This might work if we have completely new harnesses for this, because the current one’s intentionally introduce non-reproducible context and tools.

The current way we’re doing SDD is extremely pointless. You reach a decision point that is under specified, make a decision, update the spec and continue the implementation. There’s no guarantee that the implementation followed from the updated spec, and not from the decision.

I agree, another issue I have with existing SDD frameworks I've seen like spec-kit or openspec, is they seem to miss the mark by coupling specification (what the software should be) and execution (how to get there). If the description of execution is needed, it means the spec is weak. As a rule of thumb if a framework defines execution concepts like tasks or tickets, that is a red flag for me.