AI EngineerGuide

stop isn’t supported on o3 and o4-mini

Rejected Applies to o3, o3-*, o4-mini, o4-mini-* on the OpenAI Chat Completions.

OpenAI's docs describe this error but don't quote the exact message, so we don't either.

How to fix it

Remove stop and end the output with instructions or post-processing.

Before
{
  "messages": [
    {
      "content": "Summarize this ticket.",
      "role": "user"
    }
  ],
  "model": "gpt-4o-2024-08-06",
  "stop": [
    "\n\n"
  ]
}
After (o3)
{
  "messages": [
    {
      "content": "Summarize this ticket.",
      "role": "user"
    }
  ],
  "model": "o3"
}
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Official source

Not supported with latest reasoning models o3 and o4-mini.

https://developers.openai.com/api/reference/resources/chat/subresources/completions/methods/create (last checked 2026-11-02)

All migration errors

FAQ

How do I fix this when moving to o3, o3-*, o4-mini, o4-mini-*?

Remove stop and end the output with instructions or post-processing.

Where is this documented?

In OpenAI’s official docs: https://developers.openai.com/api/reference/resources/chat/subresources/completions/methods/create, last checked 2026-11-02.