The agent loop had no place to refuse a provider request. A host that needs to
act on the assembled context before it is billed, checking that the prompt still
fits the window, that a budget boundary has not been crossed, or that the
session should hand off instead of spending, could only observe the request
after the fact, when the tokens were already committed.
Add `AgentLoopConfig.beforeModelCall`, asked once per turn beside the deadline
check and before `turn_start` is emitted. A `stop` result ends the stream with
no turn open, so nothing has to synthesise a cancellation event and no consumer
is left holding a half-open turn. Placing it there also keeps `turn_end`'s
contract intact: that event carries the assistant message for a completed turn,
and a gated stop has no assistant message to report.
`syncContextBeforeModelCall` keeps its existing void contract and its job of
refreshing prompt and tool state, so implementations typed as returning void are
unaffected.
`Agent.setBeforeModelCall` installs the host's callback, and `addBeforeModelCall`
registers an additional callback without displacing the host's, returning a
disposer so an extension can attach and detach independently. A supplied
`reason` is logged where the loop stops.
Signed-off-by: Christian Stewart <christian@aperture.us>
Christian Stewart
e79eabc3b6