One chat message — the role-tagged unit of conversation between the user, the assistant, and (in the case of tool calls) the tools.
Body of the message; markdown-flavoured plain text.
Base64-encoded image payloads for multimodal models.
Speaker role — typically "user", "assistant", "system", or "tool".
"user"
"assistant"
"system"
"tool"
Raw chain-of-thought emitted by the model; surfaced for debugging only.