WBS: asking a tool is through the seam; a build is a different shape

03-DESIGN/01-to-be/28-building-the-bus.md

@@ -259,8 +259,19 @@ pays for itself furthest away.
       of the declaration it is about, so supersession becomes a check rather than something the
       controller remembers. Pure and tested without a bus, a store or a clock.
 
-      Still outstanding: wiring that decision into the loop, `RequestBuild` and `Ask`, enrolment,
-      and serving.
+      **Asking a tool is through the seam and loses two problems**: there is no reply queue to
+      declare and no correlation to check, because each account has one inbox prefix and an
+      answer cannot reach the wrong asker — which settles a cost the build code records having
+      paid, where every asker saw every result. And a tool nobody serves says so at once instead
+      of after the whole wait, which is the difference between "that module is down" and "that
+      tool is slow".
+
+      **A build is a different shape, not the same one.** It takes minutes, so it is work
+      submitted to a queue with the outcome returning to a reply subject the request carries —
+      the pattern design 25 §2 already sets for anything crossing a stream. It touches the
+      builder as well, so it travels with that conversion in step 4.
+
+      Still outstanding: wiring the window decision into the loop, enrolment, and serving.
 - [~] 3.5 the host's link on NATS — **the outbound half is through a seam**, mirroring the
       controller's and still importing nothing of the mesh's own (ADR 0005): the host's own
       interface over its own libraries, agreeing with the controller only because a fixture holds