PreCompact

Runs before Claude Code is about to run a compact operation.

The matcher value indicates whether compaction was triggered manually or automatically:

Matcher	When it fires
manual	/compact
auto	Auto-compact when the conversation reaches the auto-compact window

Exit with code 2 to block compaction. For a manual /compact, the stderr message is shown to the user. You can also block by returning JSON with "decision": "block".

Blocking automatic compaction has different effects depending on when it fires. If compaction was triggered proactively before the context limit, Claude Code skips it and the conversation continues uncompacted. If compaction was triggered to recover from a context-limit error already returned by the API, the underlying error surfaces and the current request fails.

Claude Code discards a PreCompact hook's systemMessage and continue fields.

PreCompact input

In addition to the common input fields, PreCompact hooks receive trigger and custom_instructions. For manual, custom_instructions contains what the user passes into /compact and is null when they pass nothing. For auto, custom_instructions is null.

{
  "session_id": "abc123",
  "transcript_path": "/Users/.../.claude/projects/.../00893aaf-19fa-41d2-8238-13269b9b3ca0.jsonl",
  "cwd": "/Users/...",
  "hook_event_name": "PreCompact",
  "trigger": "manual",
  "custom_instructions": null
}