Preview web apps live as your coding agent builds them with Amazon Bedrock AgentCore runtime

Coding agents now build working web apps: a dashboard, a product page, a prototype that a product team wants to try. The fastest way to review that work is to open the app, use it, ask for a change, and check the result. That loop works well when the app keeps running where the agent edits it, so each change shows up in the browser within seconds. When the app must be copied out and deployed somewhere else first, every change waits for a deployment, and the review slows down to that pace.

Amazon Bedrock AgentCore is a platform to build, connect, and optimize agents at scale, with your choice of agent framework and foundation model (FM). AgentCore runtime , a capability of Amazon Bedrock AgentCore, runs each agent session in its own isolated microVM. With Session endpoints , you can open an app running in that session from your own browser. Session Endpoints give each session an authenticated URL that forwards each request’s HTTP method, path, query string, and body, and WebSocket connections, to the ports you allow. You can watch the page update as the agent saves each file.

In this post, you deploy a coding agent that keeps a Vite website running in its session, then open one local page with a chat on the left and the live website on the right. You ask the agent to switch the site to a dark color theme, follow its steps as they stream in, and watch the website change without a refresh. Finally, you look at what happens internally: a second port in the same session, and the requests the Session endpoint refuses.

The agent’s file tools resolve every path inside the site folder and refuse paths outside it, so a prompt can’t make the agent change other files in the session. If other people can send prompts to your coding agent, consider Amazon Bedrock Guardrails to filter harmful prompts and responses, and review the agent’s changes before you publish them.

The complete sample, including the notebook this post follows, is in the 14-remote-access folder of the agentcore-samples GitHub repo .

Each session gets its own Session endpoint, and six details shape how you use it.

You choose which ports it reaches. You set allowedPorts when you call CreateAgentRuntime : 1–5 entries, each a single port or a range within 8000–12000, excluding the reserved platform ports . Allowing a port doesn’t start a server, so your application must listen on it, bound to 0.0.0.0 .