Pydantic AliasGenerator causes computed

The Fix

pip install pydantic==2.6.2

Based on closed pydantic/pydantic issue #8768 · 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

@@ -973,7 +973,7 @@ def _apply_alias_generator_to_field_info(
         # 1. An alias is not specified
         # 2. An alias is specified, but the priority is <= 1
-        if alias_generator and (
+        if (
             field_info.alias_priority is None

repro.py

from pydantic import BaseModel, computed_field, ConfigDict, AliasGenerator, AliasChoices


class Box(BaseModel):
    model_config = ConfigDict(
        populate_by_name=True,
        alias_generator=AliasGenerator(
            validation_alias=lambda field_name: AliasChoices(
                field_name.lower(),
                field_name.upper(),
                field_name.title(),
                field_name.capitalize(),
            )
        ),
    )

    width: float
    height: float
    depth: float
    extra_field: str | None

    @computed_field
    def volume(self) -> float:
        return self.width * self.height * self.depth


if __name__ == "__main__":
    b = Box(WIDTH=1, Height=2, depth=3, Extra_Field="extra")
    print(b.model_dump())

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==2.6.2\nWhen NOT to use: Do not use this fix if the alias_generator is expected to return non-string values.\n\n

Why This Fix Works in Production

Trigger: class Box(BaseModel):
Mechanism: AliasGenerator does not return a string when used with computed_field decorators, causing schema generation errors
Why the fix works: Fixes the issue with AliasGenerator causing errors when used with computed_field decorators in Pydantic models. (first fixed release: 2.6.2).

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).
AliasGenerator does not return a string when used with computed_field decorators, causing schema generation errors
Surfaces as: Traceback (most recent call last):

Proof / Evidence

GitHub issue: #8768
Fix PR: https://github.com/pydantic/pydantic/pull/8806
First fixed release: 2.6.2
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.32

Discussion

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

Jump to Sources Open on GitHub

“@soggychips, No worries. It'll definitely get released in 2.7, which will be in mid-March. Perhaps we could include this in 2.6.2, which will hopefully be…”

@sydney-runkle · 2024-02-20 · confirmation · source

“@soggychips, Thanks for reporting this bug! Seems like this would be a pretty good first issue. Any interest in contributing a fix?”

@sydney-runkle · 2024-02-12 · source

“@soggychips, Are you testing on main? The following works on the current main branch:”

@sydney-runkle · 2024-02-20 · source

“Hi, My apologies. I haven't cloned the repository as standalone. I thought this was pushed to the latest version of pydantic on pypi. Any idea…”

@soggychips · 2024-02-20 · source

Failure Signature (Search String)

class Box(BaseModel):

Error Message

Stack trace

error.txt

Error Message
-------------
Traceback (most recent call last):
  File "/Users/soggy/Developer/pyt/pydantic_test.py", line 4, in <module>
    class Box(BaseModel):
  File "/Users/soggy/.pyenv/versions/pyt/lib/python3.12/site-packages/pydantic/_internal/_model_construction.py", line 183, in __new__
    complete_model_class(
  File "/Users/soggy/.pyenv/versions/pyt/lib/python3.12/site-packages/pydantic/_internal/_model_construction.py", line 517, in complete_model_class
    schema = cls.__get_pydantic_core_schema__(cls, handler)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/soggy/.pyenv/versions/pyt/lib/python3.12/site-packages/pydantic/main.py", line 584, in __get_pydantic_core_schema__
    return __handler(__source)
           ^^^^^^^^^^^^^^^^^^^
  File "/Users/soggy/.pyenv/versions/pyt/lib/python3.12/site-packages/pydantic/_internal/_schema_generation_shared.py", line 82, in __call__
    schema = self._handler(__source_type)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/soggy/.pyenv/versions/pyt/lib/python3.12/site-packages/pydantic/_internal/_generate_schema.py", line 501, in generate_schema
    schema = self._generate_schema(obj)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/soggy/.pyenv/versions/pyt/lib/python3.12/site-packages/pydantic/_internal/_generate_schema.py", line 739, in _generate_schema
    schema = self._post_process_generated_schema(self._ge
... (truncated) ...

Minimal Reproduction

repro.py

from pydantic import BaseModel, computed_field, ConfigDict, AliasGenerator, AliasChoices


class Box(BaseModel):
    model_config = ConfigDict(
        populate_by_name=True,
        alias_generator=AliasGenerator(
            validation_alias=lambda field_name: AliasChoices(
                field_name.lower(),
                field_name.upper(),
                field_name.title(),
                field_name.capitalize(),
            )
        ),
    )

    width: float
    height: float
    depth: float
    extra_field: str | None

    @computed_field
    def volume(self) -> float:
        return self.width * self.height * self.depth


if __name__ == "__main__":
    b = Box(WIDTH=1, Height=2, depth=3, Extra_Field="extra")
    print(b.model_dump())

Environment

Python: 3.12
Pydantic: 2

What Broke

Schema generation fails, leading to application crashes or incorrect behavior in production.

Why It Broke

AliasGenerator does not return a string when used with computed_field decorators, causing schema generation errors

Fix Options (Details)

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

pip install pydantic==2.6.2

When NOT to use: Do not use this fix if the alias_generator is expected to return non-string values.

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/8806

First fixed release: 2.6.2

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

When NOT to Use This Fix

Do not use this fix if the alias_generator is expected to return non-string values.

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?

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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
2.6.2	Fixed

Related Issues

No related fixes found.

Cluster: pydantic:configuration-error Pydantic hub Pydantic best practices All hubs All clusters

Related clusters: Data consistency OpenAPI schema Serialization

Sources

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

Pydantic AliasGenerator causes computed_field to throw an error when generating schema (Fix)