Code
Build a dev utility
A small single-page tool for the thing you keep doing by hand.
Fill it in
The input, the output, and what makes it useful.
One file, no network, nothing leaves the browser.
Your prompt
Build this tool. [your what it should do] Live feedback as they type. The whole reason these get used instead of a language REPL is that the answer updates while you are still forming the question. Show the failure clearly. A regex that does not compile, a query that will not parse: say where, at the character, rather than clearing the output. One self-contained file. No CDN, no network calls, nothing typed into it leaves the browser. People paste production data into these. Keyboard first, with the input focused on load.
Use Build a dev utilityOpens with everything above already filled in.
Why this works
These get used instead of a REPL because the answer updates while you are still forming the question. It is one self-contained file with no network calls, which matters because people paste production data into tools like this.