Summary

The cost/token display attached to the last assistant message shows cumulative session totals (S.session.input_tokens, S.session.output_tokens, S.session.estimated_cost). This means the number attached to message 5 is the total cost of the entire conversation, not just that turn — which reads as misleading.

Proposed behaviour

Two acceptable options:

Option A: Show per-turn cost from the usage object delivered with each done SSE event, attached to that specific message bubble. Each message shows only what it cost.

Option B: Move the cumulative total out of the message bubble entirely and into the context ring / topbar badge. The composer footer context indicator already shows live token counts — cost could live there instead.

Either approach eliminates the misleading "last message has cumulative total" pattern.

Current behaviour

[user msg 1]
[assistant reply 1]
[user msg 2]
[assistant reply 2]  ← shows "4,217 in · 832 out · ~$0.0041" (TOTAL, not this turn)

Implementation notes

• The per-turn usage object arrives in the done SSE event and is currently stored in S.lastUsage but only used to sync the context ring
• S.lastUsage could also be used to set a per-turn cost annotation on that specific message
• The context ring / topbar already shows live totals so moving the cumulative number there wouldn't lose information

Identified in Sprint 24 planning notes.