Pydantic Serializing computed fields returning enums (Fix)

The Fix

pip install pydantic==1.10.18

Based on closed pydantic/pydantic issue #9394 · PR/commit linked

Jump to Verify Open PR/Commit

@@ -2201,6 +2201,7 @@ def _apply_field_serializers(
                 ref = typing.cast('str|None', schema.get('ref', None))
                 if ref is not None:
+                    self.defs.definitions[ref] = schema
                     schema = core_schema.definition_reference_schema(ref)

repro.py

from enum import Enum
from pydantic import BaseModel, computed_field, field_serializer

class MyEnum(Enum):
    A = "a"
    B = "b"

class MyModel(BaseModel):
    
    @computed_field
    @property
    def computed_a_or_b(self) -> MyEnum:
        return MyEnum.B

    @field_serializer("computed_a_or_b")
    def serialize_my_enum(self, a_or_b: MyEnum) -> str:
        return a_or_b.value


if __name__ == "__main__":
    m = MyModel()
    assert m.model_dump()['computed_a_or_b'] == "b"

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 serializer behavior is expected to remain unchanged.\n\nOption C — Workaround\nis to annotate the enum with a WrapSerializer.\nWhen NOT to use: This fix should not be applied if the serializer behavior is expected to remain unchanged.\n\n

Why This Fix Works in Production

Trigger: KeyError: '__main__.MyEnum:5602143824'
Mechanism: A key error occurs due to an internal schema generation bug with custom serializers for computed fields returning enums
Why the fix works: Fixes a key error that occurs when using a custom serializer for computed fields returning enums. (first fixed release: 1.10.18).

Production impact:

If left unfixed, this can cause silent data inconsistencies that propagate (bad cache entries, incorrect downstream decisions).

Why This Breaks in Prod

Shows up under Python 3.12 in real deployments (not just unit tests).
A key error occurs due to an internal schema generation bug with custom serializers for computed fields returning enums
Surfaces as: KeyError: '__main__.MyEnum:5602143824'

Proof / Evidence

GitHub issue: #9394
Fix PR: https://github.com/pydantic/pydantic/pull/10200
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

“FWIW: I don't use the Sequence as well (as mentioned above), and it seems like it has been introduced after I added the SkipValidation to…”

@resurtm · 2024-08-12 · source

“One other observation to note, if I add a static field to the model with the same enum type, the field_serializer on the computed field…”

@graydenshand · 2024-05-05 · source

“Another workaround is to annotate the enum with a WrapSerializer.”

@graydenshand · 2024-05-05 · source

“@graydenshand, Thanks for reporting. Adding to our 2.7 issues milestone. Looks similar (at least in terms of the error) to https://github.com/pydantic/pydantic/issues/9319.”

@sydney-runkle · 2024-05-14 · source

Failure Signature (Search String)

KeyError: '__main__.MyEnum:5602143824'

Error Message

Stack trace

error.txt

Error Message
-------------
KeyError: '__main__.MyEnum:5602143824'

Minimal Reproduction

repro.py

from enum import Enum
from pydantic import BaseModel, computed_field, field_serializer

class MyEnum(Enum):
    A = "a"
    B = "b"

class MyModel(BaseModel):
    
    @computed_field
    @property
    def computed_a_or_b(self) -> MyEnum:
        return MyEnum.B

    @field_serializer("computed_a_or_b")
    def serialize_my_enum(self, a_or_b: MyEnum) -> str:
        return a_or_b.value


if __name__ == "__main__":
    m = MyModel()
    assert m.model_dump()['computed_a_or_b'] == "b"

Environment

Python: 3.12
Pydantic: 2

What Broke

Serialization of computed fields fails, causing application errors and potential data handling issues.

Why It Broke

A key error occurs due to an internal schema generation bug with custom serializers for computed fields returning enums

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 serializer behavior is expected to remain unchanged.

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

Option C — Workaround Temporary workaround

is to annotate the enum with a WrapSerializer.

When NOT to use: This fix should not be applied if the serializer behavior is expected to remain unchanged.

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

Fix reference: https://github.com/pydantic/pydantic/pull/10200

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 serializer behavior is expected to remain unchanged.

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:data-consistency Pydantic hub Pydantic best practices All hubs All clusters

Related clusters: Configuration error OpenAPI schema Serialization

Sources

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