Long Claude Code sessions eventually compact: the conversation gets summarized to free up context. Usually that's fine, but sometimes the detail you needed (an error message, a decision, a file list) gets squashed. A PreCompact hook lets you save a copy first.

What a PreCompact hook does

PreCompact runs right before Claude Code compacts, whether you typed /compact (manual) or the context filled up (auto). The hook gets JSON on stdin that includes transcript_path and trigger.

Add this to .claude/settings.json:

{ "hooks": { "PreCompact": [ { "matcher": "manual|auto", "hooks": [ { "type": "command", "command": "mkdir -p .claude/backups && jq -r '.transcript_path' | xargs -I{} cp {} .claude/backups/pre-compact-$(date +%Y%m%d-%H%M%S).jsonl" } ] } ] } }

Every time the session compacts, the full transcript gets copied to .claude/backups/ with a timestamp. If the summary loses something, grep the backup.

Tips

  • Add .claude/backups/ to .gitignore. Transcripts can contain secrets and file contents.
  • Clear old backups now and then: find .claude/backups -mtime +7 -delete.
  • Want only manual compacts? Set the matcher to manual.
  • Run /hooks to check the hook is loaded.

Shared settings, safety hooks, a short CLAUDE.md and a read-only reviewer subagent ship ready to copy in Vildanden Agent Config. The free sample is enough to get started today.