Accumulate token usage across every LLM round of an agent turn and
expose the aggregate everywhere a turn is observable:
- AgentState.TurnUsage accumulates each round's reported usage,
including the final-answer synthesis call; TokenUsage.Accumulate
keeps the cache counters as prompt subsets and recomputes the
combined cache status (unsupported survives only while every call
was itself unsupported).
- The agent completion event carries the aggregate, and the complete
stream event exposes it both as data.usage and as a typed
StreamEvent.Usage that buildStreamResponse promotes into the
previously-unused StreamResponse.Usage wire field.
- The assistant message persists it as a nullable usage column on
both migration targets (postgres 000085 jsonb, sqlite 000012 text),
for web and IM channels alike, so history reads still carry the
turn's cost after the live stream is gone.
- "[LLM Usage]" log lines gain ", session_id=..., principal=type:id"
when the context carries them; calls outside sessions stay
byte-identical.
Usage from failed retry attempts is not accumulated: the per-call
usage log remains the ground truth, while the turn aggregate serves
as the operational signal.
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