An Agent Hub row reported a subagent as having run on a model that never
spoke. All 97 of its requests, 421K tokens and $6.19 of cost were served
by the primary; a transient stall then armed a fallback, that fallback
errored on its first request with an exhausted quota, and the run died.
Attribution followed the routing switch rather than the output.
Three surfaces lied independently, each re-deriving "the current model"
and calling it the run's model: the executor's progress snapshot, the
session's fallback selector that the hub row reads first, and the
transcript walk behind a settled row.
Sessions now own attribution. `AgentSession.servingModel` names the model
that produced this session's output, holding the last model that actually
served while a candidate is armed but unproven. A switch is a routing
decision, not evidence the target can produce anything, so the answer only
moves once a turn on the target settles.
Consumers read it instead of reconstructing it. The executor's observer
dropped its own event bookkeeping: that bus also carries advisor turns
running on a different model, and it was reading `retry_fallback_applied`
as proof of service. The hub row reads the same getter, so the main
session — which has no executor progress and no persisted history — stops
rendering an unproven candidate as its plain configured model. A fallback
armed before anything has served is still shown, marked as a fallback,
because there is no earlier work to miscredit there.
One predicate decides "this turn produced output", shared by the live
session and the offline replay so they cannot disagree. `error` and
`aborted` are both failures — a stalled stream is finalized as `aborted`
with its partial block still attached, so a stop reason alone proves
nothing — and a turn needs actionable content, which a `length` stop
burning its budget on unsigned thinking does not have. It tolerates
malformed content blocks: transcripts outlive the shapes that wrote them,
and one bad line previously blanked a whole row's history.
Ordering matters at two swap sites. Both the chain advance and the
cooldown-expiry restore move the model and fan `model_changed` out to
subscribers synchronously, so each now updates fallback state before the
swap rather than after; otherwise an observer reading attribution inside
that window sees the incoming candidate carrying the outgoing one's proof.
A startup-selected fallback owns the run from its first request only on a
fresh session. A resumed transcript already holds turns another model
produced, so there the candidate stays unproven until it answers.
`retryFallbackModel` is removed: every consumer reads `servingModel`, and
keeping a parallel derived getter alive for tests is the duplicate surface
this change set exists to remove.
enieuwy
0a075662ac