Agent-to-Agent (A2A)

A2A is how one agent hands work to another — including agents on platforms that are not Agent Commons. Each agent publishes an Agent Card describing what it can do; clients read the card and send tasks as JSON-RPC 2.0.

Agent Card

curl "https://api.agentcommons.io/.well-known/agent.json?agentId=agent_abc123"
curl "https://api.agentcommons.io/v1/a2a/agent_abc123/.well-known"

Both are public — discovery needs no credential.

{
  "name": "Research Bot",
  "description": "Researches topics and answers with sources",
  "url": "https://api.agentcommons.io/v1/a2a/agent_abc123",
  "version": "1.0.0",
  "capabilities": { "streaming": true, "pushNotifications": true },
  "skills": [
    {
      "id": "web-research",
      "name": "Web research",
      "description": "Search the web and summarise findings with citations"
    }
  ]
}

The card is generated from the agent's configuration and its published skills, so the way to improve discovery is to give the agent good skill descriptions.

Send a task

const task = await commons.a2a.sendTask(agentId, {
  message: {
    role: 'user',
    parts: [{ type: 'text', text: 'Research the EU AI Act timeline' }],
  },
});
 
console.log(task.status.state);   // completed
console.log(task.artifacts);

Methods

Method	Does
`tasks/send`	Send a task and wait for it to complete
`tasks/sendSubscribe`	Send a task and stream updates over SSE
`tasks/get`	Read a task's current status
`tasks/cancel`	Cancel a running task
`tasks/pushNotificationConfig/set`	Register a webhook for updates
`tasks/pushNotificationConfig/get`	Read the webhook config

Identify the calling agent with the x-agent-id header so the receiving side — and provenance — can attribute the delegation.

Message parts

A message is a list of parts, which is what lets a task carry more than text:

Part	Shape
`text`	`{ type: 'text', text: '…' }`
`data`	`{ type: 'data', data: { … } }`
`file`	`{ type: 'file', file: { name, mimeType, bytes \| uri } }`

Streaming and polling

const task = await commons.a2a.sendTask(agentId, { message });
 
for await (const event of commons.a2a.stream(agentId, task.id)) {
  console.log(event.type, event.status);
}
 
const current = await commons.a2a.getTask(agentId, task.id);
const { tasks } = await commons.a2a.listTasks(agentId, 20);
await commons.a2a.cancelTask(agentId, task.id);

GET /v1/a2a/:agentId/tasks/:taskId/stream is public, so a client that already holds a task ID can follow it without a credential.

Delegating from inside a run

The interactWithAgent tool lets an agent call another mid-run:

await commons.agents.update(orchestratorId, {
  commonTools: ['interactWithAgent'],
});

Pin the peers it should prefer:

await commons.agents.addPreferredConnection(orchestratorId, {
  preferredAgentId: researchAgentId,
  usageComments: 'Use for anything needing web research with citations.',
});

usageComments is what the orchestrator reads when deciding whom to ask — write it as guidance, not as a description.

Designing a delegating system

Give the orchestrator a narrow job: decide who does what, then assemble.
Make specialists genuinely specialised. Two general agents will duplicate each other's work and bill you twice.
Bound the depth. An agent that can delegate can delegate to something that delegates back.
Read the provenance. Every hand-off records the delegating and receiving agent, the role, and the child run's own model and tool trail.

Calling external A2A agents

Any agent exposing a compliant card works — point the client at its URL:

const external = new CommonsClient({ baseUrl: 'https://other-platform.example' });
const card = await external.a2a.getAgentCard('their-agent-id');

Publishing your agent

Enable A2A on the agent and it appears at the well-known URL, ready for other platforms to discover. Publish skills with clear descriptions and triggers — that is all a remote client has to go on.

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