Looking at the source - it looks like it's not friendly with CoffeeScript in particular at the moment.

It'd be quite possible to add the ability to output these annotations to the CoffeeScript compiler itself though. Could be an interesting fork.

It looks like Flow should work to the extent that it can infer types from your CS output, but it will be tricky to embed explicit types into your CoffeeScript source.

You can probably hack it using the backtick operator though.

    `function foo(x: string, y: number): string {`
    x.length * y
    `}`