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# Agent2Agent (A2A)

Go Micro speaks the [Agent2Agent (A2A) protocol](https://a2a-protocol.org) — the open standard for agents on different frameworks to discover and call each other over HTTP. The A2A gateway is the agent-side analogue of the [MCP gateway](../mcp.html): MCP exposes your services as tools, A2A exposes your agents as agents.

There is nothing to add to an agent. An agent already registers in the registry with `type=agent` metadata; the gateway discovers it, generates an **Agent Card** from that metadata, and translates incoming A2A tasks to the agent's existing `Agent.Chat` RPC — the same call `delegate` and flows use.

## Run it

```bash
micro a2a serve --address :4000 --base_url https://agents.example.com
micro a2a list     # agents and their Agent Card URLs
```

Or embed the gateway next to a service:

```go
go a2a.Serve(a2a.Options{
    Registry: service.Options().Registry,
    Address:  ":4000",
    BaseURL:  "https://agents.example.com",
})
```

## Gateway, or directly on the agent

A2A is JSON-RPC over HTTP — a different wire protocol from go-micro's RPC — so *something* always translates between the two. That something doesn't have to be a separate process. There are two ways to run it:

- **A gateway** (above) fronts every agent in the registry behind one endpoint. Use it for a single front door, centralized discovery, and shared policy.
- **Directly on the agent.** `AgentA2A(addr)` makes the agent serve its own A2A endpoint when it runs — no separate gateway, and the task is handled in-process (no extra RPC hop):

  ```go
  agent := micro.NewAgent("task-mgr",
      micro.AgentServices("task"),
      micro.AgentProvider("anthropic"),
      micro.AgentA2A(":4000"),   // also reachable at http://host:4000 over A2A
  )
  agent.Run()
  ```

  The agent stays a normal go-micro service; this adds a second, A2A-native HTTP endpoint. Now any A2A client can `curl` it directly. Use it when each agent should be independently addressable without a gateway.

Both reuse the same handler; the only difference is whether the agent is reached over RPC (gateway) or in-process (embedded).

## Discovery: cards from the registry

Every registered agent gets an Agent Card, generated from its registry metadata (name, the services it manages). Cards are not published by the agent — they are derived, the same way MCP tools are derived from service endpoints.

| Endpoint | Returns |
|---|---|
| `GET /agents` | a directory of all Agent Cards |
| `GET /agents/{name}` | one agent's card |
| `GET /agents/{name}/.well-known/agent.json` | one agent's card (well-known path) |
| `POST /agents/{name}` | the agent's JSON-RPC endpoint |
| `GET /.well-known/agent.json` | the single agent's card, when exactly one is registered |

A card looks like:

```json
{
  "name": "task-mgr",
  "description": "Go Micro agent managing: task,project",
  "url": "https://agents.example.com/agents/task-mgr",
  "version": "1.0.0",
  "protocolVersion": "0.3.0",
  "capabilities": { "streaming": true, "pushNotifications": true },
  "defaultInputModes": ["text/plain"],
  "defaultOutputModes": ["text/plain"],
  "skills": [
    { "id": "task", "name": "Task", "tags": ["task"] },
    { "id": "project", "name": "Project", "tags": ["project"] }
  ]
}
```

Each managed service is advertised as its own typed skill. Clients can call the
whole agent at `/agents/task-mgr`, or address one skill directly at
`/agents/task-mgr/skills/task`; the skill endpoint serves a focused card and
routes the request to the same agent with that skill selected.

## Calling an agent

A2A uses JSON-RPC 2.0 over HTTP. Send a message with `message/send`; the gateway runs the agent and returns a completed `Task`:

```bash
curl -s https://agents.example.com/agents/task-mgr \
  -H 'content-type: application/json' \
  -d '{
    "jsonrpc": "2.0", "id": 1, "method": "message/send",
    "params": { "message": {
      "role": "user", "kind": "message", "messageId": "m1",
      "parts": [{ "kind": "text", "text": "What tasks are overdue?" }]
    }}
  }'
```

```json
{
  "jsonrpc": "2.0", "id": 1,
  "result": {
    "id": "…", "contextId": "…", "kind": "task",
    "status": { "state": "completed", "timestamp": "…" },
    "artifacts": [{ "artifactId": "…", "parts": [{ "kind": "text", "text": "Two: …" }] }]
  }
}
```

Retrieve a task later with `tasks/get` (`params: { "id": "…" }`). To continue
the same piece of work, send another `message/send` with the previous `taskId`
and `contextId`. The gateway preserves the task id, context id, and prior
history, then appends the new user turn and agent reply. That makes a remote
A2A task fit the Go Micro lifecycle: services are still invoked through the
agent's normal tools, the agent keeps task context across turns, and a workflow
can poll one task id as the conversation progresses.

