Developer Tools That Run Entirely in Your Browser
These tools handle the things you would otherwise paste into a random online formatter: production schemas, auth tokens and API responses. Every one of them runs locally, so none of that leaves your machine.
Pick a tool
- JSON to ERDPaste your database schema JSON and get an interactive ERD.
- API TesterSend an HTTPS request from your browser and read the JSON response.
- JSON Field ExtractorPaste a JSON array and a field path to get a flat array of the matching values.
- JWT InspectorDecode a JWT, read its claims in local time, and verify an HMAC signature offline.
What each tool covers
- Inheriting a database you have never seen, and wanting to know what the tables are and how they join, is what JSON to ERD is for.
- Checking what an endpoint actually returns, without installing a desktop client for a one-off test, is what API Tester is for.
- Telling an expired token from one that never carried the right role is what JWT Inspector is for.
- Pulling one field out of a large response into a flat list you can paste into a spreadsheet or a fixture is what JSON Field Extractor is for.
They chain together more often than not: check the token in JWT Inspector, call the endpoint in API Tester, paste the response into the field extractor to pull out the values you are comparing, and open the ERD when you need the table structure behind them.
Why uploading matters more for this category
The inputs here are almost always things that should not travel: production table structures, unreleased endpoints, tokens sitting in a header, real data in a response. Pasting any of that into a tool that posts to a server hands it to a third party you have no way to audit.
The approach here is to keep the work inside your tab and leave no outbound path at all. Press F12, open the network tab in developer tools, run through the tool, and look for a request carrying your content. There is not one.
Limits they all share
- None of them connects to your database. The ERD reads JSON you exported yourself, not a connection string.
- None of them relays a request. API Tester uses your browser's fetch, so the target API has to allow cross-origin calls.
- None of them stores settings on a server. Saves go to browser storage and do not follow you to another machine.
- Capacity is bounded by tab memory, so a very large schema or response gets noticeably slower.
Other categories
- General tools — Generate codes, recolour SVG artwork, strip metadata from photos and clean up pasted text, without installing anything or creating an account.
- Finance tools — Work out compound growth, a minimum viable freelance rate, how long cash stays tied up in a business or a household, and which pack on the shelf is actually cheaper.