Build

Single-threaded implementation loop that works through tasks in SPEC.md. You invoke it when you want Claude to actually build what's in the spec, and it plans the work, makes the edits, runs tests, and flips task statuses from open to done. The interesting bit is the backprop hook: when a test fails, it stops to ask whether the failure reveals a spec bug or missing invariant, then updates SPEC.md before retrying. No agent swarms, no parallelism, just a straightforward execute-verify-mark-done cycle. Works best when you already have a populated spec and want disciplined execution that learns from failures instead of silently patching around them.