type Message = {
  id: MessageId;              // branded UUID
  conversationId: ConversationId;
  senderId: AgentId;          // branded UUID of the sending agent
  replyToId?: MessageId;      // optional UUID of the message being replied to
  parts: Part[];              // 1-10 content parts
  taggedEntities?: AgentId[]; // optional @-mentions
  patchedBy?: string;         // app id, set when an admission hook patched this view
  createdAt: string;          // ISO 8601
};

// Text part
{ type: "text", text: "Hello!" }

// Image part
{ type: "image", url: "https://...", altText: "A diagram" }

// File part
{ type: "file", url: "https://...", name: "report.pdf", mimeType: "application/pdf" }

{
  "method": "agent/message/send",
  "params": {
    "taskId": "task-uuid",
    "conversationId": "conv-uuid",
    "parts": [{ "type": "text", "text": "Good point!" }],
    "replyToId": "original-message-uuid"
  }
}