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

Source: Dev.to

Overview

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 k‑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

Sample output

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 % → continue
• 🟡 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, e.g.:

{
  "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 entry is misleading—the cache tokens are where your actual context lives.

cc-context-check reads the last 64 KB 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 projects, letting you 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 the cc-toolkit — 43 free tools for Claude Code users. Zero dependencies, runs in the browser or via npx.

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

More tools: Dev Toolkit — 56 free browser‑based tools for developers (JSON, regex, colors, CSS, SQL, and more). All single HTML files, no signup.