I built a JSON toolkit that never sends your data anywhere

A developer built JSONLinter, a client-side JSON toolkit with 42 tools across six categories, ensuring data never leaves the browser. The project uses React 19, TypeScript, Vite, Tailwind v4, and CodeMirror 6, and includes a prerendering solution using headless Chromium to serve real HTML to crawlers without SSR. An optional AI assistant supports bring-your-own-key for OpenAI or Anthropic, with no backend involved.

Most "paste your JSON here" tools online send that JSON to a server to process it. For internal API responses, config files, or anything with real data in it, that's not something I wanted to do — so I built JSONLinter https://jsonlinter.io , a JSON toolkit where literally everything happens client-side. It started as a validator/formatter, then grew into 42 tools across six categories: There's also an optional AI assistant on the validator page — it's bring-your-own-key OpenAI or Anthropic , and since there's no backend at all, the key and your JSON go straight from your browser to the provider. I never see either. Stack is React 19 + TypeScript + Vite, Tailwind v4 for styling, CodeMirror 6 for the editor. A few things I had to solve that were more interesting than expected: Prerendering without SSR. I didn't want to take on a Next.js-style server just to get real HTML for crawlers. Instead, the build runs a headless Chromium pass Playwright over every route after vite build and saves the fully-rendered output to dist/<route /index.html . Crawlers get real content and correct per-page meta tags on first paint; once JS loads, React takes over exactly like a normal SPA. No server, no hydration mismatches to worry about. Structural JSON diff. A text diff on two JSON documents is mostly useless because key order doesn't matter semantically. The diff tool parses both sides and compares the actual structure, so reordering keys shows as no change, but changing a value does. Selection highlighting, CodeMirror edge case. CodeMirror renders its selection layer behind the text layer z-index -2, deliberately, so glyph rendering stays crisp , while the active-line highlight paints on the text layer itself. If the active-line background is opaque, it completely hides the selection highlight on whatever line your cursor is on — which is every line right after a double-click. Fix was making the active-line background translucent instead of a solid fill, so it blends with the selection color underneath instead of occluding it. Took a while to track down since "selection isn't visible" and "double-click doesn't work" looked like different bugs at first. It's live at jsonlinter.io https://jsonlinter.io — no signup, no account, nothing to install. Would genuinely appreciate feedback, especially on anything that feels missing compared to tools you already reach for.