diff --git a/cmd/mesh-controller/merge_gate.go b/cmd/mesh-controller/merge_gate.go
index fb781bda..a1c473b8 100644
--- a/cmd/mesh-controller/merge_gate.go
+++ b/cmd/mesh-controller/merge_gate.go
@@ -742,6 +742,21 @@ func shelfWithChange(base map[string]catalogue.Manifest, sources map[string]inve
 				"of one module name (issue 239)", orRoot(dir), m.Module, s.Repository))
 			return nil
 		}
+		// A module lives whole in one repository (novox/hq ADR 0420 §1, issue 1019): no artifact builds from another.
+		if m.Build != nil {
+			own := link.SourceMoved{Owner: ownerOf(repository), Repo: repoOf(repository)}
+			other := false
+			for _, a := range m.Build.Artifacts {
+				if a.Context != nil && a.Context.Repository != "" && !sameRepository(a.Context.Repository, own) {
+					failures = append(failures, fmt.Sprintf("%s builds artifact %s from %s: a module lives whole in "+
+						"one repository (novox/hq ADR 0420 §1)", m.Module, a.Name, a.Context.Repository))
+					other = true
+				}
+			}
+			if other {
+				return nil
+			}
+		}
 		resolved, err := standInBuild(written, texts)
 		if err == nil {
 			// As registration reads a built manifest: strictly, again (takeIn).
diff --git a/cmd/mesh-controller/merge_gate_test.go b/cmd/mesh-controller/merge_gate_test.go
index 9852431d..38c3ae17 100644
--- a/cmd/mesh-controller/merge_gate_test.go
+++ b/cmd/mesh-controller/merge_gate_test.go
@@ -559,3 +559,32 @@ func TestADeliveryGroupMovingAModuleMachinesRunPassesTheGate(t *testing.T) {
 		t.Errorf("the removal of a module machines run passed alone:\n%s", v.Report())
 	}
 }
+
+// **A module lives whole in one repository** (novox/hq ADR 0420 §1, issue 1019): an artifact whose build
+// context names another repository is refused by the gate, and one naming its own passes it.
+func TestTheGateRefusesAnArtifactContextNamingAnotherRepository(t *testing.T) {
+	withContext := func(repo string) string {
+		return `{"module":"proxy","version":"1","build":{"artifacts":[{"name":"server","kind":"image","from":"Dockerfile",` +
+			`"context":{"seat":"git","repository":"` + repo + `","ref":"main"}}]}}`
+	}
+	judge := func(repo string) []string {
+		t.Helper()
+		tree := aTree(t, map[string]string{"proxy": withContext(repo)})
+		_, failures, err := shelfWithChange(map[string]catalogue.Manifest{}, map[string]inventory.Source{},
+			snapshot.Facts{}, "novox/mesh-controller", tree, map[string]string{}, nil)
+		if err != nil {
+			t.Fatal(err)
+		}
+		return failures
+	}
+	const want = "proxy builds artifact server from novox/mesh-catalog: a module lives whole in one repository (novox/hq ADR 0420 §1)"
+	failures := judge("novox/mesh-catalog")
+	if len(failures) != 1 || !strings.Contains(failures[0], want) {
+		t.Errorf("a context naming another repository: %v, want it to say %q", failures, want)
+	}
+	for _, f := range judge("novox/mesh-controller") {
+		if strings.Contains(f, "ADR 0420 §1") {
+			t.Errorf("a context naming the module's own repository was refused: %s", f)
+		}
+	}
+}