Some implementations cant merge schemas defined in `allOf` property.

The following exists in the openapi schema:

response_format:
  allOf:
    - $ref: "#/components/schemas/AssistantsApiResponseFormatOption"
    - nullable: true

Which means merging the schemas in the list (detailed here).

In AssistantsApiResponseFormatOption:

AssistantsApiResponseFormatOption:
  description: >
    Specifies the format that the model must output. Compatible with
    [GPT-4o](/docs/models#gpt-4o), [GPT-4
    Turbo](/docs/models#gpt-4-turbo-and-gpt-4), and all GPT-3.5 Turbo models
    since `gpt-3.5-turbo-1106`.


    Setting to `{ "type": "json_schema", "json_schema": {...} }` enables
    Structured Outputs which ensures the model will match your supplied JSON
    schema. Learn more in the [Structured Outputs
    guide](/docs/guides/structured-outputs).


    Setting to `{ "type": "json_object" }` enables JSON mode, which ensures
    the message the model generates is valid JSON.


    **Important:** when using JSON mode, you **must** also instruct the
    model to produce JSON yourself via a system or user message. Without
    this, the model may generate an unending stream of whitespace until the
    generation reaches the token limit, resulting in a long-running and
    seemingly "stuck" request. Also note that the message content may be
    partially cut off if `finish_reason="length"`, which indicates the
    generation exceeded `max_tokens` or the conversation exceeded the max
    context length.
  oneOf:
    - type: string
      description: |
        `auto` is the default value
      enum:
        - auto
      x-stainless-const: true
    - $ref: "#/components/schemas/ResponseFormatText"
    - $ref: "#/components/schemas/ResponseFormatJsonObject"
    - $ref: "#/components/schemas/ResponseFormatJsonSchema"
  x-oaiExpandable: true

No nullable: true is defined.

This means, according to the openapi specification (detailed here) that the value of the nullable is false!

Implementations of openapi parsers that are strict to the specifications I linked to earlier, will throw an exception, because trying to merge schemas that have the same field with different values has undefined, unexpected behaviour.

I guess that in practice, many implementations decided to treat merging a schema which has a property defined and one which doesn't define it in an out-of-spec way, using the value from the schema with the value defined, and completely ignoring the one which didn't define any value. This breaks the specs I linked to earlier, and probably why nobody noticed it before.

Edit 1:
I tried it with oapi-codegen.
Here are my temp fixes:
https://github.com/lukacat10/kin-openapi/tree/nullable-fields
https://github.com/lukacat10/oapi-codegen/tree/nullable-fields

Edit 2:
Sort-of-related issues:
https://github.com/oapi-codegen/oapi-codegen/issues/1898
https://github.com/oapi-codegen/oapi-codegen/issues/1883

Start by locating response_format and AssistantsApiResponseFormatOption in the OpenAPI YAML and review how the allOf members combine nullable values. Validate the schema with a strict OpenAPI parser; done means implementations no longer reject this definition for conflicting or ambiguous nullable behavior.