Context Re-injection on Compaction Hook

Hook Type

SessionStart with compact matcher -- Fires after context window compaction.

Description

During long Claude Code sessions, the context window gets compacted to free up space. This can cause Claude to "forget" important project-specific instructions (like which package manager to use, test requirements, or current sprint focus). This hook re-injects those critical reminders after every compaction.

Patterns/Rules

• Triggers on SessionStart events that match the compact pattern
• Outputs plain text that gets injected into Claude's context
• Keep the reminder concise -- every token counts
• Focus on instructions that Claude frequently forgets after compaction
• Use a file-based approach for easy editing without touching settings.json

Configuration

Simple Inline Version

Copy
{
  "hooks": {
    "SessionStart": [
      {
        "matcher": "compact",
        "hooks": [
          {
            "type": "command",
            "command": "echo 'CRITICAL REMINDERS: Use pnpm (not npm). Run pnpm test before committing. Current sprint: auth-v2 refactor. Branch prefix: feat/ for features, fix/ for bugs.'"
          }
        ]
      }
    ]
  }
}

File-Based Version (Recommended)

Copy
{
  "hooks": {
    "SessionStart": [
      {
        "matcher": "compact",
        "hooks": [
          {
            "type": "command",
            "command": "cat .claude/compaction-context.md 2>/dev/null || echo 'No compaction context file found.'"
          }
        ]
      }
    ]
  }
}

Then create .claude/compaction-context.md:

Copy
# Post-Compaction Context

## Build & Package Manager
- Use pnpm, NEVER npm or yarn
- Run `pnpm test` before every commit
- Run `pnpm lint` before creating PRs

## Current Sprint
- Sprint: Auth V2 Refactor (ends 2025-04-01)
- Focus: Migrate from JWT to session-based auth
- Key files: src/auth/, src/middleware/session.ts

## Branch Conventions
- Features: `feat/description`
- Bugfixes: `fix/description`
- Always branch from `develop`, never from `main`

## Testing Requirements
- Unit tests required for all new functions
- Integration tests required for API endpoints
- Minimum 80% coverage on changed files

Action

After context compaction:

1. The SessionStart event fires with the compact matcher
2. The hook reads and outputs the compaction context file
3. Claude receives the reminders as fresh context
4. Project-specific rules are preserved across compaction boundaries