Feature Request: Add context usage percentage to hooks and enable automatic /compact execution

Summary

Request to add context usage monitoring capabilities to hooks, enabling automatic /compact execution when context usage reaches a threshold.

Problem

Currently, when working with Claude Code:

1. No programmatic access to context usage - There's no API, environment variable, or hook input field that provides the current context usage percentage
2. Cannot execute /compact from hooks - Slash commands cannot be invoked from bash commands in hooks
3. No context-specific hook events - No ContextWarning or similar event exists

This makes it impossible to automatically monitor and compress context before hitting the 95% auto-compact threshold, which often results in:

• Loss of important context from the beginning of the conversation (requirements, goals)
• Loss of recent progress information
• The "Conversation too long" error when trying to run /compact manually

Proposed Solution

Option 1: Add context usage to hook environment variables

# Available in hook scripts
$CONTEXT_USAGE_PERCENT  # e.g., "78"
$CONTEXT_USAGE_TOKENS   # e.g., "156000"
$CONTEXT_MAX_TOKENS     # e.g., "200000"

Option 2: Add context usage to hook input JSON

{
  "tool_name": "Bash",
  "tool_input": {...},
  "context_usage": {
    "percent": 78,
    "tokens_used": 156000,
    "tokens_max": 200000
  }
}

Option 3: Add a ContextWarning hook event

{
  "hooks": {
    "ContextWarning": [
      {
        "matcher": "threshold:75",
        "hooks": [
          {
            "type": "command",
            "command": "echo 'Context at 75%'"
          }
        ]
      }
    ]
  }
}

Option 4: Enable /compact execution from hooks

Allow hooks to trigger slash commands like /compact with custom instructions:

{
  "hooks": {
    "PostToolUse": [
      {
        "hooks": [
          {
            "type": "slash-command",
            "command": "/compact",
            "args": "Preserve initial requirements and latest progress"
          }
        ]
      }
    ]
  }
}

Use Case

As a user working on complex, multi-step tasks:

1. I want to automatically monitor context usage
2. When it reaches 75-80%, automatically run /compact with custom instructions
3. Preserve initial requirements (beginning of conversation) and recent progress (end of conversation)
4. Avoid the "Conversation too long" error that prevents manual compaction

Current Workaround

Using a combination of:

• PostToolUse hook with a bash script that estimates context from transcript file size
• Stop hook with a prompt that asks Claude to estimate context usage

This is imprecise and cannot actually execute /compact automatically.

Environment

• Claude Code version: latest
• OS: macOS

Additional Context

This would significantly improve the user experience for long-running sessions and complex tasks, especially when using parallel agents (Task tool) which consume context quickly.