I’ve been using the copilot instruction files with pretty good results, and you can make it grab the llm.txt for any res file automatically like this:
---
applyTo: '**/*.res'
---
#fetch:https://rescript-lang.org/llms/manual/v11.0.0/llm-small.txt
#fetch:https://rescript-lang.org/llms/react/latest/llm-small.txt
The newer models seem to not need much prompting to not generate reasonml syntax, but you can also use the setting “github.copilot.chat.codeGeneration.instructions” and the same markdown front-matter above in another file to remind it to not generate reasonml syntax.