Defining the spec is ofcourse also needed in an agile process.

The difference is when is this done; In a upfront discussion, while developing, or after user feedback.

For LLM we know it needs to be written down. (At least if we want human tracability)

And agile ofcourse is a shortend waterfall to get user feedback early on.

Giving enough context is important for every case.(humans and llm's alike)