WebSockets

Agents support WebSocket connections for real-time, bi-directional communication. This page covers server-side WebSocket handling. For client-side connection, refer to the Client SDK.

Lifecycle hooks

Agents have several lifecycle hooks that fire at different points:

Hook	When called
`onStart(props?)`	Once when the agent first starts (before any connections)
`onRequest(request)`	When an HTTP request is received (non-WebSocket)
`onConnect(connection, ctx)`	When a new WebSocket connection is established
`onMessage(connection, message)`	When a WebSocket message is received
`onClose(connection, code, reason, wasClean)`	When a WebSocket connection closes
`onError(connection, error)`	When a WebSocket error occurs on a connection
`onError(error)`	When a server-level error occurs (not tied to a specific connection)
`shouldSendProtocolMessages(connection, ctx)`	Whether to send protocol messages (identity, state, MCP) to this connection. Default: `true`

`onStart`

onStart() is called once when the agent first starts, before any connections are established:

export class MyAgent extends Agent {
	async onStart() {
		// Initialize resources
		console.log(`Agent ${this.name} starting...`);

		// Load data from storage
		const savedData = this.sql`SELECT * FROM cache`;
		for (const row of savedData) {
			// Rebuild in-memory state from persistent storage
		}
	}

	onConnect(connection) {
		// By the time connections arrive, onStart has completed
	}
}

export class MyAgent extends Agent {
	async onStart() {
		// Initialize resources
		console.log(`Agent ${this.name} starting...`);

		// Load data from storage
		const savedData = this.sql`SELECT * FROM cache`;
		for (const row of savedData) {
			// Rebuild in-memory state from persistent storage
		}
	}

	onConnect(connection: Connection) {
		// By the time connections arrive, onStart has completed
	}
}

Handling connections

Define onConnect and onMessage methods on your Agent to accept WebSocket connections:

import { Agent, Connection, ConnectionContext, WSMessage } from "agents";

export class ChatAgent extends Agent {
	async onConnect(connection, ctx) {
		// Connections are automatically accepted
		// Access the original request for auth, headers, cookies
		const url = new URL(ctx.request.url);
		const token = url.searchParams.get("token");

		if (!token) {
			connection.close(4001, "Unauthorized");
			return;
		}

		// Store user info on this connection
		connection.setState({ authenticated: true });
	}

	async onMessage(connection, message) {
		if (typeof message === "string") {
			// Handle text message
			const data = JSON.parse(message);
			connection.send(JSON.stringify({ received: data }));
		}
	}
}

import { Agent, Connection, ConnectionContext, WSMessage } from "agents";

export class ChatAgent extends Agent {
	async onConnect(connection: Connection, ctx: ConnectionContext) {
		// Connections are automatically accepted
		// Access the original request for auth, headers, cookies
		const url = new URL(ctx.request.url);
		const token = url.searchParams.get("token");

		if (!token) {
			connection.close(4001, "Unauthorized");
			return;
		}

		// Store user info on this connection
		connection.setState({ authenticated: true });
	}

	async onMessage(connection: Connection, message: WSMessage) {
		if (typeof message === "string") {
			// Handle text message
			const data = JSON.parse(message);
			connection.send(JSON.stringify({ received: data }));
		}
	}
}

Connection object

Each connected client has a unique Connection object:

Property/Method	Type	Description
`id`	`string`	Unique identifier for this connection
`uri`	`string \| null`	URL of the original WebSocket upgrade request. Persists across hibernation
`state`	`State`	Per-connection state object
`setState(state)`	`void`	Update connection state
`send(message)`	`void`	Send message to this client
`close(code?, reason?)`	`void`	Close the connection
`tags`	`readonly string[]`	Tags assigned via `getConnectionTags`. Always includes the connection ID as the first tag
`server`	`string`	The agent instance name (same as `this.name` on the Agent)

Per-connection state

Store data specific to each connection (user info, preferences, etc.):

export class ChatAgent extends Agent {
	async onConnect(connection, ctx) {
		const userId = new URL(ctx.request.url).searchParams.get("userId");

		connection.setState({
			userId: userId || "anonymous",
			role: "user",
			joinedAt: Date.now(),
		});
	}

	async onMessage(connection, message) {
		// Access connection-specific state
		console.log(`Message from ${connection.state.userId}`);
	}
}

interface ConnectionState {
	userId: string;
	role: "admin" | "user";
	joinedAt: number;
}

export class ChatAgent extends Agent {
	async onConnect(
		connection: Connection<ConnectionState>,
		ctx: ConnectionContext,
	) {
		const userId = new URL(ctx.request.url).searchParams.get("userId");

		connection.setState({
			userId: userId || "anonymous",
			role: "user",
			joinedAt: Date.now(),
		});
	}

	async onMessage(connection: Connection<ConnectionState>, message: WSMessage) {
		// Access connection-specific state
		console.log(`Message from ${connection.state.userId}`);
	}
}

