[BUG] Bug Report / Feature Request: Improve oneOf Handling for FastAPI/Pydantic

I am facing a similar issue:

🐞 Bug Report / Feature Request: Improve oneOf Handling for FastAPI/Pydantic
Summary:
The current OpenAPI Generator implementation for Python (FastAPI + Pydantic) generates wrapper classes for oneOf schemas (e.g., CaseEmbeddingsInputPayloadCaseInformation) that rely on a field like actual_instance and custom deserialization logic. However, this structure is incompatible with FastAPI's request parsing flow and Pydantic's native validation pipeline, leading to broken or non-functional schema resolution.

{
  "title": "Case Embeddings Input",
  "type": "object",
  "required": ["case_information"],
  "properties": {
    "case_information": {
      "oneOf": [
        {
          "$ref": "./case_information_with_case_number.json"
        },
        {
          "$ref": "./case_information_with_id_v2.json"
        }
      ]
    }
  }
}

and my individual schemas are:

{
  "title": "Case Information with case number",
  "type": "object",
  "required": [
    "partner_id",
    "case_number",
    "case_category",
    "case_description"
  ],
  "properties": {
    "partner_id": {
      "type": "string",
      "minLength": 1,
      "maxLength": 30,
      "description": "Partner Id"
    },
    "case_number": {
      "type": "string",
      "minLength": 1,
      "maxLength": 30,
      "description": "Case Id"
    },
    "id": {
      "type": "string",
      "minLength": 1,
      "maxLength": 30,
      "description": "Case Id",
      "nullable": true
    },
    "case_category": {
      "type": "string",
      "minLength": 1,
      "maxLength": 5000,
      "description": "Case Category"
    },
    "case_description": {
      "type": "string",
      "minLength": 1,
      "maxLength": 5000,
      "description": "Case Description"
    },
    "case_language_identifier": {
      "type": "string",
      "minLength": 1,
      "maxLength": 10,
      "description": "Case Language Identifier"
    }
  }
}

Schema 2

{
  "title": "Case Information with id",
  "type": "object",
  "required": [
    "partner_id",
    "id",
    "case_category",
    "case_description"
  ],
  "properties": {
    "partner_id": {
      "type": "string",
      "minLength": 1,
      "maxLength": 30,
      "description": "Partner Id"
    },
    "case_number": {
      "type": "string",
      "minLength": 1,
      "maxLength": 30,
      "description": "Case Id",
      "nullable": true
    },
    "id": {
      "type": "string",
      "minLength": 1,
      "maxLength": 30,
      "description": "Case Id"
    },
    "case_category": {
      "type": "string",
      "minLength": 1,
      "maxLength": 5000,
      "description": "Case Category"
    },
    "case_description": {
      "type": "string",
      "minLength": 1,
      "maxLength": 5000,
      "description": "Case Description"
    },
    "case_language_identifier": {
      "type": "string",
      "minLength": 1,
      "maxLength": 10,
      "description": "Case Language Identifier"
    }
  }
}

which produces

class CaseEmbeddingsInputPayload(BaseModel):
    case_information: Optional[CaseEmbeddingsInputPayloadCaseInformation] = None

Where CaseEmbeddingsInputPayloadCaseInformation wraps the oneOf logic using an actual_instance field and custom init() or from_json() methods.
However:

• FastAPI does not invoke init() or from_json() during request parsing.
• The payload is passed as keyword arguments, which do not match the expected structure.
• As a result, actual_instance is never set, and validation fails silently. unless we manually set {"actual_instance": **kwargs"} in the init def.
• The input ends up being None, even though it matches one of the schemas.

Instead of generating a wrapper class, emit a native Python Union type:

class CaseEmbeddingsInputPayload(BaseModel):
    case_information: Union[CaseInformationWithCaseNumber, CaseInformationWithIdV2]

which works.

Originally posted by @lek18 in #21892

No repository files or tests are named. Start by tracing the generated CaseEmbeddingsInputPayloadCaseInformation wrapper, especially actual_instance, init(), and from_json(), alongside the FastAPI request parsing path. Done means oneOf fields are generated as native Python Union types and valid requests pass Pydantic validation without manual actual_instance handling.