OpenAI/Blog/25/08/OpenAI Harmony Response Fromat

• OpenAI Harmony Response Format

  • The gpt-oss models were trained on the harmony response format for defining conversation structures, generating reasoning output and structuring function calls.
  • The format is designed to mimic the OpenAI Responses API, so if you have used that API before, this format should hopefully feel familiar to you.

  • Roles

    
    Every message that the model processes has a role associated with it. The model knows about five types of roles:
    
    

    Role	Purpose
    system	A system message is used to specify reasoning effort, meta information like knowledge cutoff and built-in tools
    developer	The developer message is used to provide information about the instructions for the model (what is normally considered the “system prompt”) and available function tools
    user	Typically representing the input to the model
    assistant	Output by the model which can either be a tool call or a message output. The output might also be associated with a particular “channel” identifying what the intent of the message is.
    tool	Messages representing the output of a tool call. The specific tool name will be used as the role inside a message.

    
    These roles also represent the information hierarchy that the model applies in case there are any instruction conflicts: system > developer > user > assistant > tool