If an agent writes the implementation, the specification stops being paperwork. It becomes the part of the work with your name on it.
Specifications used to be the thing you wrote afterwards, for somebody else, if there was time. That stopped being true the moment implementation became cheap.
Cheap implementation changes what is scarce
When writing the code was the expensive step, it made sense to start coding early and discover the requirements on the way. The code was where you did your thinking, and the thinking was slow enough that mistakes surfaced before they multiplied.
When implementation takes minutes, the expensive step is deciding what should exist. A specification is not documentation of that decision. It is the decision.
What goes in one
The rule I work to is that nothing important should be inferable only from tone. Exact values, not adjectives. If a brief says 'modern' or 'clean' or 'premium' or 'you decide', that is not a requirement, it is a placeholder — and the fastest way to turn it into one is to propose exact numbers in a table and get a single word back.
So: the field names, the status values, the timings, the colours, the error text, the empty state, the loading state, the acceptance criteria. Written down before generation starts, in one place, in English.
Small phases beat big ones
The other half of the practice is scope. An agent will happily produce a forty-file change, and a forty-file change cannot be reviewed honestly. I keep a phase small enough that I can read all of it and mean it when I say it is correct. If the work grows past that, it becomes the next phase rather than a bigger one.
The discipline is not about distrusting the tool. It is that review is the only place the quality is actually decided, and review does not scale by wishing.
The honest test
A specification is good if someone who was not in the conversation can build the thing from it and you would accept the result. That is a high bar, and it is a useful one, because these days the thing building from it was definitively not in the conversation.
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