# Client and hooks URL: /docs/runtimes/a2a/client-and-hooks A2AClient, useA2ARuntime options, hooks, task states, artifacts, errors. > For AI agents: a documentation index is available at [llms.txt](/llms.txt). Use `.md` for canonical markdown pages; `.mdx` is kept as a backwards-compatible alias on supported URL paths. Deep dive on the runtime's API surface. Start with [overview](/docs/runtimes/a2a/overview) and [quickstart](/docs/runtimes/a2a/quickstart) if you have not already. ## A2AClient The built-in `A2AClient` handles all communication with the A2A server: JSON serialization, SSE streaming, ProtoJSON enum normalization, and structured error handling. ``` import { A2AClient } from "@assistant-ui/react-a2a"; const client = new A2AClient({ baseUrl: "https://my-agent.example.com", headers: { Authorization: "Bearer " }, tenant: "my-org", extensions: ["urn:a2a:ext:my-extension"], fetchOptions: { credentials: "include" }, }); ``` Pass a pre-built client to `useA2ARuntime`: ``` const runtime = useA2ARuntime({ client }); ``` ### Client options | Option | Type | Description | | -------------- | ---------------------------------------------------------- | --------------------------------------------------------------------------------------------------------------------------------------------------- | | `baseUrl` | `string` | Base URL of the A2A server. | | `basePath` | `string` | Optional path prefix for API endpoints (e.g. `"/v1"`). Does not affect agent card discovery. | | `headers` | `Record` or `() => Record` | Static or dynamic headers (e.g. for auth tokens). | | `tenant` | `string` | Tenant ID for multi-tenant servers (prepended to URL paths). | | `extensions` | `string[]` | Extension URIs to negotiate via `A2A-Extensions` header. | | `fetchOptions` | `RequestInit` (partial) | Extra fetch options applied to every request (e.g. `{ credentials: "include" }`). `headers`, `body`, `method`, and `signal` are managed internally. | ### Client methods | Method | Description | | ---------------------------------------------------- | --------------------------------------------------------- | | `sendMessage(message, configuration?, metadata?)` | Send a message (non-streaming). | | `streamMessage(message, configuration?, metadata?)` | Send a message with SSE streaming. | | `getTask(taskId, historyLength?)` | Get a task by ID. | | `listTasks(request?)` | List tasks with filtering and pagination. | | `cancelTask(taskId, metadata?)` | Cancel an in-progress task. | | `subscribeToTask(taskId)` | Subscribe to SSE updates for a task. | | `getAgentCard()` | Fetch the agent card from `/.well-known/agent-card.json`. | | `getExtendedAgentCard()` | Fetch the extended (authenticated) agent card. | | `createTaskPushNotificationConfig(config)` | Create a push notification config. | | `getTaskPushNotificationConfig(taskId, configId)` | Get a push notification config. | | `listTaskPushNotificationConfigs(taskId)` | List push notification configs. | | `deleteTaskPushNotificationConfig(taskId, configId)` | Delete a push notification config. | ## useA2ARuntime options Pass either a pre-built `client` or a `baseUrl` (the runtime creates a client for you). Other options layer on top. | Option | Type | Description | | ---------------------- | ---------------------------------------------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | `client` | `A2AClient` | Pre-built A2A client instance (provide this OR `baseUrl`). | | `baseUrl` | `string` | A2A server URL (creates a client automatically). | | `basePath` | `string` | Path prefix for API endpoints. Only used with `baseUrl`. | | `tenant` | `string` | Tenant ID for multi-tenant servers. Only used with `baseUrl`. | | `headers` | `Record` or `() => Record` | Headers for the auto-created client. | | `extensions` | `string[]` | Extension URIs to negotiate. Only used with `baseUrl`. | | `fetchOptions` | `RequestInit` (partial) | Extra fetch options for the auto-created client (e.g. `{ credentials: "include" }`). `headers`, `body`, `method`, and `signal` are managed internally. Only used with `baseUrl`. | | `contextId` | `string` | Initial context ID for the conversation. | | `configuration` | `A2ASendMessageConfiguration` | Default send message configuration. | | `onError` | `(error: Error) => void` | Error callback. | | `onCancel` | `() => void` | Cancellation callback. | | `onArtifactComplete` | `(artifact: A2AArtifact) => void` | Fired when an incremental artifact finishes. | | `adapters.attachments` | `AttachmentAdapter` | Custom attachment handling. See [adapters](/docs/runtimes/concepts/adapters#attachment-adapter). | | `adapters.speech` | `SpeechSynthesisAdapter` | Text-to-speech. See [adapters](/docs/runtimes/concepts/adapters#speech-adapter). | | `adapters.feedback` | `FeedbackAdapter` | Feedback collection. See [adapters](/docs/runtimes/concepts/adapters#feedback-adapter). | | `adapters.history` | `ThreadHistoryAdapter` | Message persistence. See [adapters](/docs/runtimes/concepts/adapters#history-adapter). | | `adapters.threadList` | `UseA2AThreadListAdapter` | Thread switching. See [threads](/docs/runtimes/concepts/threads). | ## Hooks ### Task state `useA2ATask` returns the current A2A task object, including task state and status message. **React** ``` import { useA2ATask } from "@assistant-ui/react-a2a"; function TaskStatus() { const task = useA2ATask(); if (!task) return null; return (
Task {task.id}: {task.status.state}
); } ``` ### Artifacts list `useA2AArtifacts` returns the artifacts generated by the current task. **React** ``` import { useA2AArtifacts } from "@assistant-ui/react-a2a"; function ArtifactList() { const artifacts = useA2AArtifacts(); return (
    {artifacts.map((artifact) => (
  • {artifact.name}: {artifact.parts.length} parts
    • ))}
); } ``` ### Agent card `useA2AAgentCard` returns the agent card fetched from the server on initialization. **React** ``` import { useA2AAgentCard } from "@assistant-ui/react-a2a"; function AgentInfo() { const card = useA2AAgentCard(); if (!card) return null; return (

