Per-turn execution spans: per-tool-call latency, token usage, and indices into the transcript. Same span shape as the eval iteration trace.

Incremental by default — returns the LATEST turn, not the whole session, because a session's spans are the largest thing this API emits and the turn you just took is the one you are debugging. Reach older turns with turnId or afterPromptIndex; use includeSpans=false for cheap per-turn summaries.

A turn whose spans could not be read reports spansUnavailable: true rather than an empty spans array: "made no calls" and "could not fetch" are opposite conclusions.