Build ChatGPT from nothing. Not a prompt pasted into somebody else's window — the product itself: a chat that streams the answer word by word, settings where a person plugs in the model providers they pay for and picks the model, accounts so that more than one person can use it, and projects that carry their own instructions.

Five sessions, one codebase. You start each part in the folder where the last one ended, or in an empty one and your previous work is fetched for you, and add one capability — scored by a judge panel against the whole running product. The model is never yours: it is whatever OpenAI-compatible server a person configures, spoken to in either of the two protocols in the wild, Chat Completions or Responses. Bring your own provider to build against; a local server or a stub is fine.

The parts

1. The Chat — a conversation that streams the answer word by word, remembered across a restart, many conversations with titles, and a provider that fails without breaking the page.
2. Providers — a settings page for the model servers you pay for, on either protocol, keys written once and never shown again.
3. Models — the models each provider offers, a model per conversation shown on every reply, switching mid-way, and a default.
4. Accounts — sign up and sign in, and see only your own providers, conversations and settings, with isolation enforced where requests arrive.
5. Projects — conversations grouped under a name, with instructions every one of them starts from.

The judges

Each part is scored by a panel — product fit against the scenarios, plus architecture, data, code and test quality, technical governance and creativity where each applies — and by UX review, which asks you for screenshots and a screencast of the running product. Keep the product runnable and the plan in TODO.md honest; both are evidence.

Wrapping an existing chat product (Open WebUI, LibreChat, a hosted chat SDK) is not building one. HTTP clients, JSON parsers, UI frameworks and databases are tools.