Simon Willison’s Weblog

Incomplete JSON Pretty Printer. Every now and then a log file or a tool I'm using will spit out a bunch of JSON that terminates unexpectedly, meaning I can't copy it into a text editor and pretty-print it to see what's going on.

The other day I got frustrated with this and had the then-new GPT-4.5 build me a pretty-printer that didn't mind incomplete JSON, using an OpenAI Canvas. Here's the chat and here's the resulting interactive.

I spotted a bug with the way it indented code today so I pasted it into Claude 3.7 Sonnet Thinking mode and had it make a bunch of improvements - full transcript here. Here's the finished code.

In many ways this is a perfect example of vibe coding in action. At no point did I look at a single line of code that either of the LLMs had written for me. I honestly don't care how this thing works: it could not be lower stakes for me, the worst a bug could do is show me poorly formatted incomplete JSON.

I was vaguely aware that some kind of state machine style parser would be needed, because you can't parse incomplete JSON with a regular JSON parser. Building simple parsers is the kind of thing LLMs are surprisingly good at, and also the kind of thing I don't want to take on for a trivial project.

At one point I told Claude "Try using your code execution tool to check your logic", because I happen to know Claude can write and then execute JavaScript independently of using it for artifacts. That helped it out a bunch.

I later dropped in the following:

modify the tool to work better on mobile screens and generally look a bit nicer - and remove the pretty print JSON button, it should update any time the input text is changed. Also add a "copy to clipboard" button next to the results. And add a button that says "example" which adds a longer incomplete example to demonstrate the tool, make that example pelican themed.

It's fun being able to say "generally look a bit nicer" and get a perfectly acceptable result!