Compaction Hook

A Claude Code hook that improves context compaction quality by injecting preservation priorities when compaction occurs.

The Problem

When Claude Code's context window fills up, it automatically compacts (summarizes) the conversation to make room. The built-in /compact command does the same thing manually. But the default compaction often loses important context:

• Decisions get summarized without their reasoning
• Code changes lose the "why" behind them
• User-provided constraints disappear
• The thread of what you were doing gets muddied

You end up with a compacted context that has the facts but lost the understanding.

The Solution

This hook injects a compaction strategy into the compaction process, telling Claude what to prioritize preserving:

• Decisions with their rationale
• Code changes with intent
• User-provided context and constraints
• Current task state and progress
• Open questions that need resolution

And what to compress aggressively:

• Exploration that led nowhere
• Verbose tool output
• Intermediate reasoning (keep conclusions)
• Superseded information

How It Works

Claude Code has a hooks system that runs shell commands at specific events. The PreCompact hook fires right before any compaction (manual or automatic).

Context fills up
       ↓
PreCompact hook fires
       ↓
Hook script runs, outputs strategy to stdout
       ↓
Strategy is injected as additional context
       ↓
Compaction occurs with strategy in mind
       ↓
Better quality preserved context

The hook script simply cats the strategy file. The output becomes part of the context that Claude uses when deciding what to preserve.

Installation

Step 1: Copy the files

# Copy the hook script
cp pre-compact.sh ~/.claude/hooks/pre-compact.sh

# Make it executable
chmod +x ~/.claude/hooks/pre-compact.sh

# Copy the strategy file
cp compaction-strategy.md ~/.claude/compaction-strategy.md

Step 2: Configure the hook

Add the PreCompact hook to your ~/.claude/settings.json:

{
  "hooks": {
    "PreCompact": [
      {
        "matcher": "",
        "hooks": [
          {
            "type": "command",
            "command": "$HOME/.claude/hooks/pre-compact.sh"
          }
        ]
      }
    ]
  }
}

If you already have other hooks configured, add the PreCompact section alongside them:

{
  "hooks": {
    "PostToolUse": [
      // ... your existing hooks ...
    ],
    "PreCompact": [
      {
        "matcher": "",
        "hooks": [
          {
            "type": "command",
            "command": "$HOME/.claude/hooks/pre-compact.sh"
          }
        ]
      }
    ]
  }
}

Step 3: Verify installation

# Check the hook script exists and is executable
ls -la ~/.claude/hooks/pre-compact.sh

# Check the strategy file exists
ls -la ~/.claude/compaction-strategy.md

# Test the hook script runs correctly
~/.claude/hooks/pre-compact.sh
# Should output the strategy content

Customizing the Strategy

The compaction-strategy.md file is yours to customize. Edit ~/.claude/compaction-strategy.md to match your priorities.

Adding project-specific context

If you work on specific types of projects, add relevant preservation priorities:

## Domain-Specific Preservation

- **API contracts** — Endpoint signatures, request/response shapes
- **Database schema decisions** — Table structures, migration choices
- **Architecture patterns** — Why we chose X over Y

Adjusting compression aggressiveness

If you find too much is being lost, move items from "Compress or Drop" to "Always Preserve":

## Always Preserve

... 7. **Error stack traces** — Full traces, not just error messages 8. **Alternative approaches considered** — Keep the reasoning for rejected options

Changing the output format

The format section guides how compacted context is structured. Adjust it to match how you like to receive information:

## Output Format

Structure as narrative paragraphs with clear section headers.
Lead with current state, then decisions, then code changes.
End with explicit next steps.

Relationship to /handoff Skill

This hook and the /handoff skill are complementary:

Use both:

• The hook silently improves compaction quality during your session
• The /handoff skill creates explicit documents when ending a session or switching contexts

Troubleshooting

Hook doesn't seem to be running

1. Check the script is executable: ls -la ~/.claude/hooks/pre-compact.sh
2. Verify the settings.json syntax is valid JSON
3. Check for typos in the hook path (use $HOME not ~)

Compaction quality hasn't improved

The hook injects the strategy, but I can't guarantee how Claude Code's internal compaction process uses it. The strategy becomes additional context, which should influence preservation, but the exact mechanism isn't documented.

If quality hasn't improved:

1. Try making the strategy more explicit/directive
2. Use the /handoff skill for critical context you can't afford to lose
3. Manually run /compact earlier in sessions to have more control

Strategy file not found errors

The hook silently continues if the strategy file doesn't exist (it just outputs nothing). Verify the file is at exactly ~/.claude/compaction-strategy.md.

How hooks work (technical details)

Claude Code hooks are configured in settings.json under the hooks key. Each hook type has an array of matchers and commands:

{
  "hooks": {
    "HookEventName": [
      {
        "matcher": "pattern", // String pattern to match (empty = match all)
        "hooks": [
          {
            "type": "command",
            "command": "shell command to run"
          }
        ]
      }
    ]
  }
}

For PreCompact:

• The matcher is typically empty (matches all compaction events)
• The hook's stdout is injected as additionalContext
• Exit code 0 means success; non-zero could block compaction

Available hook events:

• PreToolUse / PostToolUse - Before/after tool execution
• UserPromptSubmit - When user sends a message
• SessionStart / SessionEnd - Session lifecycle
• Stop / SubagentStop - When agents complete
• PreCompact - Before context compaction
• Notification - When notifications are sent
• PermissionRequest - When permissions are requested

Files

compaction-hook/
├── README.md                 # This documentation
├── pre-compact.sh            # Hook script (copy to ~/.claude/hooks/)
└── compaction-strategy.md    # Strategy file (copy to ~/.claude/)

License

MIT - Use freely, modify as needed, share with others.