Pydantic `model_json_schema()` fails to include annotated

The Fix

pip install pydantic==1.10.16

Based on closed pydantic/pydantic issue #9525 · 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

@@ -1053,6 +1053,19 @@ def default_schema(self, schema: core_schema.WithDefaultSchema) -> JsonSchemaVal
         #     return json_schema
 
+        # we reflect the application of custom plain, no-info serializers to defaults for
+        # json schemas viewed in serialization mode
+        # TODO: improvements along with https://github.com/pydantic/pydantic/issues/8208

repro.py

from typing import Annotated, Any

from pydantic import BaseModel, model_validator
from pydantic.functional_validators import ModelWrapValidatorHandler
from typing_extensions import Self

# Pretend this is some third-party class
# we can't modify directly...
class Quantity:
    def __init__(self, value: float, unit: str):
        self.value = value
        self.unit = unit

class QuantityAnnotations(BaseModel):
    value: float
    unit: str

    @model_validator(mode="wrap")
    def _validate(value: Any, handler: ModelWrapValidatorHandler[Self]) -> Quantity:
        if isinstance(value, Quantity):
            return value
        validated = handler(value)

        if isinstance(validated, Quantity):
            return validated

        return Quantity(**dict(validated))


QuantityType = Annotated[Quantity, QuantityAnnotations]

class OurModel(BaseModel):
    quantity: QuantityType = Quantity(value=0.0, unit='m')

model_instance = OurModel()
# This works fine...
print(model_instance.model_dump_json())
# {"quantity":{"value":0.0,"unit":"m"}}

# But this throws a warning about being unable to JSON serialize the
# same default value it just dumped successfully above.
OurModel.model_json_schema()
# ...lib/python3.10/site-packages/pydantic/json_schema.py:2158: PydanticJsonSchemaWarning:
# Default value <__main__.Quantity object at 0x75fcccab1960> is not JSON serializable;
# excluding default from JSON schema [non-serializable-default]

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==1.10.16\nWhen NOT to use: This fix should not be used if the model relies on non-serializable default values.\n\nOption C — Workaround\nthat lets you customize your schema for such types, for now. I've slimmed down an example below from actual library code I've written. In this example, I want the schema to have defaults for `pathlib.Path` objects that look like the `pathlib.Path.as_posix()` stringified output of that path. Never mind that round-tripping of string paths back from a file isn't shown in the code example.\nWhen NOT to use: This fix should not be used if the model relies on non-serializable default values.\n\n

Why This Fix Works in Production

Trigger: `model_json_schema()` fails to include annotated third-party types: "Default value is not JSON serializable [non-serializable-default]"
Mechanism: The model_json_schema() function fails to serialize default values of annotated third-party types
Why the fix works: Addresses the issue where model_json_schema() fails to include annotated third-party types by applying custom serializers to defaults for JSON schemas viewed in serialization mode. (first fixed release: 1.10.16).

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 3.10 in real deployments (not just unit tests).
The model_json_schema() function fails to serialize default values of annotated third-party types
Production symptom (often without a traceback): `model_json_schema()` fails to include annotated third-party types: "Default value is not JSON serializable [non-serializable-default]"

Proof / Evidence

GitHub issue: #9525
Fix PR: https://github.com/pydantic/pydantic/pull/9624
First fixed release: 1.10.16
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.48

Discussion

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

Jump to Sources Open on GitHub

“astropy.Quantity in my case, but yes, the situation is similar. Users prefer/expect to have vanilla quantity objects, not unusual subclasses or annotated types.”

@bjmc · 2024-10-09 · source

“My suspicion here is that core schema logic supplied by annotated metadata isn't being used during json schema generation, but I haven't looked into the…”

@sydney-runkle · 2024-06-19 · source

“@blakeNaccarato If understand your code correctly, you're silencing/ignoring the "non-serializable-default" warning, and then modifying the schema directly to re-add a stringified version of the pathlib.Path…”

@bjmc · 2024-10-09 · source

“You are using Pydantic models as annotated metadata to validate a third-party type, which isn't a supported pattern”

@Viicos · 2025-04-08 · source

Failure Signature (Search String)

`model_json_schema()` fails to include annotated third-party types: "Default value is not JSON serializable [non-serializable-default]"
... and the defaults are excluded from the resulting schema.

Copy-friendly signature

signature.txt

Failure Signature
-----------------
`model_json_schema()` fails to include annotated third-party types: "Default value is not JSON serializable [non-serializable-default]"
... and the defaults are excluded from the resulting schema.

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
`model_json_schema()` fails to include annotated third-party types: "Default value is not JSON serializable [non-serializable-default]"
... and the defaults are excluded from the resulting schema.

Minimal Reproduction

repro.py

from typing import Annotated, Any

from pydantic import BaseModel, model_validator
from pydantic.functional_validators import ModelWrapValidatorHandler
from typing_extensions import Self

# Pretend this is some third-party class
# we can't modify directly...
class Quantity:
    def __init__(self, value: float, unit: str):
        self.value = value
        self.unit = unit

class QuantityAnnotations(BaseModel):
    value: float
    unit: str

    @model_validator(mode="wrap")
    def _validate(value: Any, handler: ModelWrapValidatorHandler[Self]) -> Quantity:
        if isinstance(value, Quantity):
            return value
        validated = handler(value)

        if isinstance(validated, Quantity):
            return validated

        return Quantity(**dict(validated))


QuantityType = Annotated[Quantity, QuantityAnnotations]

class OurModel(BaseModel):
    quantity: QuantityType = Quantity(value=0.0, unit='m')

model_instance = OurModel()
# This works fine...
print(model_instance.model_dump_json())
# {"quantity":{"value":0.0,"unit":"m"}}

# But this throws a warning about being unable to JSON serialize the
# same default value it just dumped successfully above.
OurModel.model_json_schema()
# ...lib/python3.10/site-packages/pydantic/json_schema.py:2158: PydanticJsonSchemaWarning:
# Default value <__main__.Quantity object at 0x75fcccab1960> is not JSON serializable;
# excluding default from JSON schema [non-serializable-default]

Environment

Python: 3.10
Pydantic: 2

What Broke

Default values are excluded from the JSON schema, leading to incomplete schema generation.

Why It Broke

The model_json_schema() function fails to serialize default values of annotated third-party types

Fix Options (Details)

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

pip install pydantic==1.10.16

When NOT to use: This fix should not be used if the model relies on non-serializable default values.

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

Option C — Workaround Temporary workaround

that lets you customize your schema for such types, for now. I've slimmed down an example below from actual library code I've written. In this example, I want the schema to have defaults for `pathlib.Path` objects that look like the `pathlib.Path.as_posix()` stringified output of that path. Never mind that round-tripping of string paths back from a file isn't shown in the code example.

When NOT to use: This fix should not be used if the model relies on non-serializable default values.

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/9624

First fixed release: 1.10.16

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

When NOT to Use This Fix

This fix should not be used if the model relies on non-serializable default 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?

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
1.10.16	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 `model_json_schema()` fails to include annotated third-party types: "Default value is not (Fix)