messageId

messageId is the required field on every A2A Message that holds a unique identifier, such as a UUID, chosen by whoever creates the message: the client for its requests, the agent for its replies and status messages.

How it differs from other IDs.

Identifier	Set by	Identifies
messageId	The message’s creator	One message
taskId / task id	The agent, when it creates a task	One unit of work
contextId	The agent, unless it accepts client-provided values	A conversation that can span several tasks
artifactId	The agent	One output, unique within its task
JSON-RPC id	The client	One request and its response, at the transport level

Duplicates and retries. Section 3.3.1 says Send Message operations may be idempotent and that agents may use the messageId to detect duplicate messages. The specification does not require deduplication. A client that times out and resends the same message should keep the same messageId, so an agent that deduplicates can recognise the retry and avoid acting twice. A genuinely new message, including each answer to a TASK_STATE_INPUT_REQUIRED question, needs a new ID.

A continuation of an existing task, over the JSON-RPC binding, illustrative:

{
  "jsonrpc": "2.0",
  "id": 42,
  "method": "SendMessage",
  "params": {
    "message": {
      "messageId": "3b8f0c2e-5d1a-4f6b-9e7c-2a4d6f8b0c1e",
      "taskId": "5f1c2d9e-8b7a-4e3c-9d21-6a0b4c7e2f13",
      "role": "ROLE_USER",
      "parts": [{"text": "Use the warehouse address on file."}]
    }
  }
}

A note on examples. Some examples in the specification omit messageId on agent messages. The proto marks the field REQUIRED for every Message, and section 5.7 says implementations should reject messages that lack required fields. Set it on every message you send.

Neighbouring terms. A Message is the object that carries the ID, and a contextId groups messages and tasks into one conversation.