Broadcasting to all clients

Use this.broadcast() to send a message to all connected clients:

export class ChatAgent extends Agent {
	async onMessage(connection, message) {
		// Broadcast to all connected clients
		this.broadcast(
			JSON.stringify({
				from: connection.id,
				message: message,
				timestamp: Date.now(),
			}),
		);
	}

	// Broadcast from any method
	async notifyAll(event, data) {
		this.broadcast(JSON.stringify({ event, data }));
	}
}

export class ChatAgent extends Agent {
	async onMessage(connection: Connection, message: WSMessage) {
		// Broadcast to all connected clients
		this.broadcast(
			JSON.stringify({
				from: connection.id,
				message: message,
				timestamp: Date.now(),
			}),
		);
	}

	// Broadcast from any method
	async notifyAll(event: string, data: unknown) {
		this.broadcast(JSON.stringify({ event, data }));
	}
}

Excluding connections

Pass an array of connection IDs to exclude from the broadcast:

// Broadcast to everyone except the sender
this.broadcast(
	JSON.stringify({ type: "user-typing", userId: "123" }),
	[connection.id], // Do not send to the originator
);

// Broadcast to everyone except the sender
this.broadcast(
	JSON.stringify({ type: "user-typing", userId: "123" }),
	[connection.id], // Do not send to the originator
);

Connection tags

Tag connections for easy filtering. Override getConnectionTags() to assign tags when a connection is established:

export class ChatAgent extends Agent {
	getConnectionTags(connection, ctx) {
		const url = new URL(ctx.request.url);
		const role = url.searchParams.get("role");

		const tags = [];
		if (role === "admin") tags.push("admin");
		if (role === "moderator") tags.push("moderator");

		return tags; // Up to 9 tags, max 256 chars each
	}

	// Later, broadcast only to admins
	notifyAdmins(message) {
		for (const conn of this.getConnections("admin")) {
			conn.send(message);
		}
	}
}

export class ChatAgent extends Agent {
	getConnectionTags(connection: Connection, ctx: ConnectionContext): string[] {
		const url = new URL(ctx.request.url);
		const role = url.searchParams.get("role");

		const tags: string[] = [];
		if (role === "admin") tags.push("admin");
		if (role === "moderator") tags.push("moderator");

		return tags; // Up to 9 tags, max 256 chars each
	}

	// Later, broadcast only to admins
	notifyAdmins(message: string) {
		for (const conn of this.getConnections("admin")) {
			conn.send(message);
		}
	}
}

Connection management methods

Method	Signature	Description
`getConnections`	`(tag?: string) => Iterable<Connection>`	Get all connections, optionally by tag
`getConnection`	`(id: string) => Connection \| undefined`	Get connection by ID
`getConnectionTags`	`(connection, ctx) => string[]`	Override to tag connections
`broadcast`	`(message, without?: string[]) => void`	Send to all connections
`isConnectionReadonly`	`(connection) => boolean`	Check if a connection is readonly
`isConnectionProtocolEnabled`	`(connection) => boolean`	Check if protocol messages are enabled for this connection

Handling binary data

Messages can be strings or binary (ArrayBuffer / ArrayBufferView):

export class FileAgent extends Agent {
	async onMessage(connection, message) {
		if (message instanceof ArrayBuffer) {
			// Handle binary upload
			const bytes = new Uint8Array(message);
			await this.processFile(bytes);
			connection.send(
				JSON.stringify({ status: "received", size: bytes.length }),
			);
		} else if (typeof message === "string") {
			// Handle text command
			const command = JSON.parse(message);
			// ...
		}
	}
}

export class FileAgent extends Agent {
	async onMessage(connection: Connection, message: WSMessage) {
		if (message instanceof ArrayBuffer) {
			// Handle binary upload
			const bytes = new Uint8Array(message);
			await this.processFile(bytes);
			connection.send(
				JSON.stringify({ status: "received", size: bytes.length }),
			);
		} else if (typeof message === "string") {
			// Handle text command
			const command = JSON.parse(message);
			// ...
		}
	}
}

