Pydantic V2.10.1 custom model_serializer causes problems

The Fix

pip install pydantic==2.10.2

Based on closed pydantic/pydantic issue #10965 · 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

@@ -10,7 +10,7 @@
 from typing import TYPE_CHECKING, Any, ClassVar
 
-from pydantic_core import MultiHostHost, PydanticCustomError, core_schema
+from pydantic_core import MultiHostHost, PydanticCustomError, SchemaSerializer, core_schema
 from pydantic_core import MultiHostUrl as _CoreMultiHostUrl

repro.py

from pydantic import AnyHttpUrl, BaseModel, ConfigDict, model_serializer
from enum import Enum

class NewBaseModel(BaseModel):
    model_config = ConfigDict(extra="forbid")

    @model_serializer
    def serialize_model(self) -> dict:
        data = {}

        for field_name, field_value in self:
            data[field_name] = field_value

            if isinstance(field_value, Enum):
                data[field_name] = field_value.name

        return data

class Test(NewBaseModel):
    url: AnyHttpUrl

t = Test(url="http://example.com")

t.model_dump_json()

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==2.10.2\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: return self.__pydantic_serializer__.to_json(
Mechanism: Fixed serialization issues with custom model serializers in Pydantic 2.10.1, allowing models to be dumped to JSON correctly.
Why the fix works: Fixed serialization issues with custom model serializers in Pydantic 2.10.1, allowing models to be dumped to JSON correctly. (first fixed release: 2.10.2).

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 3.8 in real deployments (not just unit tests).
Surfaces as: Traceback (most recent call last):

Proof / Evidence

GitHub issue: #10965
Fix PR: https://github.com/pydantic/pydantic/pull/10947
First fixed release: 2.10.2
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.44

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: 2.10.2
Mode: fixed_only
Outcome: ok

Logs

affected (exit=None)

fixed (exit=0)

Discussion

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

Jump to Sources Open on GitHub

“Hi! Thanks for the question. This was fixed via https://github.com/pydantic/pydantic/pull/10947 and will be fixed in v2.10.2 this afternoon!”

@sydney-runkle · 2024-11-25 · confirmation · source

“I am still getting the same error after I re-ran my unit tests for v2.10.1”

@rashmi-asml · 2024-11-26 · source

Failure Signature (Search String)

return self.__pydantic_serializer__.to_json(

Error Message

Stack trace

error.txt

Error Message
-------------
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "CENSORED/.venv/lib/python3.8/site-packages/pydantic/main.py", line 477, in model_dump_json
    return self.__pydantic_serializer__.to_json(
pydantic_core._pydantic_core.PydanticSerializationError: Unable to serialize unknown type: <class 'pydantic.networks.AnyHttpUrl'>

Minimal Reproduction

repro.py

from pydantic import AnyHttpUrl, BaseModel, ConfigDict, model_serializer
from enum import Enum

class NewBaseModel(BaseModel):
    model_config = ConfigDict(extra="forbid")

    @model_serializer
    def serialize_model(self) -> dict:
        data = {}

        for field_name, field_value in self:
            data[field_name] = field_value

            if isinstance(field_value, Enum):
                data[field_name] = field_value.name

        return data

class Test(NewBaseModel):
    url: AnyHttpUrl

t = Test(url="http://example.com")

t.model_dump_json()

Environment

Python: 3.8
Pydantic: 2

What Broke

Models could not be dumped to JSON, leading to serialization errors in production.

Fix Options (Details)

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

pip install pydantic==2.10.2

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.

Fix reference: https://github.com/pydantic/pydantic/pull/10947

First fixed release: 2.10.2

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
2.10.2	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 V2.10.1 custom model_serializer causes problems with dumping model to json (Fix)