That's literally what OpenSpec does (https://openspec.dev/). It's quite nice. I've only exceptionally rarely seen claude do something wrong based on spec docs when it's fully spec'd out. More often it's because something wasn't nailed down and claude was forced to make assumptions.

The downside is the ospx markdown specs sometimes end up too granular, focusing on the wrong or less important details, so reading the specs feels like a slog.

Also at times aspects of the english language spec end up way more verbose than just giving a code example would be.