A2A Protocol (Agent-to-Agent)

Rule

Agents call other agents over A2A, a JSON-RPC protocol for discovery and delegation. Use it when work belongs to a different agent entirely — not the local agent chat.

No workarounds when A2A feels flaky. The strong default is ask_app (or call-agent) working reliably, full stop — not apps reaching around it. Do not have app A generate and execute raw SQL against app B's database, and do not expose B's internal tools directly to A as a substitute for delegation. The receiving agent has context, skills, and guardrails the caller doesn't; bypassing it to work around a flaky A2A call reintroduces exactly the bugs A2A exists to prevent, and makes the real reliability problem invisible instead of fixing it. If A2A delegation is unreliable, fix A2A — file it as a bug in the delegation path (timeout handling, retries, typed terminal states), don't route around it app by app.

Connecting app A to an Agent-Native app B is two independent things, and both must be true:

1. B is registered on A as a remote-agents/<id>.json resource.
2. A and B share a secret, so A's signed JWT verifies on B.

Neither is a code change, and neither is symmetric. Registering B on A does not let B call A. A hosted peer uses provider-specific endpoint and credential metadata in place of the shared Agent-Native secret, but it still does not make the provider's model or SDK an A2A endpoint.

A2A is already mounted

createAgentChatPlugin calls mountA2A for every app, so a generated app already serves:

• GET /.well-known/agent-card.json — public discovery, with a larger authenticated capability view for verified sibling callers
• POST /_agent-native/a2a — JSON-RPC, authenticated

Do not add a mountA2A server plugin to enable A2A; it is on. Hand-mounting is only for a bespoke card or a custom handler (see Custom mount below).

The client falls back to POST /a2a for external peers that only expose that path. New agent-native apps should call /_agent-native/a2a.

Registering a remote agent

A remote agent is a row in the resources table, not a file on disk. Path remote-agents/<id>.json, owner SHARED_OWNER, content:

{
  "id": "analytics",
  "name": "Analytics",
  "description": "Queries analytics data across providers",
  "url": "https://analytics.example.com",
  "color": "#6B7280",
  "cardUrl": "https://analytics.example.com/.well-known/agent-card.json",
  "auth": {
    "type": "bearer",
    "credentialRef": "ANALYTICS_A2A_TOKEN"
  }
}

url is the only required endpoint field. cardUrl is the optional discovery URL for providers that do not serve /.well-known/agent-card.json. The client reads the protocol version from the agent card. Pass protocolVersion directly to A2AClient when a provider's card omits it. The optional auth descriptor is non-secret connection metadata. Use { "type": "bearer", "credentialRef": "..." } for a vault-backed bearer credential, or { "type": "oauth-client-credentials", "tokenUrl": "...", "clientId": "...", "clientSecretRef": "...", "scope": "..." } when the peer issues OAuth client-credentials tokens. credentialRef and clientSecretRef are references, never secret values.

Do not add apiKey, env, skills, or token values to a manifest. Resolve credentials server-side from the workspace connection or vault. A direct A2AClient call can pass cardUrl and protocolVersion while a provider adapter is being used, but browser code must never receive the credential.

Four ways to create it, all writing the same row:

The last one matters for template apps with no Code tab: "connect me to https://analytics.example.com" is a complete instruction. remote-agents/ is whitelisted as a durable control path, so the write is workspace-scoped, and it takes effect on the next discoverAgents() call with no restart.

First-party templates are seeded automatically and overlaid on top of manifests, so Mail, Calendar, and friends appear without registration. In dev they resolve to http://localhost:<devPort>; a manifest pointing at localhost is ignored in production.

call-agent also accepts a raw URL, which skips registration entirely — useful for one-off calls, useless for @-mentions.

Checking a peer

GET /_agent-native/agents/probe?url=<base> reads the peer's card and makes one authenticated no-op call, returning reachable and authorized as independent fields. Omit url to probe every registered peer. Reachable-but-unauthorized is the failure worth looking for: local dev runs the receiver unauthenticated, so a mismatched secret only surfaces after deploy. The Connected Agents settings section calls this on add and on open.

