The privacy-first toolkit for JSON & developers
A fast, privacy-first developer toolkit. Format, validate, convert and inspect JSON - plus dozens of developer utilities. Everything runs 100% in your browser. No uploads, no tracking, no backend.
Nothing is uploaded
Your data never leaves your device. Every tool runs entirely in your browser.
Works offline
Installable as a PWA. After the first visit, every tool works without a connection.
Instant & fast
Web Workers and virtualized rendering keep even large files smooth.
Your data stays private
The content you process is never uploaded, logged, or tracked - it never leaves your browser.
Why a browser-only toolkit exists
Most online developer tools are thin front-ends over a server. You paste a payload, the page POSTs it to an API, and formatted text comes back. That is invisible in the UI and completely normal - and it is also the moment your data leaves your machine.
This matters more than it sounds, because the payloads developers debug are rarely boring. A failing request body contains a bearer token. A production log line contains a customer email. A Stripe webhook contains a payment record. A JWT you paste to check its expiry contains the signed claims of a live session. Once that has been sent to a third party you cannot tell whether it was logged, cached in a CDN, retained for abuse-prevention, or simply kept.
Meanwhile, none of these tasks actually need a server. Parsing JSON, diffing two objects, computing a SHA-256 digest and decoding Base64 are all things browsers have done natively for years. LocalOnly is built on that observation: if the work can happen locally, sending the data anywhere is an unnecessary risk.
How “runs locally” actually works here
LocalOnly has no application backend. The whole site is a static export - HTML, CSS and JavaScript files on a CDN. There is no API route to receive a payload, which is a stronger guarantee than a privacy promise: the endpoint does not exist.
Every transformation runs in your browser tab. Heavy work - parsing multi-megabyte JSON, diffing large trees, hashing files - is moved onto Web Workers so the interface never freezes, and results render through virtualized lists so a 50,000-node tree stays scrollable. Because nothing is uploaded, there is no request size limit and no queue; your machine is the only constraint.
A service worker caches the app after your first visit, so the tools keep working on a plane, on hotel Wi-Fi, or behind a corporate firewall. You can install LocalOnlyas a standalone app from your browser's address bar.
To be precise about the one exception: the content you process never leaves your browser, but the site itself does load Google Analytics and Google-served ads, which set their own cookies and see ordinary request data like your IP address and which page you opened. That is how the tools stay free. It is described in full in our Privacy Policy and Cookie Policy. We would rather say that plainly than claim a blanket “we collect nothing” that is not true.
Popular tools
Online JSON Formatter & Validator
Pretty-print JSON with configurable indentation and instant validation.
JSON Validator
Validate JSON syntax and pinpoint the exact line and column of any error.
JSON Compare & Diff
Compare two JSON documents and see a precise structural diff.
JSON to YAML Converter
Turn JSON into clean, readable YAML for configs, pipelines and manifests.
JSON Tree Viewer
Explore JSON as a collapsible, interactive tree.
JWT Decoder
Decode a JWT to inspect its header, payload, and claims instantly.
What is in the toolkit
37 tools in three groups. Every one has a written reference on its page - what the format actually is, the mistakes people hit, and when to reach for a different tool instead.
JSON Tools
Format, validate, transform and inspect JSON - everything you need to work with JSON, running entirely in your browser.
All 19 json toolsConverters
Convert JSON to and from YAML, CSV, XML and more. Bidirectional, lossless where possible, and 100% local.
All 6 convertersDeveloper Utilities
A fast, privacy-first collection of everyday developer utilities - encoders, generators, decoders and converters.
All 12 developer utilitiesReference material, not just widgets
A tool that silently fixes your input teaches you nothing about why it broke. Each tool page carries a written explanation of the format it handles - why Base64 is an encoding and not encryption, why nested JSON cannot map cleanly onto CSV rows, or why decoding a JWT is not the same as verifying one.
Longer write-ups live in the guides, and the documentation covers keyboard shortcuts, the command palette and offline installation.
Developer tools that never touch a server
Install LocalOnly and keep every utility one keystroke away - online or off, always private.
Browse all tools