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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 registered automatically during npx @opengsd/gsd-core installation — no manual steps are needed under normal circumstances. For hook configuration details, threshold overrides, and manual registration examples, see Configuration.

As a brief reference: the statusline hook registers as statusLine in settings.json; the context monitor (gsd-context-monitor.js) registers as a PostToolUse hook (or AfterTool for Gemini CLI). Both entries use the absolute Node executable path that ran the installer. On Windows PowerShell, prefix quoted executable paths with &.

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)

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