Authentication

A2A authenticates with a short-lived JWT the caller signs, not a stored bearer key. Authorization: Bearer <jwt>; claims carry the caller's email and org domain; HS256; 15-minute default TTL. There is no per-peer API key anywhere in the system, which is why the manifest has no field for one.

Two secrets can sign it:

The org secret is the managed path: Team page → A2A secret (owner only) reveals, copies, regenerates, and pushes it to every discovered peer. Rotation is tolerated — the caller tries both secrets in order and sticks to whichever worked.

A2A_SECRET is the deploy-level path. workspace-deploy refuses a production workspace deploy without it and prints the generator command. Peers that must trust each other need the same value on both sides.

With no secret configured at all:

"Production" is detected broadly (NODE_ENV, Netlify, Lambda, Vercel, Render, Fly, Cloud Run, …) and unrecognized deployed hosts fail closed unless the request is genuine loopback or A2A_ALLOW_UNSIGNED_INTERNAL=1.

A2AConfig.apiKeyEnv still exists for static bearer auth against non-agent-native peers, but the framework's own mount never sets it. Do not reach for it when debugging a connection between two agent-native apps — the answer there is always the shared secret. For a provider that uses OAuth, resolve and refresh the access token in a server-side adapter. A2A_SECRET is not a substitute for the provider's Entra, Google, or other OAuth credential.

Hosted providers

Foundry, Gemini Enterprise, and other hosted services can be A2A peers only when they expose a compatible protocol endpoint. Foundry hosted agents run agent code in managed containers and can expose A2A through Agent Service. Keep the Agent-Native UI, actions, and PostgreSQL app on its normal host. Foundry callers use Microsoft Entra bearer tokens with Foundry Agent Consumer access, so configure cardUrl and obtain the token through the workspace credential provider. Foundry v1.0 is the GA JSON-RPC endpoint; v0.3 is the preview endpoint used when no version is selected. Use the v1 card URL .../agents/{agent}/endpoint/protocols/a2a/agentCard/v1.0 when available, and pass protocolVersion only when the card omits it. Foundry v1 does not provide SSE streaming. See the Foundry hosted agent overview and A2A endpoint guidance.

Gemini Enterprise managed assistants expose a standard A2A JSON-RPC endpoint under the assistant resource, such as .../assistants/default_assistant/agents/{id}/a2a, and can publish the card at a custom URL. Use the generic bearer client with that cardUrl; the caller's Google OAuth bearer needs the discoveryengine.assist permission. Custom A2A agent registration is Pre-GA, so verify that it is enabled in the target project before exposing it in the workspace picker. A model provider or SDK without an A2A endpoint still needs a server-side adapter before an Agent-Native app can call it.

Anthropic Managed Agents has no inbound A2A endpoint. Use createAnthropicManagedAgentsHandler() as the A2AConfig.handler when the Agent-Native app should own the A2A task, workflow state, and approvals while Anthropic owns the session, context, and tools. Its configuration is a native provider kind on the existing remote-agent resource:

{
  "kind": {
    "provider": "anthropic-managed-agents",
    "agentId": "agt_01...",
    "environmentId": "env_01...",
    "credentialRef": "ANTHROPIC_API_KEY"
  }
}

credentialRef is a vault reference, resolved in the caller's user and organization scope. The adapter calls POST /v1/sessions, sends user.message or user.tool_confirmation events to POST /v1/sessions/{id}/events, and reads GET /v1/sessions/{id}/events/stream with the managed-agents-2026-04-01 beta header. A session.status_idle event whose stop reason is requires_action becomes A2A input-required; each blocking tool also emits the normal approval-request runtime event. Continue only with structured { toolUseId, result: "allow" | "deny" } confirmations in the adapter metadata. Register the anthropic-managed-agents workspace connection provider to reuse existing per-app grants.

Never hardcode either secret in source, docs, prompts, app state, action descriptions, client bundles, or examples. Read them from runtime config; never log or return them.

