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30 tools that run in your browser

Every tool on this page does its work in your own browser. Nothing is uploaded, nothing is logged, and there is no account to create. Once a page has loaded you can go offline and it still works — which is the point, because most of these take input you shouldn't be handing to a stranger's server.

Why these exist. Catalase is a code editor that runs AI models on your own machine rather than sending your code somewhere. These tools follow the same principle: a diff checker that posts both sides of your diff to a server, or a JWT decoder that receives your live token, is solving a local problem remotely for no good reason. These don't.

Convert

Move data between formats and encodings.

Base64 Encode / DecodeText to Base64 and back, with URL-safe output and full UTF-8 support. URL Encode / DecodePercent-encoding, with the component-versus-whole-URI distinction handled right. Epoch ConverterUnix timestamps to ISO, UTC and local time, and back again. Case ConvertercamelCase, PascalCase, snake_case, kebab-case and more, all at once. Hex to RGBColour conversion with HSL and WCAG contrast ratios. JSON to CSVFlatten a JSON array into spreadsheet rows, with RFC 4180 quoting. XML to JSONConvert XML with a documented, predictable mapping. YAML to JSONConvert either direction, and see how your YAML is actually typed. Markdown to HTMLRender Markdown with a live preview and escaped output.

Format

Make unreadable text readable, or strip it back down.

SQL FormatterBreak a long query onto readable lines, or minify it. HTML FormatterIndent markup by depth, or minify it for production. XML FormatterIndent XML and check it is well formed. YAML FormatterNormalise indentation and confirm your YAML parses. Python FormatterFix mangled indentation and tidy spacing. Markdown Table GeneratorPaste CSV, get an aligned Markdown table.

Inspect

Compare, decode and measure.

Diff CheckerCompare two texts line by line with a proper LCS diff. JSON DiffCompare JSON structurally by key path, ignoring key order. JWT DecoderDecode a token's claims and expiry without sending it anywhere. Regex TesterLive match highlighting with capture groups and named groups. Subnet CalculatorIPv4 CIDR, masks and host ranges — correct for /31 and /32. LLM Token CounterEstimate prompt tokens, context usage and cost, locally. Character CounterCharacters, words, lines, paragraphs and UTF-8 bytes.

Generate

Identifiers, secrets and slugs, from your browser's cryptographic random source.

UUID GeneratorRandom version 4 UUIDs, up to 1,000 at a time. API Key GeneratorPrefixed keys in hex, base64url or alphanumeric form. Random String GeneratorAny length and character set, with the entropy shown in bits. Hash GeneratorSHA-256, SHA-1, SHA-384 and SHA-512 via Web Crypto. Slug GeneratorURL-safe slugs with accent folding and stop-word removal.

Reference

The things worth looking up, explained rather than just listed.

Regex Cheat SheetEvery token, quantifier and group, plus the dialect differences. Git Cheat SheetCommands by task, and which are safe on a shared branch. Markdown Cheat SheetFull syntax, with GitHub extensions marked.

How these tools handle your data

All thirty run as JavaScript in your browser. There is no server component, so:

  • Nothing is uploaded. Your input never crosses the network. You can verify this — open your browser's network tab, use any tool, and watch that no request is made.
  • Nothing is stored. No cookies for tracking, no accounts, no history. Reload the page and it is empty.
  • They work offline. Once a page has loaded, disconnect and it keeps working.
  • No sign-up, no rate limits, no paywall.

This matters most for the tools that take secrets. A JWT is a live credential; an API key is a secret; a prompt often contains proprietary instructions or customer data. Pasting any of those into a tool that posts them to a server means disclosing them to whoever runs it. These don't.

Where each tool is honest about its limits

Several of these take deliberate shortcuts, and each page says so plainly rather than leaving you to find out:

  • The token counter is a heuristic estimate, within roughly 10–15% — an exact count needs each model's own multi-megabyte vocabulary file.
  • The SQL formatter is keyword-driven, not a parser, so it doesn't re-indent nested subqueries by depth.
  • The Python formatter tidies indentation and spacing; it is not Black and won't reflow long lines or sort imports.
  • The YAML converter covers the config-file subset, not anchors, aliases or multi-document streams.
  • The JWT decoder decodes but deliberately does not verify signatures, because that would mean asking you to paste a signing secret into a web page.

The same idea, in an editor

These tools keep your data on your machine because there is no reason not to. Catalase takes that further: it is a free, open-source code editor that runs language models locally through Ollama, so your code is not sent to a provider unless you explicitly pick a cloud model. No account, no telemetry, Apache-2.0 licensed, for Windows and Linux.

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