I Had No Idea How Full My Claude Code Context Window Was. So I Built a Tool.

You know that feeling when Claude starts giving weird, truncated answers? Or when it seems to "forget" earlier context in a long session?

That's your context window filling up.

Claude Sonnet has a 200,000 token context window. When it's full, older messages get dropped. Your carefully set-up context — the files you loaded, the constraints you established, the codebase you explained — silently disappears.

I've been running Claude Code for 60+ days in an autonomous loop. Last week I realized I had no idea how full my context was at any given moment. So I built cc-context-check.

What it does

npx cc-context-check

cc-context-check — Context window usage across sessions

Context limit: 200.0k tokens (Claude Sonnet/Opus)

🟢 ~/projects/my-app    [a3f9c12] just now · 12.4 MB
   ████████████░░░░░░░░░░░░░░░░░░ 40.1% used
   80.2k input · 1.2k output · 119.8k remaining

🟡 ~/                   [b7d44e1] 2h ago · 5.9 MB
   █████████████████████░░░░░░░░░ 71.5% used
   143.0k input · 89 output · 57.0k remaining
   △ Warning: Context is getting full — consider /compact

Color coded: 🟢 under 70% → 🟡 70-85% (consider compacting) → 🔴 85%+ (compact now)

How it works

Claude Code stores session transcripts as .jsonl files in ~/.claude/projects/. Each line is a message. Each assistant message includes a usage object:

{
  "usage": {
    "input_tokens": 1,
    "cache_read_input_tokens": 79927,
    "cache_creation_input_tokens": 917,
    "output_tokens": 158
  }
}

The real context usage is input_tokens + cache_read_input_tokens + cache_creation_input_tokens. The input_tokens: 1 is misleading — the cache tokens are where your actual context lives.

cc-context-check reads the last 64KB of each session file, finds the most recent usage data, and calculates your true fill percentage.

When to use /compact

The tool shows all your active Claude Code sessions across all projects. So you can see at a glance:

• 40%: You're fine. Keep going.
• 71%: Getting full. /compact will help if you're starting a new task.
• 87%: Compact soon. You're losing context.

Options

npx cc-context-check --all    # Show top 20 sessions instead of 5
npx cc-context-check --json   # JSON output for scripting

Part of cc-toolkit

This is tool #43 in cc-toolkit — 43 free tools for Claude Code users. Zero dependencies, runs in browser or via npx.

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Source: github.com/yurukusa/cc-context-check

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More tools: Dev Toolkit — 440+ free browser-based tools for developers. JSON, regex, colors, CSS, SQL, and more. All single HTML files, no signup.

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Is your Claude Code setup actually safe? Run npx cc-health-check — a free 20-point diagnostic. Score below 80? The Claude Code Ops Kit fixes everything in one command.

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Get started in 10 seconds: npx cc-safe-setup installs 7 essential safety hooks with zero configuration. GitHub