Advertising what this agent can do

Card skills are derived from actions marked publicAgent; do not maintain a second capability registry. Anonymous callers see only explicitly public-safe reads. Verified sibling callers see two concise kinds of capability:

• Authenticated read-only actions selected by connector policy include their input schemas and may be invoked directly.
• Authenticated writes marked publicAgent: { expose: true, readOnly: false, requiresAuth: true } are advertised without schemas as message-only capabilities. A sibling delegates an objective; the receiving agent chooses and validates its own local actions.

agentTool: false and externalAgents.denyActions remove an action from both authenticated paths. An app that marks no actions still publishes "skills": [], and natural-language delegation still works because the receiver loads its own instructions, skills, data dictionary, credentials, and tools. Expose stable user-facing capabilities, not internal implementation actions.

Calling another agent

Natural-language delegation is the default for agent-to-agent work. Tell the receiving specialist the objective, relevant IDs/date range, and desired result shape. The receiver owns source selection, schemas, queries, joins, SQL, and provider-specific details. Do not switch to a direct action merely because a delegated run is slow or flaky; fix the A2A path instead.

Simple: callAgent() (text in, text out)

import { callAgent, resolveA2ACallerAuth } from "@agent-native/core/a2a";

const auth = await resolveA2ACallerAuth();
const answer = await callAgent(
  "https://analytics.example.com",
  "What were last week's signups?",
  auth,
);
// answer is a plain string

callAgent signs nothing on its own — with no userEmail/orgSecret in opts it calls unauthenticated, which silently works in local dev and 401s in production. resolveA2ACallerAuth() pulls the authenticated email, org domain, and org secret out of request context; pass them explicitly only from CLI or cron, where there is no request.

Explicit machine-contract exception: invokeAgentAction()

Use direct invocation only when a trusted integration already specifies the exact receiver-owned semantic read action and complete arguments. It is an optional machine API, not an agent-performance shortcut or fallback for failed message delegation:

import { invokeAgentAction } from "@agent-native/core/a2a";

const { result } = await invokeAgentAction({
  target: "analytics",
  action: "gong-calls",
  input: { company: "Acme", days: 90, includeTranscripts: true },
  userEmail,
  orgDomain,
  orgSecret,
});

The receiver still owns schema validation, credentials, access scoping, audit attribution, and exposure policy. Direct invocation is available only for cataloged, authenticated, explicitly exposed read-only actions that do not require approval. Its JWT is audience-bound to the receiving app's exact base URL, including a workspace path such as /content. Use normal message delegation whenever the receiver must interpret the request, choose a source, consult its data dictionary, plan, synthesize, join data, or perform a multi-step workflow.

Inside an agent loop, call-agent exposes the same path with action + input; omit message and taskId in that mode.

Retry safety and trace linkage

call-agent automatically derives an owner-scoped idempotency key from the originating turn, target, and exact message. If a retry reaches the receiver after the caller timed out, asynchronous message/send returns the existing active or completed task instead of starting a duplicate agent run. Failed and canceled tasks release the key for an intentional retry; synchronous calls are never deduplicated. Lower-level clients may pass an idempotencyKey explicitly; keep it stable for the same logical submission and change it when the work changes. Dedupe is scoped to the JWT-authenticated owner and verified org, and keys are limited to 128 characters.

The caller also forwards bounded correlation metadata (callerApp, selectedReceiverApp, callerThreadId, parentRunId, parentTurnId, and direct-read invocationId). selectedReceiverApp lets the matching receiver prioritize its declared local capabilities before loading cross-app tools; the other fields remain telemetry hints. Receivers must continue to derive identity, data ownership, org scope, access, and approval from the verified request context. Delegated model loops emit $ai_generation with A2A/MCP lineage, while direct reads emit the content-free $a2a_read_invoke event; neither event includes action arguments or results.

