for this this issue?

When NOT to use: Do not use if it changes public behavior or if the failure cannot be reproduced.

Use only if you cannot change versions today. Treat this as a stopgap and remove once upgraded.

Fix reference: https://github.com/pydantic/pydantic-core/pull/1456

First fixed release: 1.10.1

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

When NOT to Use This Fix

Do not use if it changes public behavior or if the failure cannot be reproduced.

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
2.7	Broken
1.10.1	Fixed

Related Issues

No related fixes found.

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Related clusters: Data consistency OpenAPI schema Serialization

Sources

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

Pydantic Pydantic v2.7+ fails to validate MultiValueEnum fields (Fix)