Dev tools that
run in your tab.
JSON, Base64, hashes, regex, JWTs, cron, diffs, markdown, color, timestamps, and an AI token counter — all client-side, no server, no account.
// tools
Pick one — everything runs locally in this tab.
JSON Formatter & Validator
Pretty-print, minify, or validate JSON. Errors point to exactly what's wrong.
Input
Output
Other tools
Base64 / URL Encoder-Decoder
Encode or decode standard Base64, URL-safe Base64, and URL-component strings.
Input
Output
Other tools
Hash Generator
Compute MD5, SHA-1, and SHA-256 hashes of any text, entirely in your browser.
Input
Output
Other tools
Regex Tester
Test a JavaScript regular expression against sample text and see every match.
Pattern
Matches
Other tools
UUID Generator
Generate RFC 4122 version-4 (random) UUIDs, one at a time or in bulk.
Generate
Output
Other tools
Cron Expression Parser
Turn a standard 5-field cron expression into a plain-English description.
Expression
Description
Other tools
Text / Code Diff Checker
Line-by-line comparison of two blocks of text or code.
Input
Diff
Other tools
Markdown Live Previewer
Live-render common Markdown syntax to HTML as you type. All output is safely escaped.
Other tools
Color Converter
Convert between HEX, RGB, and HSL, with a live swatch preview.
Input
Other tools
JWT Decoder
Decode a JSON Web Token's header and payload. This never verifies the signature.
Token
Header
Payload
Other tools
Unix Timestamp Converter
Convert between Unix time (seconds or milliseconds) and human-readable dates.
Unix → Date
Date → Unix
Other tools
AI Token & Word Counter
Estimate GPT/Claude-style token count for a prompt before you send it.
Text
Other tools
How DevKit works
What runs where, and the honest limits of each tool.
Everything runs in your browser
DevKit has no backend. Every tool — JSON formatting, hashing, regex testing, JWT decoding — runs as plain JavaScript in this tab. Nothing you type or paste is sent anywhere, which is also why the whole site is free with no account.
Two honesty notes worth reading
The JWT decoder only decodes the header and payload — it never verifies a signature, because that needs the signing secret or public key, which this tool never asks for. A decoded JWT tells you what a token claims, not whether it's authentic; that check has to happen server-side with the real key.
The AI token counter is an approximation, not a real tokenizer. GPT and Claude models use their own byte-pair-encoding vocabularies, and an exact count requires that model's own tokenizer library. This tool uses the common ~4-characters-per-token heuristic for English text, which is close enough for rough context-window planning but not an exact bill.
MD5 and SHA-1 are included for checksums and deduplication (common developer use cases), but both are cryptographically broken — never rely on them for password storage or security-sensitive integrity checks. Use SHA-256 or stronger for anything security-related.
Tool reference
| Tool | What it does | Runs on |
|---|---|---|
| JSON | Pretty-print, minify, validate | Native JSON.parse/stringify |
| Base64/URL | Encode/decode Base64, Base64URL, URL components | Pure-JS UTF-8-safe codec |
| Hash | MD5, SHA-1, SHA-256 | Pure-JS implementations (no server round-trip) |
| Regex | Match testing with capture groups | Native JS RegExp engine |
| UUID | RFC 4122 v4 generation | Math.random-seeded |
| Cron | 5-field expression → English | Custom field parser |
| Diff | Line-level LCS diff | Custom dynamic-programming diff |
| Markdown | Common syntax → escaped HTML | Custom minimal parser (not full CommonMark) |
| Color | HEX ⇄ RGB ⇄ HSL | Standard color-space math |
| JWT | Decode header + payload only | Base64URL decode (no signature check) |
| Timestamp | Unix ⇄ ISO/UTC | Native Date |
| Tokens | Approximate AI token count | Character/word heuristic (not a real tokenizer) |