No signup vibe coding on a phone: the form you never fill in

Most “no signup” tools mean “signup later”

Almost every builder that ranks for this topic is honest about one thing and quiet about another. The honest part: you can look at a demo or generate a preview without an account. The quiet part: the moment you try to actually build, save, or run something, a form appears. Some of them are mobile apps you have to install from a store first, which is its own signup with its own friction. “No signup” was the thumbnail, not the product.

mk0r took the harder route. There is no form on submit, no paywall on the first build, and no sleight of hand. The rest of this page is the literal path from a tapped link to a working app, and then the one mechanism that makes the empty front door possible.

Why no signup is a bigger deal on a phone

On a laptop, an account form is a mild annoyance. On a phone it is the part of onboarding most likely to make you give up. Walk through a normal signup with your thumbs: type an email without a typo, invent a password that satisfies some rule you cannot see, clear a captcha that was designed for a mouse, then leave the browser, open your mail app, find the verification message, tap the link, and hope it returns you to the right place.

Every one of those steps is slower and more error-prone on a phone. They are also the steps that have nothing to do with the app you wanted to build. Removing the form does not just save time; it removes precisely the work a phone is worst at. What is left is a single textarea, which a phone handles perfectly well.

That is the whole reason this page exists as its own topic. Vibe coding from a phone is not hard because of the model or the preview. It is hard when a tool wraps the model in an account wall. Take the wall away and the phone is a fine place to build.

There is no signup form, because there is no signup code

This is the part no other guide on the topic gets to, because it is in the source rather than the marketing. mk0r does not “defer” the signup form. There is no signup endpoint to defer. Your identity is two values, both created by code that runs before you can type anything.

// src/app/(landing)/page.tsx, line 46-52
let key = localStorage.getItem(SESSION_KEY);
if (!key) {
  key = crypto.randomUUID();        // your session id
  localStorage.setItem(SESSION_KEY, key);
}

// src/components/auth-provider.tsx, line 69-72
} else {
  await signInAnonymously(auth);    // your anonymous UID
}

Because the identity is anonymous, the server still treats you as a real user. Your sessions and projects are owned by that anonymous UID in Firestore, the sandbox warms in the background (a prewarm call fires on first paint), and the model that answers you is Claude Haiku 4.5, the same model a signed-in user gets for free turns. The only thing missing is the email mk0r never asked you for.

You can verify this yourself without reading the code. Open the site, open your browser's storage inspector, and you will see one mk0r_session_key entry with a UUID in it. That UUID appeared the instant the page loaded. That is the receipt for an account you never created.

The honest limit: six turns, and the session stays on this phone

An empty front door has exactly one cost, and pretending otherwise would be dishonest. The chat API counts your completed anonymous turns on the Firestore field users/{uid}.anonTurnsCompleted. The constant ANON_TURN_LIMIT is 6, defined at src/app/api/chat/route.ts line 23. On the seventh turn the request returns sign_in_required and you get a one-tap Google sign-in.

The second half of the limit: that anonymous UID lives in this browser, on this phone. It does not roam to your laptop, and it does not survive clearing your browser storage. Six turns is enough to build and shape a tight single-screen app. If you want to keep going, or pick the project up on another device, the one tap of Sign in with Google upgrades the same anonymous user in place and carries every project with it. Nothing resets.

So the accurate framing is not “free forever, no account ever.” It is: build first, decide later, and the deciding is one tap rather than a form. For someone who tapped a link out of curiosity on a phone, that is the right order.