The most comprehensive tool I've built so far was a JSONPath playground [0] when I was working on a game where I found myself writing a fair bit of JSONPath (well, JSONPath Plus specifically) expressions by hand and wanted to be able to test them out in the same way that I would on regex101 when writing regular expressions.

I realistically probably would have only saved myself less than an hour of crafting those expressions if the tool had already existed (with this level of detail, there are lots of many simpler ones already for it), but I would have spent a solid 40-50 hours of bouncing between manually crafting and writing detailed instructions to direct the agents to get this tool there.

[0] https://jsonpath101.com/