> ## Documentation Index
> Fetch the complete documentation index at: https://docs.auriko.ai/llms.txt
> Use this file to discover all available pages before exploring further.

# response_api_only

> HTTP 400 — Model is only available via the Response API.

## HTTP status

`400 invalid_request_error`

## Typical cause

The model is only served via the [Response API](/response-api/overview) — OpenAI pro-tier models such as `gpt-5.5-pro` — and the request uses `/v1/chat/completions` or `/v1/messages`.

`/v1/messages` returns this error in the [Anthropic error envelope](/api-reference/errors#anthropic-error-envelope), which carries the same message and `suggestion` but no `code` field.

## Resolution

Call `POST /v1/responses` with a Response API body (`input`, not `messages`):

```bash theme={null}
curl https://api.auriko.ai/v1/responses \
  -H "Authorization: Bearer $AURIKO_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{"model": "gpt-5.5-pro", "input": "Hello"}'
```

To check endpoint support before sending, read `supported_endpoints` from [`GET /v1/models?endpoint=responses`](/api-reference/list-callable-models) or from each provider entry in the [model directory](/api-reference/model-directory). If your integration only speaks Chat Completions, choose a chat-capable model instead.