JSON Web Tokens (JWTs) are everywhere. Whether you're debugging an OAuth flow, a rogue microservice, or a broken single-page application, inspecting a JWT is a daily task for most developers. But there's a massive, glaring problem with how we usually do it: We paste production tokens into random third-party websites. Many online JWT decoders send your token to their backend to parse or verify it. If that token contains sensitive claims, PII, or internal routing data - and if it hasn't expired - you've just leaked it. I was tired of wondering if a random tool was logging my tokens, so I decided to build a privacy-first JWT Inspector for my tool hub, ToolsMatic. The goal? Zero backend. Zero dependencies. 100% client-side processing. Here's how I built it using nothing but Vanilla JavaScript and the native Web Crypto API. Step 1: Safely Decoding Base64URL in the Browser A JWT is just three strings separated by dots ( header.payload.signature ), encoded in Base64URL.…