Sandboxing is a completely orthogonal issue and WASM is probably not a good direct target for LLMs.

Of course writing a language that compiles to Wasm is certainly a way, but you would have to sandbox also all the other tools that is used during development (e.g. agents can just call grep/find/etc).

WASM isn't a language you'd want to program with. you can't verify outputs nor is there any proper training data aside from examples and such