Jul 2026
COS opens the door. COSF closes it.
Design Notes Design
An assistant that anyone can text is an assistant anyone can misuse. Two words decide who it works for — and the off switch is the one that had to be perfect.
An agent reachable by text is reachable by everyone with the number. So the first question is not what it can do; it is who it answers to, and how a person says yes and no.
Texting COS turns the chief of staff on for you. Texting COSF turns her off. That is the entire mechanism, and almost all the engineering went into making those two words impossible to trip over by accident.
Three letters, only because matching is strict
A short activation word is only safe if it cannot appear by accident. Lowercase 'cos' hides inside because, cost, cosy and tacos — a forgiving matcher would have enrolled strangers out of ordinary sentences.
So matching is uppercase-exact and whole-word. You cannot activate an assistant by mentioning the price of something.
The off switch is evaluated first
COSF is checked before activation and can never read as COS. An off switch that occasionally turns things on is the one failure you cannot apologise your way out of, so it gets the stricter path.
Revoking also keeps the person and what the agent knows about them. Closing your assistant is not amnesia — if you come back, she still knows who you are.
"An off switch that sometimes turns things on is the one failure you cannot recover from."
A door with the handle on the inside
One early version only listened while somebody was enrolled, which meant that revoking the last person shut the door with the handle on the outside — no message could ever reopen it, and every health check reported the system fine.
It now listens whenever the gate exists, whether or not anyone is currently enrolled. Empty and listening is a valid state; silent and unreachable is not.
Design Notes writes product notes for Rivesa, focusing on consent, activation, and the craft of keeping delegated work accountable.
