my org at work adopted openspec, and i strongly dislike it. every change, medium or larger, turns into a large set of multiple markdown documents, each that need review. and they are never handwritten - always slop, filled with the all the tells of ai writing, which i personally find grating.
i find a small, human written spec to be much more effective than these large spec documents.
the idea is that you iterate with your agent to write the spec, you implement, then eventually that spec gets merged into a "spec corpus" that describes all the behavior of the repository. but i don't think that prose can ever enumerate all the behaviors required of code, nor should it. the spec almost immediately becomes out of date.