## Push notifications

Operators can register a task callback with
`tasks/pushNotificationConfig/set`:

```bash
curl -s https://agents.example.com/agents/task-mgr \
  -H 'content-type: application/json' \
  -d '{
    "jsonrpc": "2.0", "id": 2,
    "method": "tasks/pushNotificationConfig/set",
    "params": {
      "id": "task-id",
      "pushNotificationConfig": {
        "url": "https://workflow.example.com/a2a/tasks",
        "token": "optional-bearer-token"
      }
    }
  }'
```

The gateway stores one callback per retained task and POSTs the latest task
snapshot to that URL whenever the task changes. Delivery is best effort: failures
do not fail the agent turn, and there is no retry queue in the in-memory gateway.
Use `tasks/get` as the source of truth after a missed callback or receiver
outage. If a token is configured, it is sent as `Authorization: Bearer <token>`.

## Calling out to other agents

The gateway makes your agents reachable *from* the A2A ecosystem. The
client (`a2a.Client`) is the other direction: it lets a Go Micro agent or
flow call an agent on any framework, by URL.

```go
reply, err := a2a.NewClient("https://other.example.com/agents/research").
    Send(ctx, "Summarize the latest on X")
```

It's wired into the two places that hand off work:

- **A flow step** — `flow.A2A(url)` is the cross-framework counterpart to
  `flow.Dispatch(name)` (which dispatches to a local agent):

  ```go
  flow.Step{Name: "research", Run: flow.A2A("https://other.example.com/agents/research")}
  ```

- **Agent delegate** — when an agent's `delegate` target is an `http(s)`
  URL, the subtask is sent to that external agent over A2A instead of to a
  locally registered one. Nothing else changes; the model just delegates
  to a URL.

`Send` handles the task lifecycle: if the remote returns a task that isn't
yet terminal, it polls `tasks/get` until it completes.

## Scope

This is the JSON-RPC binding for task execution:

- **`message/send`** runs the agent and returns a completed `Task`.
- **`message/stream`** streams the completed `Task` as an SSE `data:` event, giving A2A clients a streaming-compatible path while the underlying agent call remains synchronous.
- **`tasks/get`** returns a recent task by id.
- **Multi-turn continuation** keeps task state when a new message includes the previous `taskId`.
- **`tasks/pushNotificationConfig/set` / `get`** stores and reads a task callback for best-effort update delivery.
- **`tasks/resubscribe`** reconnects to an existing task stream, immediately emits the current task snapshot, then streams subsequent updates until the task reaches a terminal state.
- **`input-required`** task state carries human-input handoffs (for example checkpointed approval pauses) in task status, artifacts, and history; continue the task by sending a follow-up message with the same `taskId` and `contextId`.
- **Agent Card** discovery, generated from the registry.

Both directions work: the gateway exposes your agents, and `a2a.Client` (via `flow.A2A` or `delegate` to a URL) calls external ones. The task binding is what makes a Go Micro agent both reachable from, and able to reach, the A2A ecosystem today.

## AP2 mandate layer (opt-in)

AP2 sits above A2A as a verifiable-intent and audit layer. Go Micro keeps the
A2A envelope separate from payment settlement: an A2A message can carry signed
AP2 checkout or payment mandates, and the resulting task can retain the stable
mandate reference plus verification result. Payment settlement state remains in
the payment rail. For x402, use an AP2 payment mandate with an `x402` rail
reference to name the payment requirement; the existing x402 facilitator still
performs verification and settlement.

## See also

- [MCP & AI Agents](../mcp.html) — exposing services as tools
- [Agents and Workflows](agents-and-workflows.html) — the agent model
- [A2A protocol specification](https://a2a-protocol.org)