Error and close handling

Handle connection errors and disconnections. The onError method has two overloads — one for WebSocket connection errors and one for server-level errors:

export class ChatAgent extends Agent {
	// WebSocket connection error

	// Server-level error (not tied to a specific connection)

	onError(connectionOrError, error) {
		if (error) {
			console.error(`Connection ${connectionOrError.id} error:`, error);
		} else {
			console.error("Server error:", connectionOrError);
		}
	}

	async onClose(connection, code, reason, wasClean) {
		console.log(`Connection ${connection.id} closed: ${code} ${reason}`);

		this.broadcast(
			JSON.stringify({
				event: "user-left",
				userId: connection.state?.userId,
			}),
		);
	}
}

export class ChatAgent extends Agent {
	// WebSocket connection error
	onError(connection: Connection, error: unknown): void;
	// Server-level error (not tied to a specific connection)
	onError(error: unknown): void;
	onError(connectionOrError: Connection | unknown, error?: unknown) {
		if (error) {
			console.error(
				`Connection ${(connectionOrError as Connection).id} error:`,
				error,
			);
		} else {
			console.error("Server error:", connectionOrError);
		}
	}

	async onClose(
		connection: Connection,
		code: number,
		reason: string,
		wasClean: boolean,
	) {
		console.log(`Connection ${connection.id} closed: ${code} ${reason}`);

		this.broadcast(
			JSON.stringify({
				event: "user-left",
				userId: connection.state?.userId,
			}),
		);
	}
}

The default onError implementation logs the error and rethrows it. Override it to add custom error handling, reporting, or recovery logic.

Message types

Type	Description
`string`	Text message (typically JSON)
`ArrayBuffer`	Binary data
`ArrayBufferView`	Typed array view of binary data

Hibernation

Agents support hibernation — they can sleep when inactive and wake when needed. This saves resources while maintaining WebSocket connections.

Enabling hibernation

Hibernation is enabled by default. To disable:

export class AlwaysOnAgent extends Agent {
	static options = { hibernate: false };
}

export class AlwaysOnAgent extends Agent {
	static options = { hibernate: false };
}

How hibernation works

Agent is active, handling connections
After a period of inactivity with no messages, the agent hibernates (sleeps)
WebSocket connections remain open (handled by Cloudflare)
When a message arrives, the agent wakes up
onMessage is called as normal

What persists across hibernation

Persists	Does not persist
`this.state` (agent state)	In-memory variables
`connection.state`	Timers/intervals
SQLite data (`this.sql`)	Promises in flight
Connection metadata	Local caches

Store important data in this.state or SQLite, not in class properties:

export class MyAgent extends Agent {
	initialState = { counter: 0 };

	// Do not do this - lost on hibernation
	localCounter = 0;

	onMessage(connection, message) {
		// Persists across hibernation
		this.setState({ counter: this.state.counter + 1 });

		// Lost after hibernation
		this.localCounter++;
	}
}

export class MyAgent extends Agent<Env, { counter: number }> {
	initialState = { counter: 0 };

	// Do not do this - lost on hibernation
	private localCounter = 0;

	onMessage(connection: Connection, message: WSMessage) {
		// Persists across hibernation
		this.setState({ counter: this.state.counter + 1 });

		// Lost after hibernation
		this.localCounter++;
	}
}

Common patterns

Presence tracking

Track who is online using per-connection state. Connection state is automatically cleaned up when users disconnect:

export class PresenceAgent extends Agent {
	onConnect(connection, ctx) {
		const url = new URL(ctx.request.url);
		const name = url.searchParams.get("name") || "Anonymous";

		connection.setState({
			name,
			joinedAt: Date.now(),
			lastSeen: Date.now(),
		});

		// Send current presence to new user
		connection.send(
			JSON.stringify({
				type: "presence",
				users: this.getPresence(),
			}),
		);

		// Notify others that someone joined
		this.broadcastPresence();
	}

	onClose(connection) {
		// No manual cleanup needed - connection state is automatically gone
		this.broadcastPresence();
	}

	onMessage(connection, message) {
		if (message === "ping") {
			connection.setState((prev) => ({
				...prev,
				lastSeen: Date.now(),
			}));
			connection.send("pong");
		}
	}

	getPresence() {
		const users = {};
		for (const conn of this.getConnections()) {
			if (conn.state) {
				users[conn.id] = {
					name: conn.state.name,
					lastSeen: conn.state.lastSeen,
				};
			}
		}
		return users;
	}

	broadcastPresence() {
		this.broadcast(
			JSON.stringify({
				type: "presence",
				users: this.getPresence(),
			}),
		);
	}
}

type UserState = {
	name: string;
	joinedAt: number;
	lastSeen: number;
};

export class PresenceAgent extends Agent {
	onConnect(connection: Connection<UserState>, ctx: ConnectionContext) {
		const url = new URL(ctx.request.url);
		const name = url.searchParams.get("name") || "Anonymous";

		connection.setState({
			name,
			joinedAt: Date.now(),
			lastSeen: Date.now(),
		});

		// Send current presence to new user
		connection.send(
			JSON.stringify({
				type: "presence",
				users: this.getPresence(),
			}),
		);

