Microsoft has released Bot Framework Composer in May 2020, and since then the tool was under active development. It allows to rapidly create rich conversational bots that leverage adaptive dialogs, language generation, skills and more.
In working with the Composer I have found that one of practical challenges was the "teaching" LUIS to recognize intents and entities: the labelling was quite verbose, the number of training examples was in dozens per intent (at least), and on top of that the documentation of the new LUDOWN format could be improved, even though the format is by far more convenient than JSON.
I've done some digging thanks to Composer being released as open source, and found a library that the Composer uses for parsing and validating the .lu files: @microsoft/bf-lu.
I thought it was easier to validate the .lu files in CLI mode, so I ended up writing my own CLI to help with quick and verbose validation. As a "bonus" it can also create a temporary LUIS app from an .lu file since the @microsoft/bf-lu validation may miss certain errors, which LUIS would complain about when attempting to create an app.
Here is the utility: https://www.npmjs.com/package/@softforte/lu
While the new Bot Framework CLI offers similar and comprehensive features, I still find this utility handy for day-to-day LUIS development and hope that it will save you some troubleshooting time. Check it out!