Claude Code's auto-compact fires late and its generic summary often drops the files, decisions, and blockers you needed. The post-compact session then veers off course. prep-compact nudges you earlier: a UserPromptSubmit hook watches your transcript's byte delta since the last compact. When it crosses a configurable threshold (default 4 MB ≈ 450K tokens on Opus 4.7), Claude is prompted to invoke the prep-compact skill, which drafts a tailored /compact <mini-schema> covering goal/next/files/decisions/state. You paste, run /compact, and the post-compact session resumes correctly. Invoke /prep-compact:prep-compact manually any time to refresh the draft before compacting.