diff --git a/Dockerfile b/Dockerfile
index 9bc18a5..84c032a 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -1,3 +1,4 @@
+ARG GO_BASE=golang:1.25-alpine
 # The control plane's image.
 #
 # novox/hq ADR 0006: this image is pinned by digest in the bundle the host carries, fetched on a
@@ -11,7 +12,7 @@
 # (novox/hq ADR 0004). Adding them "just in case" would put a trust store in the one image whose
 # whole argument is that it contains nothing to reason about.
 
-FROM golang:1.25-alpine AS build
+FROM ${GO_BASE} AS build
 WORKDIR /src
 
 # Dependencies first, so a change to the source does not refetch them.
diff --git a/cmd/mesh-builder/Dockerfile b/cmd/mesh-builder/Dockerfile
index 3c6cc5f..3951536 100644
--- a/cmd/mesh-builder/Dockerfile
+++ b/cmd/mesh-builder/Dockerfile
@@ -1,17 +1,19 @@
+ARG ALPINE_BASE=alpine:3
+ARG GO_BASE=golang:1.25-alpine
 # The builder, as a module ships one.
 #
 # Not FROM scratch, unlike the control plane: this one runs git and a container client, so it
 # needs a filesystem with them on it. That is the honest cost of a machine whose job is to build —
 # and it is why building is a MODULE on a machine that has a runtime rather than something the
 # control plane does (novox/hq ADR 0005).
-FROM golang:1.25-alpine AS build
+FROM ${GO_BASE} AS build
 WORKDIR /src
 COPY go.mod go.sum ./
 RUN go mod download
 COPY . .
 RUN CGO_ENABLED=0 go build -trimpath -ldflags '-s -w' -o /mesh-builder ./cmd/mesh-builder
 
-FROM alpine:3
+FROM ${ALPINE_BASE}
 # git to clone what it is asked to build, and the docker client to build and push it. The daemon
 # is the machine's, reached through its socket — a build machine shares the runtime it was given
 # rather than running one inside itself.
diff --git a/internal/builder/builder.go b/internal/builder/builder.go
index ca7c3d6..f78ef31 100644
--- a/internal/builder/builder.go
+++ b/internal/builder/builder.go
@@ -397,13 +397,13 @@ func one(ctx context.Context, run Runner, publish Publisher,
 				module, a.From, strings.Join(copies, ", "))
 		}
 		if len(bases) > 0 {
-			// Said, not yet refused: the mesh's own images start FROM a public base — the control
-			// plane's, the builder's, the tool runtime's — and refusing those refuses genesis.
-			// They declare their bases next; until then a base fetched on its own is named here,
-			// with the remedy, every build.
-			say("recipe", "UNDECLARED base(s) %s in %s — declare each under build.on as "+
-				"{arg, image@sha256:…} and read it from that argument (novox/hq ADR 0097)",
-				strings.Join(bases, ", "), a.From)
+			// Refused, since the mesh's own images declare theirs (ADR 0097): a base fetched on
+			// its own is a build that works when a public registry answers, which is sometimes.
+			return catalogue.Built{}, fmt.Errorf(
+				"%s: the recipe %s starts FROM %s, which the manifest does not declare. Declare "+
+					"each under build.on as {\"arg\": \"<NAME>\", \"image\": \"<image>@sha256:…\"} "+
+					"and start FROM ${<NAME>} (novox/hq ADR 0097)",
+				module, a.From, strings.Join(bases, ", "))
 		}
 		invocation := append([]string{"build", "-f", a.From, "-t", local}, args...)
 		if a.Target != "" {
diff --git a/internal/builder/standing_on_test.go b/internal/builder/standing_on_test.go
index 61a94ab..9f87782 100644
--- a/internal/builder/standing_on_test.go
+++ b/internal/builder/standing_on_test.go
@@ -124,8 +124,7 @@ FROM golang:1.25-alpine AS go
 	if strings.Join(copies, "|") != "${MC_BASE} (a build argument the manifest does not declare)|vendor/tool:latest" {
 		t.Fatalf("copies out of undeclared images: %v", copies)
 	}
-	// A base fetched on its own is named apart: the mesh's own images still start FROM one, so
-	// it is said rather than refused until they declare theirs.
+	// A base fetched on its own is named apart, and refused like a copy (ADR 0097).
 	if strings.Join(bases, "|") != "golang:1.25-alpine" {
 		t.Fatalf("undeclared bases: %v", bases)
 	}
diff --git a/module.json b/module.json
index 3b80760..dbb7f0c 100644
--- a/module.json
+++ b/module.json
@@ -66,6 +66,12 @@
         "kind": "image",
         "from": "Dockerfile"
       }
+    ],
+    "on": [
+      {
+        "arg": "GO_BASE",
+        "image": "golang@sha256:1ae0735f00daffa3aaf1363a5184c0d2dc55c78e3db4ec70241cdac97bf84b59"
+      }
     ]
   }
 }