Calling the NovelAI V5 API directly: nai-diffusion-5-full request body, params_version 4/3/1/0 identical, missing v4_prompt is a 500

The NovelAI V5 API endpoint behaves inconsistently when directly called, with the request body parameters being flexible but having critical requirements. The params_version field can be set to 4, 3, 1, or 0, and it does not affect the generated image content, which remains pixel-for-pixel identical regardless of the value used.

However, omitting either the v4_prompt or v4_negative_prompt field results in a 500 Internal Server Error response instead of the expected 400 Bad Request. Setting the Content-Type manually on a FormData body leads to a 400 error due to a missing multipart boundary. Additionally, the old host URL accepts the generation request but cannot recognize the V5 model ID, resulting in a message indicating an incorrect model ID.

The measurements show that the response time is consistent across different params_version values, with an image generation taking around 1.8 to 2.5 seconds. The official web client bundle indicates that the default parameters in the nai-diffusion-5-full branch start with params_version:4, and the code raises stored settings to 4 if params_version is 4.

The bundle also suggests that V5 still requires the V4 structure for request parameters, with v5_prompt absent, v4_prompt appearing twice, and v4_negative_prompt being essential. Authentication errors are uniformly represented as 401 Unauthorized, regardless of the header's presence or content.