LocalDevTools

Free developer tools that never upload your data

Format JSON, decode JWTs, test regular expressions, compare text and more. Every tool on LocalDevTools runs locally in your browser, so tokens, logs and API responses stay on your machine.

  • No sign-up
  • No uploads
  • Works offline after loading
  • Dark mode

Data formats

JSON Formatter & ValidatorPretty-print, minify and validate JSON with exact line and column errors.JSON ⇄ CSV ConverterTurn JSON arrays into CSV for spreadsheets, or CSV back into JSON. Handles nested objects.

Security & encoding

JWT DecoderDecode JSON Web Tokens locally, check expiry, and verify HMAC signatures.Base64 Encode / DecodeEncode and decode Base64 and Base64URL with full UTF-8 support, or turn a file into a data URI.URL Encode / DecodePercent-encode and decode URLs and query strings, and break a URL into its parts.Hash Generator (SHA-256, MD5…)Compute SHA-256, SHA-512, SHA-1 and MD5 of text or files, and verify checksums.

Text & code

Regex TesterTest JavaScript regular expressions with live highlighting, groups and replace preview.Diff CheckerCompare two texts or code files line by line with word-level highlights.

Generators

UUID / ULID GeneratorGenerate UUID v4, time-ordered UUID v7 and ULIDs in bulk, and inspect existing IDs.

Date & time

Unix Timestamp ConverterConvert epoch seconds or milliseconds to dates in any time zone, and back.Cron Expression ExplainerTranslate cron schedules into plain English and preview the next run times.

AI & LLM

LLM Token Counter & Cost EstimatorCount GPT tokens exactly with the o200k tokenizer, check context limits and estimate API cost.

Why local-only tools?

Developers paste sensitive things into online tools every day: production JWTs, customer records inside a JSON payload, internal URLs, API keys hidden in a config file. Many online utilities send that input to a server to process it. The tools here are written so the work happens in JavaScript on your own device. You can verify this yourself: open your browser's developer tools, switch to the Network tab and use any tool. You will not see your input leave the page.

What makes these tools different

  • Precise error messages. The JSON validator points to the line and column of the problem, and the cron explainer tells you which field is invalid.
  • Explained, not just converted. Each page includes a short guide, worked examples and answers to the questions people actually ask, such as why a JWT is readable without a key or why a regex behaves differently in Python.
  • Fast on any device. Plain HTML, CSS and a little JavaScript. No frameworks, no trackers, no pop-ups.