What is a product spec?
A product spec is a short, build-ready description of bounded work.
It captures the problem, desired behavior, affected surface and acceptance criteria. It works well for bugs, improvements and other changes where the decision is already clear.
A spec isn't a product requirements document. A spec covers known work. A PRD is useful for a new capability where strategy, scope and open questions still need to be settled.
Why it matters for product builders
A good spec lets the builder start without guessing what success means. It keeps the customer problem attached to the implementation.
What changes in AI-first products
A spec used to align people before a build. It now also acts as an execution interface for an agent.
That changes what carries weight. Clear boundaries, real codebase context and testable criteria matter more than narrative length because the reader can start building immediately.