ChatCompletionMessageParam:
    | ChatCompletionDeveloperMessageParam
    | ChatCompletionSystemMessageParam
    | ChatCompletionUserMessageParam
    | ChatCompletionAssistantMessageParam
    | ChatCompletionToolMessageParam
    | ChatCompletionFunctionMessageParam

Developer-provided instructions that the model should follow, regardless of messages sent by the user. With o1 models and newer, developer messages replace the previous system messages.