In-call tools

Call control is expressed the same way as every other agent capability: as tools the agent is explicitly given. An agent that should be able to hang up gets cosmo.end_call. Nothing is implied.

Hanging up: cosmo.end_call

const agent = client.agent({
  instructions: '…End the call politely once the delivery window is confirmed.',
  tools: [{ kind: 'server', name: 'cosmo.end_call' }],
});

When the model calls it, the server ends the phone leg and the session winds down — your event stream finishes with the terminal session-ended item like any other shutdown. Without this tool the agent cannot hang up; the call ends when the human does, a silence-timeout EndCall hook fires, or your code calls session.end().

Give the model clear hang-up criteria in the instructions. The tool grants the ability; the instructions supply the judgment.

Guarding call control

Call-control tools are server tools, so PreToolUse hooks don't intercept them — the guarantee is in which tools you grant, plus the server hooks you declare:

from cosmo_ai.hooks import SilenceTimeout, Say, EndCall

agent = client.agent(
    instructions="…",
    hooks=[
        SilenceTimeout(timeout_seconds=30,
                       action=Say(prompt="Check if the caller is still there."),
                       max_count=2, reset_mode="on_user_speech"),
        SilenceTimeout(timeout_seconds=90,
                       action=EndCall(farewell="I'll let you go — call back anytime.")),
    ],
)

This combination — explicit grants, described policy, declarative timeouts — is the whole call-control story: no hidden telephony mode, just composed primitives you can read back off the agent config.