Agents SDK build guide

Cloudflare Agents SDK

Your knowledge of the Agents SDK may be outdated. Prefer retrieval over pre-training for any Agents SDK task.

Retrieval Sources

Cloudflare docs: https://developers.cloudflare.com/agents/

Capabilities

The Agents SDK provides:

• Persistent state — SQLite-backed, auto-synced to clients via setState
• Callable RPC — @callable() methods invoked over WebSocket
• Scheduling — One-time, recurring (scheduleEvery), and cron tasks
• Workflows — Durable multi-step background processing via AgentWorkflow
• Durable execution — runFiber() / stash() for work that survives DO eviction
• Queue — Built-in FIFO queue with retries via queue()
• Retries — this.retry() with exponential backoff and jitter
• MCP integration — Connect to MCP servers or build your own with McpAgent
• Email handling — Receive and reply to emails with secure routing
• Streaming chat — AIChatAgent with resumable streams, message persistence, tools
• Server-driven messages — saveMessages, waitUntilStable

FIRST: Verify Installation

npm ls agents  # Should show agents package

If not installed:

npm install agents

For chat agents:

npm install agents @cloudflare/ai-chat ai @ai-sdk/react

Wrangler Configuration

{
  "compatibility_flags": ["nodejs_compat"],
  "durable_objects": {
    "bindings": [{ "name": "MyAgent", "class_name": "MyAgent" }]
  },
  "migrations": [{ "tag": "v1", "new_sqlite_classes": ["MyAgent"] }]
}

Gotchas:

• Do NOT enable experimentalDecorators in tsconfig (breaks @callable)
• Never edit old migrations — always add new tags
• Each agent class needs its own DO binding + migration entry
• Add "ai": { "binding": "AI" } for Workers AI

Agent Class

import { Agent, routeAgentRequest, callable } from "agents";

type State = { count: number };

export class Counter extends Agent<Env, State> {
  initialState = { count: 0 };

  validateStateChange(nextState: State, source: Connection | "server") {
    if (nextState.count < 0) throw new Error("Count cannot be negative");
  }

  onStateUpdate(state: State, source: Connection | "server") {
    console.log("State updated:", state);
  }

  @callable()
  increment() {
    this.setState({ count: this.state.count + 1 });
    return this.state.count;
  }
}

export default {
  fetch: (req, env) => routeAgentRequest(req, env) ?? new Response("Not found", { status: 404 })
};

Routing

Requests route to /agents/{agent-name}/{instance-name}:

Client: useAgent({ agent: "Counter", name: "user-123" })

Custom routing: use getAgentByName(env.MyAgent, "instance-id") then agent.fetch(request).

Core APIs

React Client

import { useAgent } from "agents/react";

function App() {
  const [state, setLocalState] = useState({ count: 0 });

  const agent = useAgent({
    agent: "Counter",
    name: "my-instance",
    onStateUpdate: (newState) => setLocalState(newState),
    onIdentity: (name, agentType) => console.log(`Connected to ${name}`)
  });

  return (
    <button onClick={() => agent.setState({ count: state.count + 1 })}>
      Count: {state.count}
    </button>
  );
}

References

Core

• [references/state-scheduling.md](references/state-scheduling.md) — State persistence, scheduling, SQL
• [references/callable.md](references/callable.md) — RPC methods, streaming, timeouts
• [references/routing.md](references/routing.md) — URL patterns, custom routing, getAgentByName
• [references/configuration.md](references/configuration.md) — Wrangler config, bindings, Vite setup

Chat & Streaming

• [references/streaming-chat.md](references/streaming-chat.md) — AIChatAgent, resumable streams, tools
• [references/client-sdk.md](references/client-sdk.md) — useAgent, useAgentChat, AgentClient
• [references/server-driven-messages.md](references/server-driven-messages.md) — Trigger patterns, saveMessages
• [references/human-in-the-loop.md](references/human-in-the-loop.md) — Approval flows, needsApproval

Background Processing

• [references/workflows.md](references/workflows.md) — Durable Workflows integration
• [references/durable-execution.md](references/durable-execution.md) — runFiber, stash, surviving eviction
• [references/queue-retries.md](references/queue-retries.md) — Built-in queue, retry with backoff

Integrations

• [references/mcp.md](references/mcp.md) — MCP client and server, transports, securing
• [references/email.md](references/email.md) — Email routing and handling
• [references/webhooks-push.md](references/webhooks-push.md) — Webhooks, push notifications
• [references/observability.md](references/observability.md) — Diagnostics-channel events

Experimental

• [references/think.md](references/think.md) — @cloudflare/think higher-level chat agent
• [references/voice.md](references/voice.md) — @cloudflare/voice STT/TTS
• [references/codemode.md](references/codemode.md) — Code Mode for tool orchestration
• [references/browse-the-web.md](references/browse-the-web.md) — CDP browser tools