durable-actors 0.2.5

Standalone regional durable-object control plane, host, and durability runtime
Documentation

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 1.4.2+. Install the CLI once:

pnpm add --global durable-actors

Create your actor project in your directory of choice

durable-actors init my-actors
cd my-actors
pnpm install
durable-actors dev // this will 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 SDK:

pnpm add durable-actors

Copy the three settings printed by dev into that application's .env file:

DURABLE_ACTORS_PROJECT_ID=local
DURABLE_ACTORS_CONTROL_PLANE_URL=http://127.0.0.1:7100
DURABLE_ACTORS_SECRET='<paste the secret printed by dev>'

Then generate your client from the same application directory:

durable-actors generate

This contract will match perfectly the actor you have defined!

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

Define and export actors in your actor project’s src/durable-objects.ts. 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 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>
        </>
    )
}

Host it yourself

Follow the self-hosting guide to connect your backend with an API key and deploy your actors.

See bucket authority and replication for ownership, leases, storage layout, and replica placement.

Test Modal hosts locally

To test Modal hosts against a control plane on your laptop, use the repository's pnpm run start:cloud command.

Reserve a dedicated ngrok domain for your control plane. Substitute your domain in these values in the repository root .env, alongside the Modal, GCS, database, and authentication settings:

NGROK_DOMAIN=YOUR_DOMAIN.ngrok.app
DURABLE_ACTORS_CONTROL_PLANE_URL=https://YOUR_DOMAIN.ngrok.app
DURABLE_ACTORS_CONTROL_PLANE_BIND=127.0.0.1:7200

pnpm run start:cloud

Keep this domain separate from the Terse backend's tunnel. Other ngrok accounts should reserve their own domain and substitute it above. The command waits for the tunnel, then starts the control plane; Ctrl+C stops both.

To keep the Terse backend's ngrok API on port 4040, give this tunnel its own local config. Create .durable-actors/ngrok.yml (already gitignored):

version: 3
agent:
  web_addr: 127.0.0.1:4041

Add NGROK_CONFIG=.durable-actors/ngrok.yml to the root .env, along with NGROK_AUTH_TOKEN for authentication. This config replaces ngrok's default config for this process only. Restart pnpm run start:cloud to apply it; the public endpoint and control-plane port stay the same. Leave NGROK_CONFIG unset to use ngrok's default config.

Reference

License

MIT © 2026 Terse