Pydantic validate_by_alias does not work with validate

The Fix

pip install pydantic==2.12.0

Based on closed pydantic/pydantic issue #11962 · 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

@@ -169,7 +169,7 @@
 
 VALIDATE_CALL_SUPPORTED_TYPES = get_args(ValidateCallSupportedTypes)
-UNSUPPORTED_STANDALONE_FIELDINFO_ATTRIBUTES = [
+UNSUPPORTED_STANDALONE_FIELDINFO_ATTRIBUTES: list[tuple[str, Any]] = [
     ('alias', None),

repro.py

from pydantic import Field, ValidationError, validate_call, ConfigDict
from typing import Annotated

@validate_call
def how_many(num: Annotated[int, Field(gt=10)]):
    return num  

try:
    print(how_many(number = 20))
    print('alias test 1 failed')
except Exception as e:
    print('alias test 1 passed')


@validate_call
def how_many(num: Annotated[int, Field(gt=10, alias = 'number')]):
    return num

try:
    print(how_many(number = 20))
except Exception as e:
    print(f'alias test 2 : {e}')


@validate_call
def how_many(num: Annotated[int, Field(gt=10, validation_alias = 'number')]):
    return num

try:
    print(how_many(number = 20))
except Exception as e:
    print(f'validation_alias test 1 : {e}')


@validate_call(config = ConfigDict(validate_by_alias = True))
def how_many(num: Annotated[int, Field(gt=10, validation_alias = 'number')]):
    return num

try:
    print(how_many(number = 20))
except Exception as e:
    print(f'validation_alias test 2 : {e}')

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==2.12.0\nWhen NOT to use: This fix is not applicable if validate_call is not used with validation_alias.\n\n

Why This Fix Works in Production

Trigger: validate_by_alias does not work with validate_call
Mechanism: The validate_call decorator does not recognize the validate_by_alias setting
Why the fix works: Respects the `validation_alias` in the `@validate_call` decorator, addressing the issue where `validate_by_alias` did not function as expected. (first fixed release: 2.12.0).

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

The validate_call decorator does not recognize the validate_by_alias setting
Production symptom (often without a traceback): validate_by_alias does not work with validate_call

Proof / Evidence

GitHub issue: #11962
Fix PR: https://github.com/pydantic/pydantic/pull/12340
First fixed release: 2.12.0
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.95
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.43

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: 2.12.0
Mode: fixed_only
Outcome: ok

Logs

affected (exit=None)

fixed (exit=0)

alias test 1 passed
20
20
20

Discussion

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

Jump to Sources Open on GitHub

“yup , agree with that”

@mg3146 · 2025-06-20 · source

Failure Signature (Search String)

validate_by_alias does not work with validate_call
does validate_by_alias work with validate_call() ? I'm getting a validation error in the below code snippet. My ultimate goal was to use `AliasChoices ` so that I could do a many

Copy-friendly signature

signature.txt

Failure Signature
-----------------
validate_by_alias does not work with validate_call
does validate_by_alias work with validate_call() ? I'm getting a validation error in the below code snippet.  My ultimate goal was to use `AliasChoices ` so that I could do a many : 1 mapping of a keyword argument.

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
validate_by_alias does not work with validate_call
does validate_by_alias work with validate_call() ? I'm getting a validation error in the below code snippet.  My ultimate goal was to use `AliasChoices ` so that I could do a many : 1 mapping of a keyword argument.

Minimal Reproduction

repro.py

from pydantic import Field, ValidationError, validate_call, ConfigDict
from typing import Annotated

@validate_call
def how_many(num: Annotated[int, Field(gt=10)]):
    return num  

try:
    print(how_many(number = 20))
    print('alias test 1 failed')
except Exception as e:
    print('alias test 1 passed')


@validate_call
def how_many(num: Annotated[int, Field(gt=10, alias = 'number')]):
    return num

try:
    print(how_many(number = 20))
except Exception as e:
    print(f'alias test 2 : {e}')


@validate_call
def how_many(num: Annotated[int, Field(gt=10, validation_alias = 'number')]):
    return num

try:
    print(how_many(number = 20))
except Exception as e:
    print(f'validation_alias test 1 : {e}')


@validate_call(config = ConfigDict(validate_by_alias = True))
def how_many(num: Annotated[int, Field(gt=10, validation_alias = 'number')]):
    return num

try:
    print(how_many(number = 20))
except Exception as e:
    print(f'validation_alias test 2 : {e}')

Environment

Pydantic: 2

What Broke

Validation errors occur when using aliases in function parameters.

Why It Broke

The validate_call decorator does not recognize the validate_by_alias setting

Fix Options (Details)

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

pip install pydantic==2.12.0

When NOT to use: This fix is not applicable if validate_call is not used with validation_alias.

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/12340

First fixed release: 2.12.0

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

When NOT to Use This Fix

This fix is not applicable if validate_call is not used with validation_alias.

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
2.12.0	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 validate_by_alias does not work with validate_call (Fix)