> Not sure why this doesn't seem to be a common practice TBH, might be missing something.

Yeah... I've been deep in this problem space myself. The two big points of friction are: 1. Requiring a build-step to generate runtime code from the TS types 2. TS doesn't officially support compiler transforms that do it

That said, the two most promising approaches I've found so far: 1. https://github.com/GoogleFeud/ts-runtime-checks -- it does exactly what you describe 2. https://arktype.io/ -- a very interesting take on the Zod model, but feels like writing native Typescript

Congrats on the launch, really exciting to see a way to get capabilities into the JS ecosystem!