Two years ago I introduced a one-page tech spec template at a startup growing fast enough that nobody had time to read longer ones. We're still using it. Engineers have rewritten it, but the shape has held.
The template
Problem. What are we solving, in two sentences. No history.
Proposed solution. The headline, then the smallest diagram that explains it.
Alternatives considered. Two or three, with why we're not choosing them.
Open questions. The genuine ones.
Rollout. What ships first, what's reversible, who's on call.
That's it. One page, five headers, no appendices. The discipline isn't writing the spec — it's resisting the urge to make it two pages.