Free developer tools that run in your browser

15 small, fast utilities for everyday dev work: JSON, YAML and CSV, JWTs, cron, regex, hashes, timestamps, colors, plus a few you won't find elsewhere, like an x402 payment-header decoder and a robots.txt tester for AI crawlers. No sign-up. Your input never leaves your device.

Data

JSON Formatter & ValidatorFormat, minify and validate JSON in your browser.JSON ↔ YAML ConverterConvert JSON to YAML or YAML to JSON instantly in your browser.CSV ↔ JSON ConverterConvert CSV to JSON or JSON to CSV with quoted fields, auto-detected delimiters (comma, semicolon, tab, pipe) and optional type detection.

Security

JWT DecoderDecode a JSON Web Token in your browser: header, payload, exp/iat/nbf as readable dates, expiry status, and optional HS256 signature check.Hash GeneratorCompute MD5, SHA-1, SHA-256, SHA-384, SHA-512, SHA3-256 and Keccak-256 hashes of text or files, plus HMAC, entirely in your browser.

Time

Cron Expression ExplainerTranslate a cron expression like */15 9-17 * * 1-5 into plain English and see the next run times in your time zone or UTC.Unix Timestamp ConverterConvert Unix epoch timestamps to human dates and back.

Text

Regex TesterTest JavaScript regular expressions live: highlighted matches, capture and named groups, flags (g, i, m, s, u, y) and a replace preview.

Generators

UUID Generator (v4 & v7)Generate random UUID v4 or time-ordered UUID v7 in bulk with the browser's secure random generator, and inspect any UUID's version, variant and timestamp.

Encoding

Base64 Encoder / DecoderEncode text to Base64 or decode Base64 to text with full UTF-8 support, URL-safe Base64URL, MIME line wrapping, and file to data URI.URL Encoder / Decoder & ParserPercent-encode or decode URL components, and break any URL into protocol, host, path and a decoded table of query parameters.

Design

Color Converter & Contrast CheckerCheck WCAG 2.2 contrast ratio between text and background colors (AA/AAA), with live preview and HEX, RGB and HSL conversion.

Web3

x402 Payment Header DecoderDecode x402 HTTP 402 payment headers (PAYMENT-REQUIRED, X-PAYMENT, PAYMENT-SIGNATURE, PAYMENT-RESPONSE) into readable price, network, payee and expiry.Ethereum Address Checksum (EIP-55)Validate and convert Ethereum, Base and other EVM addresses to EIP-55 mixed-case checksum format.

Web

robots.txt Tester (incl. AI crawlers)Test whether a URL is allowed or blocked by robots.txt for any user-agent, and see a matrix for AI crawlers like GPTBot, ClaudeBot, Google-Extended and PerplexityBot.

Why ParseJar

  • Private by design. Every tool is plain JavaScript that runs locally. Tokens, keys and data you paste are not sent to a server.
  • Correct first. The tools follow the actual standards: RFC 8259 JSON, YAML 1.2, RFC 4180 CSV, RFC 9562 UUIDs, RFC 9309 robots.txt, WCAG 2.x contrast and EIP-55. Edge cases are explained on each page.
  • Fast. No frameworks, and each page loads only the code it needs.
ParseJar is free. Its sister project ScrapeMole sells pay-per-call data APIs for AI agents.