Pydantic Change JSON schema (and maybe config) default for

The Fix

pip install pydantic==2.12.0

Based on closed pydantic/pydantic issue #10785 · PR/commit linked

Production note: This usually shows up under retries/timeouts. Treat it as a side-effect risk until you can verify behavior with a canary + real traffic.

Jump to Verify Open PR/Commit

@@ -62,7 +62,12 @@
 from ..json_schema import JsonSchemaValue
 from ..version import version_short
-from ..warnings import ArbitraryTypeWarning, PydanticDeprecatedSince20, UnsupportedFieldAttributeWarning
+from ..warnings import (
+    ArbitraryTypeWarning,

repro.py

from pydantic import ValidationError, BaseModel
from devtools import debug
from pydantic.json_schema import GenerateJsonSchema
from pydantic_core import SchemaValidator, core_schema

class Model(BaseModel):
    a: int
    b: int


schema = core_schema.model_schema(
    Model,
    core_schema.model_fields_schema(
        {
            'a': core_schema.model_field(core_schema.int_schema()),
            'b': core_schema.model_field(core_schema.int_schema()),
        },
    ),
    extra_behavior='forbid',
)

json_schema = GenerateJsonSchema().generate(schema)
debug(json_schema)
"""
json_schema: {
        'properties': {
            'a': {
                'title': 'A',
                'type': 'integer',
            },
            'b': {
                'title': 'B',
                'type': 'integer',
            },
        },
        'required': ['a', 'b'],
        'title': 'Model',
        'type': 'object',
    }
"""

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 you require TypedDicts to allow additional properties.\n\n

Why This Fix Works in Production

Trigger: Change JSON schema (and maybe config) default for `extra` for `dataclass`es and `TypedDict`s
Mechanism: The default behavior for TypedDicts and dataclasses regarding additional properties in JSON schema was inconsistent
Why the fix works: Adds support for PEP 728 and modifies the default behavior for TypedDicts and dataclasses regarding additional properties in JSON schema. (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 default behavior for TypedDicts and dataclasses regarding additional properties in JSON schema was inconsistent
Production symptom (often without a traceback): Change JSON schema (and maybe config) default for `extra` for `dataclass`es and `TypedDict`s

Proof / Evidence

GitHub issue: #10785
Fix PR: https://github.com/pydantic/pydantic/pull/12179
First fixed release: 2.12.0
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.47

Discussion

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

Jump to Sources Open on GitHub

“@sydney-runkle I'd add that Doesn't solve the problem — with both 2.9 and 2.10 additionalProperties is missing.”

@samuelcolvin · 2024-11-07 · source

“@Viicos I bet you're aware, but PEP 728 has been accepted[^1] [^1]: https://discuss.python.org/t/pep-728-typeddict-with-typed-extra-items/45443/159”

@edgarrmondragon · 2025-08-20 · source

“In some ways, this is more consistent with models and dataclasses, where the extra_behavior spec on core_schema wasn't considered when generating json schema in previous…”

@sydney-runkle · 2024-11-07 · source

“We can chat about final behavior in our oss sync this morning 👍”

@sydney-runkle · 2024-11-07 · source

Failure Signature (Search String)

Change JSON schema (and maybe config) default for `extra` for `dataclass`es and `TypedDict`s
You'll see that `'additionalProperties': False` is included in `2.9.2`, but missing with `2.10.0b1`. I think this change comes from #10675.

Copy-friendly signature

signature.txt

Failure Signature
-----------------
Change JSON schema (and maybe config) default for `extra` for `dataclass`es and `TypedDict`s
You'll see that `'additionalProperties': False` is included in `2.9.2`, but missing with `2.10.0b1`. I think this change comes from #10675.

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
Change JSON schema (and maybe config) default for `extra` for `dataclass`es and `TypedDict`s
You'll see that `'additionalProperties': False` is included in `2.9.2`, but missing with `2.10.0b1`. I think this change comes from #10675.

Minimal Reproduction

repro.py

from pydantic import ValidationError, BaseModel
from devtools import debug
from pydantic.json_schema import GenerateJsonSchema
from pydantic_core import SchemaValidator, core_schema

class Model(BaseModel):
    a: int
    b: int


schema = core_schema.model_schema(
    Model,
    core_schema.model_fields_schema(
        {
            'a': core_schema.model_field(core_schema.int_schema()),
            'b': core_schema.model_field(core_schema.int_schema()),
        },
    ),
    extra_behavior='forbid',
)

json_schema = GenerateJsonSchema().generate(schema)
debug(json_schema)
"""
json_schema: {
        'properties': {
            'a': {
                'title': 'A',
                'type': 'integer',
            },
            'b': {
                'title': 'B',
                'type': 'integer',
            },
        },
        'required': ['a', 'b'],
        'title': 'Model',
        'type': 'object',
    }
"""

Environment

Pydantic: 2

What Broke

Users experienced unexpected validation errors when using TypedDicts with additional properties.

Why It Broke

The default behavior for TypedDicts and dataclasses regarding additional properties in JSON schema was inconsistent

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 you require TypedDicts to allow additional properties.

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/12179

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 you require TypedDicts to allow additional properties.

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.

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 Change JSON schema (and maybe config) default for `extra` for `dataclass`es and (Fix)