diff --git a/modules/amqp-ping/Dockerfile b/modules/amqp-ping/Dockerfile
index 3f4a7b1..8072911 100644
--- a/modules/amqp-ping/Dockerfile
+++ b/modules/amqp-ping/Dockerfile
@@ -5,9 +5,12 @@
 # other artifact. That is what makes this buildable by the mesh from a repository and a path
 # (novox/hq ADR 0069) rather than only on a workstation that happens to have the siblings.
 #
-# The base is named by ARG so it can be pinned to a digest the mesh's registry assigned. A tag here
-# would make the runtime's contents depend on what somebody last pushed under that name.
-ARG RUNTIME_BASE=127.0.0.1:5000/mesh-tools/runtime@sha256:44b5d8bc30107fdc3bffdaebc1ca2de97615053853f826dfccce253a01a163fd
+# Named, not pinned. The mesh answers this with the copy of the runtime it holds, because a
+# fingerprint written here would name one particular copy — the one on whichever machine the person
+# typing it was using — and on any other mesh that copy has never existed (novox/hq issue 044).
+# Declared in module.json's `build.on`; there is deliberately no default, so a build nobody told
+# stops here rather than on a reference that resolves to nothing.
+ARG RUNTIME_BASE
 
 FROM ${RUNTIME_BASE} AS build
 # Compiled under /app/modules, so resolving `@novox/mesh-sdk` walks up to the base's own
diff --git a/modules/amqp-ping/module.json b/modules/amqp-ping/module.json
index 5e7184c..f50d554 100644
--- a/modules/amqp-ping/module.json
+++ b/modules/amqp-ping/module.json
@@ -58,6 +58,13 @@
     }
   ],
   "build": {
+    "on": [
+      {
+        "arg": "RUNTIME_BASE",
+        "module": "mesh-tools",
+        "artifact": "runtime"
+      }
+    ],
     "artifacts": [
       {
         "name": "runtime",
diff --git a/modules/mesh-catalog/Dockerfile b/modules/mesh-catalog/Dockerfile
index 5331f6b..9f1bd60 100644
--- a/modules/mesh-catalog/Dockerfile
+++ b/modules/mesh-catalog/Dockerfile
@@ -5,9 +5,12 @@
 # nothing is copied out of a neighbouring checkout — which is what lets the mesh build this from a
 # repository and a path (novox/hq ADR 0069) rather than only on a workstation with the siblings.
 #
-# The base is named by ARG so it can be pinned to a digest the mesh's registry assigned. A tag would
-# make this runtime's contents depend on what somebody last pushed under that name.
-ARG RUNTIME_BASE=127.0.0.1:5000/mesh-tools/runtime@sha256:44b5d8bc30107fdc3bffdaebc1ca2de97615053853f826dfccce253a01a163fd
+# Named, not pinned. The mesh answers this with the copy of the runtime it holds, because a
+# fingerprint written here would name one particular copy — the one on whichever machine the person
+# typing it was using — and on any other mesh that copy has never existed (novox/hq issue 044).
+# Declared in module.json's `build.on`; there is deliberately no default, so a build nobody told
+# stops here rather than on a reference that resolves to nothing.
+ARG RUNTIME_BASE
 
 FROM ${RUNTIME_BASE} AS build
 # Compiled under /app/modules so `@novox/mesh-sdk` resolves upward into the base's own
diff --git a/modules/mesh-catalog/module.json b/modules/mesh-catalog/module.json
index 6165f8b..b6f0047 100644
--- a/modules/mesh-catalog/module.json
+++ b/modules/mesh-catalog/module.json
@@ -78,6 +78,13 @@
     }
   ],
   "build": {
+    "on": [
+      {
+        "arg": "RUNTIME_BASE",
+        "module": "mesh-tools",
+        "artifact": "runtime"
+      }
+    ],
     "artifacts": [
       {
         "name": "runtime",
diff --git a/modules/postgres/Dockerfile b/modules/postgres/Dockerfile
index aa74659..5851eed 100644
--- a/modules/postgres/Dockerfile
+++ b/modules/postgres/Dockerfile
@@ -6,9 +6,12 @@
 # repository and a path (novox/hq ADR 0069) rather than only on a workstation that happens to have
 # the siblings.
 #
-# The base is named by ARG so it can be pinned to a digest the mesh's registry assigned. A tag would
-# make this runtime's contents depend on what somebody last pushed under that name.
-ARG RUNTIME_BASE=127.0.0.1:5000/mesh-tools/runtime@sha256:44b5d8bc30107fdc3bffdaebc1ca2de97615053853f826dfccce253a01a163fd
+# Named, not pinned. The mesh answers this with the copy of the runtime it holds, because a
+# fingerprint written here would name one particular copy — the one on whichever machine the person
+# typing it was using — and on any other mesh that copy has never existed (novox/hq issue 044).
+# Declared in module.json's `build.on`; there is deliberately no default, so a build nobody told
+# stops here rather than on a reference that resolves to nothing.
+ARG RUNTIME_BASE
 
 FROM ${RUNTIME_BASE} AS build
 # Compiled under /app/modules so `@novox/mesh-sdk` resolves upward into the base's own
diff --git a/modules/postgres/module.json b/modules/postgres/module.json
index a161300..20ec55f 100644
--- a/modules/postgres/module.json
+++ b/modules/postgres/module.json
@@ -122,6 +122,13 @@
     }
   ],
   "build": {
+    "on": [
+      {
+        "arg": "RUNTIME_BASE",
+        "module": "mesh-tools",
+        "artifact": "runtime"
+      }
+    ],
     "artifacts": [
       {
         "name": "runtime",