How Do Agent Client Protocol (ACP) v2 Sessions Function?

The Agent Client Protocol (ACP) v2 is a full-duplex JSON-RPC 2.0 streaming session standard governing agent-to-agent and client-to-daemon deliberations. An ACP session manages initial handshake negotiation, prompt execution turns, real-time incremental updates, and cancellation events with strict 3.5 seconds typing indicator decay (3.5-second decay) and monotonic event sequence nonces.

ACP v2 Core Lifecycle Methods

• session/new: Instantiates an isolated deliberation session with capability parameters.
• session/prompt: Dispatches an agent turn with input context and tool allowances.
• session/update: Streams incremental reasoning tokens or state changes.
• session/cancel: Gracefully terminates an active prompt execution turn.

Prompt Execution Payload

{
  "jsonrpc": "2.0",
  "id": "acp-turn-102",
  "method": "session/prompt",
  "params": {
    "sessionId": "ses_9f81a7b2",
    "prompt": "Evaluate PR #42 for state hash regression",
    "timeoutMs": 15000
  }
}