Durable Actors

Managing state is hard! Back in the pre-agent era, building a multiplayer app showed just how hard this could be. You had to lock resources, deal with websockets at scale, handle peak loads etc...

Now with AI, we've got agents working with agents and agents working with people to worry about. Furthermore, we have agent swarms coming!

Durable Actors is a primitive to help developers build the next generation of collaborative software. We provide a mechanism to serve shared, concurrency safe state to your app.

Based on the Actor principle from Erlang, all state is durably persisted for you. Only one Agent/person can be in the actor at a time, protecting you from race conditions.

We are fully horizontally scalable, and instances go dormant when not in use. Only pay for what your users are using.

We offer a clean API to manage webSocket connections, Swift inspired syntax for building your actor and full observability into your deployed actors.

Local development

Install Node.js 22.19+, pnpm, and Bun 1.3.9+.

Create your actor project in your directory of choice

npx durable-actors init my-actors
cd my-actors
pnpm install
# Or with npm:
npm install
npx durable-actors dev # Run the server locally on your machine

Running dev will also start a watch, every-time you make a change to an actor and save, metadata changes will be stored automatically.

Connect your application

In your separate application project's directory (ex: node server), install the generator as a development dependency:

pnpm install --save-dev durable-actors
# Or with npm:
npm install --save-dev durable-actors

Then generate your client from the same application directory:

npx durable-actors generate

Now you may call your actor and access the state.

import { actors } from "./generated/index.js"

const counter = actors.Counter.get("example")
console.log(await counter.increment())

For complete sample applications, see AI Chat, Collaborative documents, and Chatroom.

Define an Actor

Install ai in your actor project:

pnpm install ai
# Or with npm:
npm install ai

Define and export actors in your actor project’s src/actors.ts, the default entrypoint loaded by durable-actors dev. For example, a chat history actor:

import type { UIMessage } from "ai"
import { Actor, Persisted } from "durable-actors"

export class ChatHistory extends Actor {
    @Persisted private messages: UIMessage[] = []

    async load() {
        return this.messages
    }

    async append(message: UIMessage) {
        this.messages.push(message)
        return this.messages
    }
}

Stream from the backend (Express)

After adding ChatHistory, rerun npx durable-actors generate in your application and use its generated client:

import { openai } from "@ai-sdk/openai"
import { convertToModelMessages, generateId, pipeUIMessageStreamToResponse, streamText, toUIMessageStream, validateUIMessages } from "ai"
import express from "express"

import { actors } from "./generated/index.js"

const app = express()
app.use(express.json())

app.get("/api/chat/:id", async (request, response) => {
    response.json(await actors.ChatHistory.get(request.params.id).load())
})

app.post("/api/chat", async (request, response) => {
    const [message] = await validateUIMessages({ messages: [request.body.messages.at(-1)] })
    if (message.role !== "user") return response.sendStatus(400)
    const chat = actors.ChatHistory.get(request.body.id)
    const messages = await chat.append(message)
    const result = streamText({
        model: openai("gpt-5-mini"),
        messages: await convertToModelMessages(messages)
    })
    await pipeUIMessageStreamToResponse({
        response,
        stream: toUIMessageStream({
            stream: result.stream,
            originalMessages: messages,
            generateMessageId: generateId,
            onEnd: async ({ responseMessage, outcome }) => {
                if (outcome.status === "completed") await chat.append(responseMessage)
            }
        })
    })
})

Connect the frontend (React)

import { useChat } from "@ai-sdk/react"
import type { UIMessage } from "ai"

const history: UIMessage[] = await fetch("/api/chat/lobby").then(response => response.json())

function Chat() {
    const { messages, sendMessage, status } = useChat({ id: "lobby", messages: history })
    const busy = status === "submitted" || status === "streaming"

    return (
        <>
            {messages.map(message => (
                <p key={message.id}>
                    {message.role}: {message.parts.map(part => (part.type === "text" ? part.text : "")).join("")}
                </p>
            ))}
            <form
                action={async form => {
                    await sendMessage({ text: String(form.get("message")) })
                }}
            >
                <input name="message" aria-label="Message" required disabled={busy} />
                <button disabled={busy}>Send</button>
            </form>
        </>
    )
}

durable-actors 0.4.3