Feature Request: PreContextCompaction hook event

Feature Request

Add a PreContextCompaction hook event that triggers before the context window is automatically compacted/summarized.

Use Case

I created a /retro skill that analyzes session collaboration patterns and recommends improvements to CLAUDE.md and workflows. Currently, this skill must be run manually via slash command.

Ideally, this retrospective analysis would run automatically before context compaction to ensure:

1. Session insights are captured before context is summarized
2. Collaboration improvements are identified even when the user forgets to run /retro
3. A history of retrospectives is maintained across sessions

Current Hook Events

The current hook system supports these events:

• bash - Bash tool commands
• file - Edit, Write, MultiEdit tools
• stop - When agent wants to stop
• prompt - When user submits prompts
• all - All events

Proposed Addition

Add a new hook event:

• PreContextCompaction - Triggers before context window is automatically compacted

Example Usage

{
  "hooks": {
    "PreContextCompaction": [
      {
        "matcher": "*",
        "hooks": [
          {
            "type": "command",
            "command": "echo 'Context compaction starting...'"
          }
        ]
      }
    ]
  }
}

Or for skill invocation, the hook could trigger a slash command or skill before compaction occurs.

Benefits

• Enables automatic session retrospectives
• Allows saving session state/notes before context loss
• Supports automated logging or metrics collection
• Gives users a chance to preserve important context

Environment

• Claude Code CLI
• macOS Darwin 25.2.0