The keyboard rejects the whole sentence, not the last word

The vendor's installer is an ncurses wizard. No flags, no answer file. An agent has to drive it.

It sends POST /api/v1/terminal/press with keys: ["arrow_down", "arrow_down", "tab", "enter"]. libvterm encodes each key for the mode the program is actually in, application cursor mode inside the wizard and normal mode back at the shell, so arrow_down is the right bytes in both. The agent never tracks DECCKM itself.

The array is validated before any key is sent. A misspelled arrow_dwn returns 400 with the full supported list and zero keystrokes delivered. The wizard is not left two arrows deep with a Tab pending. The agent reads the list, fixes the name, resends.

Expect scripts fail mid-sequence. This one fails before the sequence, or completes it.