Localhost

The first time an agent says "your site is running on localhost:3333," you open a browser, type it in, and there it is. A real app. You almost paste the link in the group chat.

Then you open it on your phone. Nothing. Dead.

That is the whole lesson. Localhost is a dress rehearsal in an empty theater. Full lights. Full cast. No audience. You run it, you watch it, you fix the parts that wobble, and nobody has bought a ticket yet.

When you scaffold with the CLI and run bun dev:web, your laptop quietly becomes its own little server. It serves the page back to you. The address looks like:

http://localhost:3333

Two parts worth knowing:

• localhost is a nickname your computer uses for itself. You will also see 127.0.0.1. Same door. When you type it, you are not reaching the internet. You are knocking on your own front door.
• :3333 is the port — the specific door number. Your machine can run several things at once, so each gets its own number so they do not collide. You will see :3000, :5173, :8080. The exact number does not matter. It is just which door this show is behind.

The privacy is the feature.

Nobody else can see it. Not your teammate across the table. Not a judge. Not someone who guesses the address. There is no public URL pointing here. Close the lid and the show ends. Open it, start the rehearsal again.

That is why localhost is where all the building and breaking happens. Change a button, hit refresh, see it instantly. Ugly? Fix it. Broken? Break it more, read the error, fix it. I have had localhost look like a crime scene — half the page missing, red text everywhere — and it cost me nothing, because the only witness was me.

Where you will actually trip

1. "It works on my machine." Oldest joke in software. The thing runs fine on localhost and nobody has proved it runs anywhere else. Private rehearsal and opening night are different tests.

2. Your agent says it is "running locally." Same idea. It scaffolded the project and started the rehearsal on your disk so you can look before it goes public.

3. The link dies on your phone. Of course. localhost on your phone means your phone, not your laptop. The address only ever points at the machine you typed it on.

So how does rehearsal become a show the room can actually see? You push it to a public address anyone can reach. That move — empty theater finally filling up — is deploy.

Build it private. Rehearse on localhost. Open the doors when the show is ready.