Yes on both counts. Implementation plan is a second layer after the spec is written, at which point, spec can't be changed by agents. I then launch a planner agent that writes a phased plan file and each builder can only work on a single phase from that file.

So it's spec (human in the loop) > plan > build. Then it cycles autonomously in plan > build until spec goals are achieved. This orchestration is all managed by a simple shell script.

But even with the implementation plan file, a new agent has to orient itself, load files it may later decide were irrelevant, the plan may have not been completely correct, there could have been gaps, initial assumptions may not hold, etc. It then starts eating tokens.

And it feels like this can be optimized further.

And yes on deterministic tooling as well.