Why can't I create messages with role 'assistant' using the Assistants API?

Based on the current documentation and implementation details, the Assistants API (using methods like client.beta.threads.messages.create) is built to accept new user messages rather than allow creating messages with role "assistant" directly. In other words, it’s not possible to inject assistant messages as historical context using that endpoint.

For scenarios where you need to include previous message history (including assistant messages) as part of a conversation, you have a couple of approaches:

• Client-Side History Management: Maintain the complete conversation history (both user and assistant messages) within your application state. When sending a new message using the API (as a user message), you can include this history in your internal logic so that the UI reflects the entire conversation.

• Using Chat API for Context: The chat API (see sendMessage and thread state management functions) is designed to handle conversation history. With this approach, you would create or update the thread state to include historical messages. This way, the context is preserved even if the Assistants API itself only accepts user messages when sending a new message.

So, if your use case requires sending a full conversation history (including assistant messages), it’s recommended to rely on the chat API where you build and manage the thread state rather than trying to directly create an assistant message via client.beta.threads.messages.create.

This design choice aligns with the intended flow of a conversation—the assistant responses are meant to be generated by the system in reaction to user input, while the historical context should be maintained as part of your application’s state or by using the appropriate API meant for chat history management.