Free Online Developer Tools

These are the utilities developers reach for a dozen times a day: pretty-printing a JSON payload, decoding a JWT to check its claims, testing a regular expression against real input, turning a Unix timestamp into a readable date, or diffing two config files to see what actually changed. Every tool on this page runs locally in your browser. Nothing you paste is uploaded, logged or stored, which matters when the thing you are debugging is a production token, a customer record or a private API response.

The hub is organised by job rather than by file type. Formatters & Validators clean up and check JSON, XML, YAML and SQL, and include a JSON Schema generator and a Mermaid diagram editor. Encoders, Security & Auth covers Base64, URL and HTML entity encoding, hashing, JWT inspection (including multi-tenant issuer debugging), UUIDs, password generation, Stripe-style test card numbers, cron expressions and timestamps. Data Converters move data between JSON, CSV, XML and Excel, translate SQL schemas into ORM models, and convert CI/CD pipelines between GitHub Actions, GitLab CI, CircleCI and Bitbucket.

If you build with large language models, the AI & LLM group estimates token costs across providers, sizes GPU memory for self-hosted models, prices RAG pipelines and vector databases, and generates rules files for AI coding assistants. DevOps, Web & Network tools handle .gitignore templates, service status checks, subnet maths, SEO audits, CSS gradients and WCAG colour contrast. Finally, the cheatsheets give you searchable references for Git, Linux, Vim, CSS and React — plus a small game for learning Vim motions.

Each tool page explains what the tool does, walks through a real example, lists common errors and how to fix them, and links to related tools so you can chain steps together — for example, validate JSON, convert it to CSV, then open it in a spreadsheet converter.