{card.name}

{card.description}

Skills: {card.skills.map((s) => s.name).join(", ")}
); } ``` ## Task states The A2A protocol defines 9 task states. The runtime maps them to assistant-ui message statuses: | A2A task state | Description | Message status | | ---------------- | ------------------------- | ------------------------ | | `unspecified` | Unknown or default state. | `running` | | `submitted` | Task acknowledged. | `running` | | `working` | Task in progress. | `running` | | `completed` | Task finished. | `complete` | | `failed` | Task errored. | `incomplete (error)` | | `canceled` | Task cancelled. | `incomplete (cancelled)` | | `rejected` | Agent declined task. | `incomplete (error)` | | `input_required` | Agent needs user input. | `requires-action` | | `auth_required` | Authentication needed. | `requires-action` | > [!info] > > When a task enters `input_required`, the user can continue the conversation normally. The runtime sends the next message with the same `taskId` to resume the task. ## Artifacts A2A agents can produce artifacts (files, code, data) alongside their responses. Artifacts are accumulated during streaming and accessible via `useA2AArtifacts`. The runtime supports: - **Incremental artifact streaming** via `append` mode. - **Artifact completion notification** via the `onArtifactComplete` callback. - **Automatic reset** of artifacts on each new run. ``` const runtime = useA2ARuntime({ baseUrl: "http://localhost:9999", onArtifactComplete: (artifact) => { console.log("Artifact ready:", artifact.name); }, }); ``` ## Streaming vs non-streaming The runtime automatically selects the communication mode based on the agent's capabilities: - If the agent card indicates `capabilities.streaming: true` (or the field is unset), the runtime uses `POST /message:stream` with SSE. - If `capabilities.streaming: false`, the runtime falls back to `POST /message:send`. ## Error handling The client throws `A2AError` instances with structured error information following the `google.rpc.Status` format: ``` import { A2AError } from "@assistant-ui/react-a2a"; const runtime = useA2ARuntime({ baseUrl: "http://localhost:9999", onError: (error) => { if (error instanceof A2AError) { console.log(error.code); // HTTP status code console.log(error.status); // e.g. "NOT_FOUND" console.log(error.details); // google.rpc.ErrorInfo details } }, }); ``` ## Multi-tenancy For multi-tenant A2A servers, pass a `tenant` option: ``` const client = new A2AClient({ baseUrl: "https://agent.example.com", tenant: "my-org", }); ``` This prepends `/{tenant}` to all API paths (e.g. `/my-org/message:send`). ## Feature support | Feature | Supported | | ---------------------------- | ----------------------------------------------------------------------- | | Streaming (SSE) | Yes | | Non-streaming fallback | Yes | | All 9 task states | Yes | | Artifacts (text, data, file) | Yes | | Agent card discovery | Yes | | Multi-tenancy | Yes | | Structured errors | Yes | | Push notifications CRUD | Yes | | Extension negotiation | Yes | | Task cancellation | Yes | | Message editing | Yes | | Message reload | Yes | | History persistence | Via [history adapter](/docs/runtimes/concepts/adapters#history-adapter) | | Thread list management | Via [thread list adapter](/docs/runtimes/concepts/threads) | ## Related - [A2A overview](/docs/runtimes/a2a/overview) — What A2A is and when to pick it. - [Quickstart](/docs/runtimes/a2a/quickstart) — Minimal runtime + Thread setup. - [Adapters](/docs/runtimes/concepts/adapters) — Attachments, speech, feedback, history, threads.