diff --git a/modules/build-agent/README.md b/modules/build-agent/README.md
index e44dd4ee..7f253b8e 100644
--- a/modules/build-agent/README.md
+++ b/modules/build-agent/README.md
@@ -13,6 +13,11 @@ that repository's main (`build.artifacts[].context`); this module ships the pack
 Assign it to every machine with a container runtime. It replaces `builder`, the one-holder form of the
 same thing; retire that once this is assigned where it was.
 
+A send that replaces the agent's container does not throw away the build it runs: the container declares
+`busy` as `/usr/local/bin/mesh-builder busy` (novox/hq ADR 0335). The node-engine asks it before replacing it; from that
+ask the agent takes no new build, the node-engine leaves it running while it says a build is in hand, and
+replaces it once it says idle, or after 15 minutes at the latest.
+
 ## The package registry (novox/hq ADR 0336)
 
 The package registry (`npm-package-registry`, provided by the gitea module) is an optional requirement.
diff --git a/modules/build-agent/module.json b/modules/build-agent/module.json
index c4b1d615..9e2a5b22 100644
--- a/modules/build-agent/module.json
+++ b/modules/build-agent/module.json
@@ -85,6 +85,9 @@
         "timeout": "5s",
         "looks": 3,
         "grace": "60s"
+      },
+      "busy": {
+        "command": "/usr/local/bin/mesh-builder busy"
       }
     }
   ],