a2a

A2A Plugin for Crush

The A2A (Agent-to-Agent) plugin lets Crush communicate with other AI agents using the A2A v1.0 protocol. It uses the official a2a-go SDK and has two halves:

• Server mode — Expose Crush as an A2A agent over JSON-RPC 2.0.
• Client mode — Give the LLM tools to discover and invoke remote A2A agents.

Both modes can run simultaneously. This plugin is mutually exclusive with the ACP plugin — use the crush-a2a build variant instead of crush-extended.

Quick Start

Expose Crush as an A2A server

CRUSH_A2A_PORT=8200 crush

Verify:

curl http://localhost:8200/.well-known/agent-card.json | jq .name
# "crush"

Call a remote A2A agent from Crush

Add server(s) to your crush.json:

{
  "options": {
    "plugins": {
      "a2a": {
        "servers": [
          {"name": "teammate", "url": "http://localhost:8201"}
        ]
      }
    }
  }
}

The LLM now has access to a2a_list_agents, a2a_send_message, a2a_get_task, and a2a_attach_file tools.

Server Mode

The a2a-server hook starts a JSON-RPC 2.0 server alongside the normal Crush TUI, implementing the full A2A v1.0 protocol.

Server Configuration

{
  "options": {
    "plugins": {
      "a2a-server": {
        "port": 8200,
        "agent_name": "crush",
        "description": "Crush AI coding assistant",
        "skills": [
          {
            "id": "code",
            "name": "Code Assistant",
            "description": "Write, review, and debug code",
            "tags": ["coding"]
          }
        ]
      }
    }
  }
}

Environment Variables

Agent Card

Served at /.well-known/agent-card.json with all A2A v1.0 required fields:

• name, version, description
• supportedInterfaces (JSON-RPC transport)
• capabilities (streaming: true)
• skills (configurable)
• defaultInputModes, defaultOutputModes

JSON-RPC Methods

Task Lifecycle

submitted → working → completed
                 ├──→ failed
                 └──→ canceled

Client Mode

Client Configuration

{
  "options": {
    "plugins": {
      "a2a": {
        "servers": [
          {"name": "local", "url": "http://localhost:8201"},
          {"name": "remote", "url": "https://agent.example.com", "headers": {"Authorization": "Bearer sk-xxx"}},
          {"name": "disabled", "url": "http://localhost:9000", "enabled": false}
        ],
        "default_timeout_seconds": 120
      }
    }
  }
}

LLM Tools

a2a_list_agents

Discover agents on configured servers.

a2a_send_message

Send a message to a remote A2A agent.

a2a_get_task

Get task status and artifacts.

a2a_attach_file

Attach a file as an artifact to the current A2A task response.

This tool only works when Crush is processing an incoming A2A request.

Multi-Agent Architecture

Hub and Spoke

                    ┌──────────────┐
                    │ Orchestrator │  (client mode)
                    └──────┬───────┘
                           │
              ┌────────────┼────────────┐
              │            │            │
       ┌──────▼─────┐ ┌───▼────────┐ ┌─▼───────────┐
       │ Code Writer │ │  Reviewer  │ │  Test Runner │
       │   :8200     │ │   :8201    │ │    :8202     │
       └─────────────┘ └────────────┘ └──────────────┘
       (server mode)   (server mode)   (server mode)

CRUSH_A2A_PORT=8200 CRUSH_A2A_AGENT_NAME=coder crush --cwd /path/to/project &
CRUSH_A2A_PORT=8201 CRUSH_A2A_AGENT_NAME=reviewer crush --cwd /path/to/project &
CRUSH_A2A_PORT=8202 CRUSH_A2A_AGENT_NAME=tester crush --cwd /path/to/project &

Spec Compliance

This plugin targets 100% compliance with the A2A v1.0 spec-torture test suite:

• 5 discovery tests (agent card)
• 5 lifecycle tests (SendMessage, GetTask, CancelTask)
• 3 messaging tests (text parts, context sharing)
• 2 streaming tests (SSE)
• 4 error handling tests (JSON-RPC error codes)
• 1 push notification test (unsupported)

Testing

Unit tests

cd a2a && go test -short ./...

Build the A2A distro

task distro:a2a

E2E tests

task distro:all
cd a2a && go test -v -tags e2e -run 'A2A'

ACP vs A2A