OpenAPI schema generation incorrectly marks nullable required fields as not required

[cofin]

Summary

Fields declared as field: int | None (nullable, no default) are not included in the OpenAPI schema's required array. Litestar conflates "nullable" (can be None) with "optional" (has a default), causing the generated schema to diverge from both Pydantic's own model_json_schema() output and the OpenAPI 3.1.0 specification.

Reproduction

from pydantic import BaseModel
from litestar import Litestar, get


class TestSchema(BaseModel):
    required_non_nullable_field: int
    required_nullable_field: int | None
    optional_nullable_field: int | None = None


@get("/")
def get_test() -> TestSchema:
    return TestSchema(required_non_nullable_field=2, required_nullable_field=1, optional_nullable_field=3)


app = Litestar(route_handlers=[get_test])

Expected required array (matches Pydantic's model_json_schema()):

"required": ["required_non_nullable_field", "required_nullable_field"]

Actual required array from Litestar's /schema/openapi.json:

"required": ["required_non_nullable_field"]

Root Cause

The bug exists in four locations that each independently conflate nullability with optionality:

1. litestar/typing.py:242 — FieldDefinition.is_required returns not self.is_optional and ..., but is_optional (line 289) returns True whenever NoneType is in the type args — i.e., whenever the type is nullable, regardless of whether a default exists.

2. litestar/_openapi/schema_generation/plugins/dataclass.py:32 — The required-field comprehension includes not is_optional_union(type_hints[field.name]), which excludes nullable fields even when they have no default.

3. litestar/_openapi/schema_generation/plugins/struct.py:48 — Same not is_optional_union(...) pattern as the dataclass plugin.

4. litestar/plugins/pydantic/utils.py:251 — Only wraps fields in NotRequired when default_factory is set, but the downstream FieldDefinition.is_required check (used at plugins/pydantic/plugins/schema.py:175) still fails due to issue Fix KeyError raised when annotating a view with return annotations #1.

Affected Model Types

All model types are affected: Pydantic (v1/v2), dataclasses, and msgspec Structs.

Versions

Confirmed on 2.16.0 and 2.21.0.

[cofin]

Related issues

• Bug: SchemaCreator.for_optional_field drops Meta() constraints and examples for Annotated optional fields #4650 — SchemaCreator.for_optional_field drops Meta() constraints for Annotated optional fields. Shares the same root cause in how nullable/optional types are handled in schema generation.
• Bug: Incorrect analysis of annotations in pydantic #4501 — Incorrect analysis of annotations in Pydantic unions. Reports the inverse symptom (a field with default=None incorrectly marked as required) due to the same is_optional logic in litestar/typing.py.
• Bug: OpenAPI schema generation for msgspec.Structs is woefully lacking #4575 — Broader msgspec.Struct OpenAPI schema generation issues, including the is_optional_union guard in struct.py identified here.