Spec first (spec-first). It is written before coding, guides the current task, and is then discarded. Drift is not a problem, but the documentation is lost as an asset.
Anchored spec (spec-anchored). It is maintained as living documentation of the system: when the code changes, the spec is updated. Drift is a real risk that demands discipline. This is where most teams work today.
Spec as source (spec-as-source). The spec is the primary artifact: only the spec is edited, and the code is regenerated. Drift disappears by design, but this approach is still experimental.
