msd-core/docs/context-monitor.md

Context Window Monitor

A post-tool hook (PostToolUse for Claude Code, AfterTool for Gemini CLI) that warns the agent when context window usage is high.

Problem

The statusline shows context usage to the user, but the agent has no awareness of context limits. When context runs low, the agent continues working until it hits the wall — potentially mid-task with no state saved.

How It Works

1. The statusline hook writes context metrics to /tmp/claude-ctx-{session_id}.json
2. After each tool use, the context monitor reads these metrics
3. When remaining context drops below thresholds, it injects a warning as additionalContext
4. The agent receives the warning in its conversation and can act accordingly

Thresholds

Level	Remaining	Agent Behavior
Normal	> 35%	No warning
WARNING	<= 35%	Wrap up current task, avoid starting new complex work
CRITICAL	<= 25%	Stop immediately, save state (/gsd-pause-work)

Debounce

To avoid spamming the agent with repeated warnings:

• First warning always fires immediately
• Subsequent warnings require 5 tool uses between them
• Severity escalation (WARNING -> CRITICAL) bypasses debounce

Architecture

Statusline Hook (gsd-statusline.js)
    | writes
    v
/tmp/claude-ctx-{session_id}.json
    ^ reads
    |
Context Monitor (gsd-context-monitor.js, PostToolUse/AfterTool)
    | injects
    v
additionalContext -> Agent sees warning

The bridge file is a simple JSON object:

{
  "session_id": "abc123",
  "remaining_percentage": 28.5,
  "used_pct": 71,
  "timestamp": 1708200000
}

Integration with GSD

GSD's /gsd-pause-work command saves execution state. The WARNING message suggests using it. The CRITICAL message instructs immediate state save.

Setup

Both hooks are automatically registered during npx @opengsd/get-shit-done-redux installation:

• Statusline (writes bridge file): Registered as statusLine in settings.json
• Context Monitor (reads bridge file): Registered as PostToolUse hook in settings.json (AfterTool for Gemini)

Manual registration should use the absolute Node executable path that ran the installer. On Windows PowerShell, prefix the command with & when that executable path is quoted.

Manual registration in ~/.claude/settings.json (Claude Code):

{
  "statusLine": {
    "type": "command",
    "command": "\"/usr/local/bin/node\" \"/Users/me/.claude/hooks/gsd-statusline.js\""
  },
  "hooks": {
    "PostToolUse": [
      {
        "hooks": [
          {
            "type": "command",
            "command": "\"/usr/local/bin/node\" \"/Users/me/.claude/hooks/gsd-context-monitor.js\""
          }
        ]
      }
    ]
  }
}

For Gemini CLI (~/.gemini/settings.json), use AfterTool instead of PostToolUse:

{
  "hooks": {
    "AfterTool": [
      {
        "hooks": [
          {
            "type": "command",
            "command": "& \"C:/Program Files/nodejs/node.exe\" \"C:/Users/me/.gemini/hooks/gsd-context-monitor.js\""
          }
        ]
      }
    ]
  }
}

Safety

• The hook wraps everything in try/catch and exits silently on error
• It never blocks tool execution — a broken monitor should not break the agent's workflow
• Stale metrics (older than 60s) are ignored
• Missing bridge files are handled gracefully (subagents, fresh sessions)