The short answers,
in one place.
Everything asked most often, with no hedging. If something is still open, the terminal has the long version: --help prints every flag and every command headless accepts.
Nothing. headless is free to run, forever. There is no trial to expire, no seat to buy, no usage cap and no account to create. Run it as often and for as long as you like, and it draws on frontier models the whole time: free here does not mean the weaker brain.
A terminal and Node 26.4 or newer. There is no separate runtime to install and no service to sign up to.
No account and no sign-in. headless runs entirely on the machine you started it on, under your own root and your own keys.
Yes, and only once. On the very first launch headless sends a single request so the people who build it can count how many people are using it, then nothing more. It never sends your prompts, your files or your workspace, and there is nothing to switch off because there is nothing recurring to stop.
The root you launched it in, and nothing outside it. Paths are resolved against that root and refused if they climb out, and the noisy directories (node_modules, .git, dist, build) are skipped when listing.
Yes, but the exec tool is closed until you pass --shell for the run. It never reaches the agent's list by accident; you open it on purpose.
Three things stop it. It asks for one move per reply, so a reply that only describes an action is asked again in the shape the loop can act on; and if the same call with the same arguments comes back twice, the loop stops rather than spinning.
Esc interrupts a reply, and ctrl+c quits. Nothing is left running behind you.
The rest is in the terminal.
Every flag, every command and every key is printed by the thing itself. Nothing about it lives only on a web page.