Advanced: A2AClient (full control)

import { A2AClient, resolveA2ACallerAuth } from "@agent-native/core/a2a";

// The second argument is the bearer token itself, not a signing secret.
const { apiKey, apiKeyFallbacks } = await resolveA2ACallerAuth();
const client = new A2AClient("https://analytics.example.com", apiKey, {
  fallbackApiKeys: apiKeyFallbacks,
});

// Discover agent capabilities
const card = await client.getAgentCard();

// Send a message and get a task back
const task = await client.send({
  role: "user",
  parts: [{ type: "text", text: "What were last week's signups?" }],
});
// task.status.state === "completed"
// task.status.message.parts[0].text === "Last week: 1,247 signups..."

// Stream responses
for await (const update of client.stream({
  role: "user",
  parts: [{ type: "text", text: "Detailed breakdown by day" }],
})) {
  console.log(update.status.state, update.status.message);
}

Agent activity in delegated chat

Agent-Native peers attach a bounded data part with kind: "agent-native/agent-activity" to in-progress and terminal task status messages. It contains the same user-visible reasoning summaries shown in the receiving app, tool names and completion states, elapsed time, and progressive response text. It never includes tool inputs, tool results, credentials, or hidden provider reasoning.

call-agent reads this optional part while it polls an asynchronous task and renders the remote work as a nested agent run. Unknown A2A peers do not need to implement the extension: their ordinary status and final text still render in the same nested block. Treat activity data as untrusted presentation content; never use it for identity, authorization, approval, routing, or artifact validation.

Carrying explicit chat authorization

When the authenticated caller has an exact consequential action that the user explicitly authorized in the originating chat, pass the tool name and complete input as approvedActions. The receiver accepts these grants only from a JWT-verified user identity, converts each one to the same content-addressed key as its local approval gate, and consumes it once:

await client.send(message, {
  async: true,
  approvedActions: [
    {
      tool: "send-email",
      input: { to, subject, body, attachments },
    },
  ],
});

Never infer authorization from request prose or broaden the input. A changed recipient, body, attachment, or tool produces a different key and follows the receiver's normal approval-required path. Static API keys and unsigned callers cannot carry these grants.

JSON-RPC Methods

Task Lifecycle

Tasks go through these states:

submitted → working → completed
                    → failed
                    → canceled
                    → input-required

• submitted — message received, not yet processing
• working — agent is processing the request
• completed — agent finished, result in status.message
• failed — agent encountered an error
• canceled — task was canceled via tasks/cancel
• input-required — agent needs more information from the caller

Message Parts

Messages contain typed parts:

Custom mount

Only when the default card is wrong for the app — a curated skill list, a non-agent handler, or static bearer auth against an external peer:

// server/plugins/a2a.ts
import { mountA2A } from "@agent-native/core/a2a";

export default defineNitroPlugin((nitro) => {
  mountA2A(nitro, {
    appId: "analytics",
    name: "Analytics Agent",
    description: "Queries analytics data across providers",
    skills: [
      {
        id: "query-data",
        name: "Query Data",
        description: "Run analytics queries across connected data sources",
        tags: ["analytics", "data"],
        examples: ["What were last week's signups?", "Show conversion rates"],
      },
    ],
    streaming: true,
  });
});

Config also carries handler, publicSkillsOnly, durableBackgroundRuns, executeReadOnlyAction, executeApproval, and apiKeyEnv. See A2AConfig in @agent-native/core/a2a for the full shape.

All Types

All types are exported from @agent-native/core/a2a:

import type {
  A2AConfig,
  A2AHandler,
  A2AHandlerContext,
  A2AHandlerResult,
  AgentCard,
  AgentSkill,
  AgentCapabilities,
  Task,
  TaskState,
  TaskStatus,
  Message,
  Part,
  TextPart,
  FilePart,
  DataPart,
  Artifact,
  JsonRpcRequest,
  JsonRpcResponse,
} from "@agent-native/core/a2a";

Related Skills

• delegate-to-agent — For work the local agent handles. Use A2A when the work goes to a different agent.
• authentication — Org creation, membership, and where the org secret lives.
• actions — A2A calls typically happen inside actions; publicAgent marks what peers can see.
• storing-data — Results from A2A calls are stored in SQL like any other data.