The preview is stuck on "Building your app"

The preview shows your app running on a real server. If it does not appear within about 15 seconds, the panel replaces the spinner with the actual reason.

What the states mean

• Starting — the server is booting. A cold project runs npm install first, which can take a minute. The elapsed time is shown so you can tell progress from a hang.
• The server stopped — it started and exited. The recent output is shown underneath; that output names the cause.
• The server has not been started — no start command was ever run for this project.

Two buttons

Start the server runs the project's start command. If none has ever run, the platform works one out from your package.json. It also self-heals the common boot failures: a missing npm package is installed, a port held by a dead process is freed, and a CommonJS/ESM mismatch is corrected.

Fix with AI hands the whole diagnosis — state, command, exit code and output — to the AI, so it starts from facts rather than "it's broken".

If it keeps stopping

Read the output in the panel. A crash on boot is almost always a missing dependency or a syntax error in a file that was just edited. Use Fix with AI; it sees the same output you do.