> when you come back later and have to remember wtf is going on."

When that happens, do you read just the spec, or do you also need to read the code? Is there a difference between "I can remember what I intended" and "I can predict what the system will do in a situation the spec didn't cover"?

Interestingly, you said the spec author must be you. What happens when you join a codebase where someone else wrote the SPEC, or where an agent wrote the code and nobody spec'd it? Is the spec still sufficient, or does the "you must write it" part mean the understanding doesn't transfer?