I Built a Chrome Extension to Track AI Token Usage — Here's How It Works

Six weeks ago I got cut off mid-debugging session by Claude's rate limit with no warning. Two hours of context gone. I started looking for a tool that would show me how close I was before it happened. Nothing existed that worked across more than one platform without requiring an API key. So I built one. TokenPulse is a Chrome extension (MV3) that injects a live token bar above the input box on Claude, ChatGPT, Gemini, DeepSeek and Grok. It tracks context window usage, rate limits, cost estimates, and daily history — all from your existing browser session, no API key required. Here's how it works technically. Architecture overview Content Scripts (per platform) ↓ Background Service Worker ↓ Chrome Storage API (local) ↓ Popup UI ↓ Desktop Notifications The extension runs a content script on each supported domain. Each script is responsible for: Reading token usage data from that platform Injecting the visual bar above the input box Sending data to the background service worker via chrome.runtime.sendMessage The service worker aggregates data, writes to chrome.storage.local , checks notification thresholds, and serves data to the popup on demand. How Claude's rate limits are read Claude is the only platform that exposes real rate limit data through its internal API. When you use claude.ai, the browser session makes requests to a usage endpoint that returns exact utilization percentages and reset timestamps. The content script intercepts this data by hooking into the platform's network requests using a MutationObserver to detect when Claude updates its state, then reading the cached response. The response looks roughly like: { five_hour : { utilization : 0.82 , reset_at : " 2026-07-15T14:14:00Z " }, seven_day : { utilization : 0.34 , reset_at : " 2026-07-21T21:00:00Z " } } This gives exact percentages — not estimates. The popup shows these directly. Client-side token estimation for other platforms ChatGPT, Gemini, DeepSeek and Grok don't expose usage data the same way.