Let your coding agent check your localhost app

Your coding agent can change the code. Checking the result is the slow part: you open the page, click through the flow, read the console, and tell the agent what broke. Here is how to let the agent do that check itself, in a browser window you can watch.

You need a local coding agent that speaks MCP, such as Claude Code, Codex or Cursor, and a dev server you can start.

1. Install and start Hronaut

Download the package for your system from the download page and start it. Hronaut is a desktop app for Windows, macOS 13 or newer, and Linux. The Windows and macOS builds are not code-signed yet, so SmartScreen or Gatekeeper may warn; the download page shows how to check the file against its published hash and GitHub build attestation. The first 10 days are free, counted from your agent's first tool call. The trial needs no card and never charges you.

2. Connect your agent

In Claude Code:

/plugin marketplace add hronaut/hronaut-claude-plugin
/plugin install hronaut@hronaut

In a terminal, for Codex CLI or the ChatGPT desktop app:

codex plugin marketplace add hronaut/hronaut-codex-plugin
codex plugin add hronaut@hronaut

The plugins read Hronaut's MCP token on their own, so there is nothing to paste. Other agents have a ready configuration on the setup page and on Hronaut Home. Start a new session and run /mcp in Claude Code or Codex: Hronaut should show as connected.

3. Start your dev server on a fixed address

For Vite: npx vite --host 127.0.0.1 --port 5173 --strictPort. Browsers keep local storage and IndexedDB per origin, that is per scheme, host and port, so http://localhost:5173, http://127.0.0.1:5173 and http://localhost:5174 do not share them. --strictPort makes Vite stop with an error instead of quietly moving to the next free port.

4. Ask for the check

Paste this, with your own address and finish line:

Create a Hronaut workspace named local-check with scratch storage.
Open http://127.0.0.1:5173 and wait for the page heading.
Submit the sign-up form empty, then with a valid email.
Report what the page shows each time, console errors,
and failed network requests.

The agent creates an isolated workspace and opens the page in a visible tab. It reads the page as an accessibility snapshot, clicks and types by element, and reads the console and network log. You watch the same tab and can pause the agent or take over at any time.

5. Edit, restart, check again

Fix what the agent found, restart the server if needed, and ask it to check again in the same tab. Cookies and storage live in the workspace, not in your dev server, so they survive the restart. If the server is down for a moment, Hronaut keeps the failed address in the tab and the agent can retry it.

A new agent session the next day can return to the same workspace with its ID and private resume key, instead of starting over.

Where this stops

• Hronaut drives Chromium on your computer. It is not a CI runner and does not cover Firefox or WebKit. Turn each bug it finds into a test in your repository.
• The agent must run on the same machine. A cloud agent sees its own localhost, not yours: see connection troubleshooting.
• Keep the dev server on 127.0.0.1. There is no need to expose it with 0.0.0.0 or a tunnel for this.

More detail is in the localhost guide.

Sources: hronaut.dev/setup (plugin commands), the localhost guide (origins, restarts, limits), Vite server options (strictPort), MDN: origin.

Written with the help of AI.