Feature request: add a PreCompact hook event #12208

What feature would you like to see?

Add a dedicated PreCompact hook event that runs immediately before conversation compaction (manual or auto).

Why this matters

Today, compaction can happen automatically when token usage crosses model_auto_compact_token_limit. There are hook events for AfterAgent and AfterToolUse, but no hook for the compaction boundary itself.

For users who maintain external memory/state, this means there is no deterministic point to capture full pre-compaction context.

Proposed behavior

Add a new hook event, for example:

• PreCompact

Triggered when compaction is about to run (both manual and auto).

Suggested payload fields:

• session_id
• thread_id
• turn_id (if applicable)
• reason (manual or auto)
• total_usage_tokens_before_compaction
• auto_compact_limit
• triggered_at
• cwd

Failure behavior

Please make failure policy explicit and configurable (same style as existing hook outcomes):

• continue on error
• abort compaction on error

Example use cases

• extract important memory facts before summary compaction
• persist custom checkpoints to external systems
• emit telemetry about compaction quality and timing
• run a user-defined “last chance” context snapshot

Additional context

This request is specifically for Codex hooks (not Claude-specific behavior).

If helpful, I can propose a concrete schema and tests aligned with the existing hook event design.