Recording: Claude Code transcript compaction can erase the /prepare-build announcement bookend

Observed on run-08 (2026-09-07): the .session breadcrumb correctly pointed at the live transcript, but by extraction time the session had gone through context compaction and the transcript JSONL no longer contained the /prepare-build announcement text ("Run 08 — building in builds/run-08/") — the default opening bookend matched nothing and extract.py sliced from the start of the surviving transcript. The closing bookend (recent text) still matched. Recovered by re-running with --from="Spec fully absorbed", a phrase that survived compaction; the final log covers 138 turns of the build but has lost the allocation/checklist turns that preceded the compaction boundary.

Implication: the post-hoc recording design (CLAUDE.md "Recording is post-hoc") assumes the transcript retains raw turns for the whole build window. Long sessions violate that: compaction can drop any pre-boundary turn, including the opening bookend and early build activity.

Options to consider (next tooling pass):

• /prepare-build writes the announcement text (or a unique token) into evals/run-NN/.session alongside the path, and extract.py falls back to slicing from the earliest surviving turn while WARNING loudly that the opening bookend was lost (today it says "no match — sliced from start" only in stdout).
• Have /prepare-build note a timestamp at allocation; slice by timestamp rather than text.
• Or extract eagerly (run /extract-build-log periodically during long builds) since re-runs are idempotent.

🤖 Generated with Claude Code

Comienza con extract.py y el flujo de grabación de /prepare-build; después, lee las indicaciones de CLAUDE.md según las cuales la grabación es post-hoc. Reproduce la extracción después de la compactación de la transcripción usando el escenario run-08 y compara las opciones de boundary disponibles. Se considera terminado cuando las sesiones largas ya no pierden silenciosamente el contexto inicial y la extracción informa claramente de qué boundary se usó cuando falta el bookend original.