Message

A Message is the fundamental communication unit in the A2A protocol used to transmit non-Artifact content between the client and the agent.

Purpose of Messages

Messages carry various types of information during the interaction process, including:

User Input: All requests, instructions, or contextual information from the client are sent as messages.
Agent Thinking: Intermediate steps, thought processes, or explanations from the agent while processing a task.
Status Updates: Information conveyed by the agent regarding task status changes.
Instructions: Requests or instructions issued by the agent to the client (e.g., requesting more information).
Error Information: Transmission of errors that occur during the interaction.
Metadata: Additional accompanying information.

Unlike Artifact, which focuses on the final result, Message is more concerned with the process and control flow of the interaction.

Message Structure

Role: The role of the message sender, typically user (client) or agent (agent).
Parts: A message can contain one or more Parts, each representing different types of content (e.g., text, file reference). This allows a single message to convey multiple types of information simultaneously, such as a user’s text description and related context files.
Metadata: An optional metadata field for carrying additional information.

Message Interface Definition

Below is an illustrative definition of the Message interface:

interface Message {
  role: "user" | "agent"; // Role of the message sender
  parts: Part[]; // Array of parts contained in the message
  metadata?: Record<string, any>; // Extended metadata
}

By using Message and Artifact, the A2A protocol clearly distinguishes between interaction process information and final results, making communication more structured and manageable.

Artifact Part