		// Notify others that someone joined
		this.broadcastPresence();
	}

	onClose(connection: Connection) {
		// No manual cleanup needed - connection state is automatically gone
		this.broadcastPresence();
	}

	onMessage(connection: Connection<UserState>, message: WSMessage) {
		if (message === "ping") {
			connection.setState((prev) => ({
				...prev!,
				lastSeen: Date.now(),
			}));
			connection.send("pong");
		}
	}

	private getPresence() {
		const users: Record<string, { name: string; lastSeen: number }> = {};
		for (const conn of this.getConnections<UserState>()) {
			if (conn.state) {
				users[conn.id] = {
					name: conn.state.name,
					lastSeen: conn.state.lastSeen,
				};
			}
		}
		return users;
	}

	private broadcastPresence() {
		this.broadcast(
			JSON.stringify({
				type: "presence",
				users: this.getPresence(),
			}),
		);
	}
}

Chat room with broadcast

export class ChatRoom extends Agent {
	onConnect(connection, ctx) {
		const url = new URL(ctx.request.url);
		const username = url.searchParams.get("username") || "Anonymous";

		connection.setState({ username });

		// Notify others
		this.broadcast(
			JSON.stringify({
				type: "join",
				user: username,
				timestamp: Date.now(),
			}),
			[connection.id], // Do not send to the joining user
		);
	}

	onMessage(connection, message) {
		if (typeof message !== "string") return;

		const { username } = connection.state;

		this.broadcast(
			JSON.stringify({
				type: "message",
				user: username,
				text: message,
				timestamp: Date.now(),
			}),
		);
	}

	onClose(connection) {
		const { username } = connection.state || {};
		if (username) {
			this.broadcast(
				JSON.stringify({
					type: "leave",
					user: username,
					timestamp: Date.now(),
				}),
			);
		}
	}
}

type Message = {
	type: "message" | "join" | "leave";
	user: string;
	text?: string;
	timestamp: number;
};

export class ChatRoom extends Agent {
	onConnect(connection: Connection, ctx: ConnectionContext) {
		const url = new URL(ctx.request.url);
		const username = url.searchParams.get("username") || "Anonymous";

		connection.setState({ username });

		// Notify others
		this.broadcast(
			JSON.stringify({
				type: "join",
				user: username,
				timestamp: Date.now(),
			} satisfies Message),
			[connection.id], // Do not send to the joining user
		);
	}

	onMessage(connection: Connection, message: WSMessage) {
		if (typeof message !== "string") return;

		const { username } = connection.state as { username: string };

		this.broadcast(
			JSON.stringify({
				type: "message",
				user: username,
				text: message,
				timestamp: Date.now(),
			} satisfies Message),
		);
	}

	onClose(connection: Connection) {
		const { username } = (connection.state as { username: string }) || {};
		if (username) {
			this.broadcast(
				JSON.stringify({
					type: "leave",
					user: username,
					timestamp: Date.now(),
				} satisfies Message),
			);
		}
	}
}

Suppressing protocol messages

By default, agents send JSON text frames (identity, state sync, MCP server lists) to every connection. Override shouldSendProtocolMessages to suppress them for specific connections — for example, binary-only clients that cannot handle JSON text frames:

export class IoTAgent extends Agent {
	shouldSendProtocolMessages(connection, ctx) {
		const url = new URL(ctx.request.url);
		return url.searchParams.get("protocol") !== "binary";
	}
}

export class IoTAgent extends Agent {
	shouldSendProtocolMessages(
		connection: Connection,
		ctx: ConnectionContext,
	): boolean {
		const url = new URL(ctx.request.url);
		return url.searchParams.get("protocol") !== "binary";
	}
}

When this returns false, the connection does not receive identity, state, or MCP server list frames — neither on connect nor via broadcasts. The connection can still send and receive regular messages, use RPC, and participate in all non-protocol communication.

Use isConnectionProtocolEnabled(connection) to check the status of any connection at runtime.

Agent properties

These properties are available on this inside any Agent method:

Property	Type	Description
`this.name`	`string`	The instance name of this agent
`this.state`	`State`	The current agent state (lazy-loaded from SQLite)
`this.env`	`Env`	Worker environment bindings
`this.ctx`	`DurableObjectState`	Durable Object context (storage, alarms, etc.)
`this.sql`	template tag	SQL template tag for executing queries against the agent's SQLite storage
`this.mcp`	`MCPClientManager`	MCP client manager for connecting to external MCP servers

Connecting from clients

For browser connections, use the Agents client SDK:

Vanilla JS: AgentClient from agents/client
React: useAgent hook from agents/react

Refer to Client SDK for full documentation.