How to preview apps an AI agent builds

On Runtime, a preview that nobody is using costs nothing but storage. With idlePauseSeconds, the sandbox pauses when visits stop. It keeps its files, memory and the running server. The next visit wakes it, behind a short "Waking up" page. While it runs, a 2 vCPU, 4 GiB sandbox costs $0.03125 an hour while it waits on requests, because Runtime bills the CPU the app uses.

The short answer

Write the agent's files, start the server with spawn, and share the port:

TypeScriptimport { Sandbox } from "withruntime";const sbx = await Sandbox.create({ idlePauseSeconds: 900, labels: { project: "p-123" } });await sbx.files.write(  "/workspace/app/index.html",  "<!doctype html><h1>Built by the agent</h1>", // what the agent wrote);await sbx.spawn("python3 -m http.server 3000", { cwd: "/workspace/app" });const preview = await sbx.previews.create(3000);console.log(preview.urlWithToken); // a link that carries its token, for the user

Pythonfrom withruntime import Sandboxsbx = Sandbox.create(idle_pause_seconds=900, labels={"project": "p-123"})sbx.files.write("/workspace/app/index.html", "<!doctype html><h1>Built by the agent</h1>")sbx.spawn("python3 -m http.server 3000", cwd="/workspace/app")preview = sbx.previews.create(3000)print(preview["urlWithToken"])  # a link that carries its token, for the user

For a real project, replace the server with the project's own dev server, such as npm run dev -- --port 3000. A process started with spawn keeps running after your call returns. A process started with exec ends when its command ends.

Private or public

On a paid sandbox, make a preview public with previews.create(3000, { visibility: "public" }). A sandbox without credit keeps its previews private. previews.rotate(port) rejects every token issued so far, and previews.delete(port) stops sharing. WebSockets work, and dev servers use them for live reload. Preview addresses are under runtimehost.com, never under Runtime's own site. So an agent's app never shares an origin with your account (security).

Start every project with the template running

Installing a framework's dependencies takes the longest part of a new project. Do it once in a template sandbox. Keep it as a snapshot, and start each new project from the snapshot. A snapshot keeps running processes, so the dev server is already up when the copy starts:

TypeScriptimport { Runtime } from "withruntime";const runtime = new Runtime();// Once: install the template and start its dev server.const template = await runtime.sandboxes.create({ diskMiB: 8192 });await template.files.upload("./template", "/workspace/app");await template.exec("cd app && npm ci", { check: true, timeoutMs: 600_000 });await template.spawn("cd app && npm run dev -- --port 3000");const snapshot = await template.snapshot({ name: "web-template", retentionDays: 30 });await template.stop();// Each new project: a copy, a preview, and the agent at work.const project = await runtime.sandboxes.create({ snapshot: snapshot.id });await project.update({ idlePauseSeconds: 900 });const preview = await project.previews.create(3000);console.log(preview.urlWithToken);

Pythonfrom withruntime import Runtimeruntime = Runtime()template = runtime.sandboxes.create(disk_mib=8192)template.files.upload("./template", "/workspace/app")template.exec("cd app && npm ci", check=True, timeout_ms=600_000)template.spawn("cd app && npm run dev -- --port 3000")snapshot = template.snapshot(name="web-template", retention_days=30)template.stop()project = runtime.sandboxes.create(snapshot=snapshot["id"])project.update(idle_pause_seconds=900)preview = project.previews.create(3000)print(preview["urlWithToken"])

To show the user two versions side by side, fork the project into copies. Let the agent change each copy, and share a preview of each one.

Find the user's project again

Name each project's sandbox. getOrCreate returns the same sandbox tomorrow, woken if it paused:

TypeScriptimport { Sandbox } from "withruntime";const sbx = await Sandbox.getOrCreate("project-p-123", { idlePauseSeconds: 900 });console.log(sbx.info.reused, sbx.id);

What an app-preview product needs

What it costs

Take 200 projects a month. Each keeps a 2 vCPU, 4 GiB sandbox running for 3 hours in all, while its user works. It uses 0.1 of a vCPU on average. It is paused the other 717 hours of a 30-day month, with 1 GB of its own disk and memory stored:

That is about 18 cents a project a month. Runtime charges $0.025 per vCPU-hour of measured CPU, $0.0075 per GiB-hour of memory while running, and $0.08 per GB per 30-day month while paused (pricing). The snapshot is billed as storage at the same $0.08 rate. Every new account gets 100 free hours of a 1 GB machine in its first 30 days, with no card.

Start

Terminalnpx withruntime sandboxes run --keep -- python3 --version

The first run prints a link to approve in your browser. From the CLI, runtime sandboxes preview <id> 3000 --public shares a port (CLI).

Related: coding agent sandbox, per-user dev environments, sandbox snapshots, pause and resume a sandbox.

Facts on this page were checked on 26 October 2026.