Of everything in SpecDevShip, approval gates are the feature most people skim past on first read and come to value most after the first time they'd have been burned without it.
The mechanism is simple
Every spec document carries a status: draft or approved. While a document is a draft, editing it is just normal iteration — nothing special happens. The moment you say "approved" — and only when you explicitly say so, never inferred from silence or the conversation moving on — the document locks. From that point, it isn't edited in place. Not automatically, not as a quick fix mid-implementation, not because a better idea came up while building.
What "locked" actually means in practice
If something genuinely needs to change after approval, the change gets proposed explicitly, with reasoning, and the document only reverts to draft with your clear agreement — the same discipline as approving it the first time. Once reverted, it's edited normally and goes back through the approval loop before implementation resumes.
The failure mode this prevents
Without this gate, the natural behavior of an AI agent mid-implementation is entirely reasonable-sounding: it notices something that could be better, and quietly makes it better. Multiply that across a dozen small "improvements" over a long implementation session and the document you approved no longer describes what got built. You're not maintaining a spec anymore — you're maintaining fiction that happens to be mostly true.
Why this changes how much you can trust the process
A document you can't trust to stay put is a document you'll eventually stop reading before you approve it — which defeats the entire point of having one. The gate is what makes "approved" mean something specific and checkable, rather than a vague gesture toward agreement. It's the single smallest habit in the whole workflow, and it's the one everything else quietly depends on.
If you adopt nothing else from this system, adopt this: say "approved" out loud, mean it as binding, and expect the workflow to hold you to it. That's the whole feature.