diff --git a/check-nested-modules.sh b/check-nested-modules.sh
new file mode 100644
index 00000000..ce37d2f9
--- /dev/null
+++ b/check-nested-modules.sh
@@ -0,0 +1,29 @@
+#!/bin/sh
+# The unit tests of every Go module of its own below this repository's root, run from the root, for merge-check.sh
+# and for the test that tries it on a tree (merge_check_nested_test.go). Its own file so that the test needs no
+# particular merge-check.sh: the check that judges a change to merge-check.sh is main's.
+set -eu
+cgo="${cgo:-0}"
+race="${race:-}"
+
+# nested_modules vets and tests every Go module of its own below the root, `modules/<name>/go.mod` (novox/hq ADR 0420
+# §5, issue 1025): `./...` stops at a nested go.mod, so a module that lives whole in this repository would have its
+# unit tests run by nobody. None is a pass. Each runs in its own directory, outside the vendor directory this
+# repository's own packages are built from; how its dependencies are found offline is issue 1019's (the move of the
+# first two modules in), and it is settled there. A failing module fails the check; the others are still run.
+nested_modules() {
+	nested=0
+	for gomod in modules/*/go.mod; do
+		[ -e "$gomod" ] || continue
+		dir=$(dirname "$gomod")
+		echo "nested module $dir:"
+		if ! (cd "$dir" && GOFLAGS= CGO_ENABLED=0 go vet ./... &&
+			GOFLAGS= CGO_ENABLED="${cgo:-0}" go test ${race:-} -count=1 -timeout 30m ./...); then
+			echo "nested module $dir FAILED"
+			nested=1
+		fi
+	done
+	return $nested
+}
+
+nested_modules
diff --git a/cmd/mesh-controller/merge_check_nested_test.go b/cmd/mesh-controller/merge_check_nested_test.go
index 933c7a5d..e8888954 100644
--- a/cmd/mesh-controller/merge_check_nested_test.go
+++ b/cmd/mesh-controller/merge_check_nested_test.go
@@ -9,10 +9,10 @@ import (
 )
 
 // **A repository's merge check runs every module's unit tests, a Go module of its own below the root included**
-// (novox/hq ADR 0420 §5, issue 1025): `./...` stops at a nested go.mod. This runs the script's own
-// `nested-modules` mode in a tree with a `go` that only records where it was run and fails for a module named bad.
+// (novox/hq ADR 0420 §5, issue 1025): `./...` stops at a nested go.mod. This runs the script
+// check-nested-modules.sh in a tree with a `go` that only records where it was run and fails for a module named bad.
 func TestTheMergeCheckVetsAndTestsEveryNestedGoModule(t *testing.T) {
-	script, err := filepath.Abs("../../merge-check.sh")
+	script, err := filepath.Abs("../../check-nested-modules.sh")
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -24,7 +24,7 @@ func TestTheMergeCheckVetsAndTestsEveryNestedGoModule(t *testing.T) {
 		t.Fatal(err)
 	}
 	run := func() (string, error) {
-		cmd := exec.Command("sh", script, "nested-modules")
+		cmd := exec.Command("sh", script)
 		cmd.Dir = tree
 		cmd.Env = append(os.Environ(), "PATH="+bin+":"+os.Getenv("PATH"), "LOG="+log, "GOFLAGS=-mod=vendor")
 		out, err := cmd.CombinedOutput()
diff --git a/cmd/mesh-controller/merge_gate_test.go b/cmd/mesh-controller/merge_gate_test.go
index e50cb483..9852431d 100644
--- a/cmd/mesh-controller/merge_gate_test.go
+++ b/cmd/mesh-controller/merge_gate_test.go
@@ -527,6 +527,10 @@ func TestADeliveryGroupMovingAModuleMachinesRunPassesTheGate(t *testing.T) {
 	f, manifests := catalogueMeshWithAnApp(t)
 	catTree := aTree(t, withEdit(manifests, "album", ""))
 	app := t.TempDir()
+	// The app module is built from this repository's root, and stays there: only album moves.
+	if err := os.WriteFile(filepath.Join(app, "module.json"), []byte(`{"module":"app","version":"1"}`), 0o644); err != nil {
+		t.Fatal(err)
+	}
 	if err := os.MkdirAll(filepath.Join(app, "modules", "album"), 0o755); err != nil {
 		t.Fatal(err)
 	}
diff --git a/merge-check.sh b/merge-check.sh
index f30cc9de..239bf8ac 100755
--- a/merge-check.sh
+++ b/merge-check.sh
@@ -36,30 +36,11 @@ judge_store_tests() {
 		missing=$((missing + 1))
 	fi
 }
-# nested_modules vets and tests every Go module of its own below the root, `modules/<name>/go.mod` (novox/hq ADR 0420
-# §5, issue 1025): `./...` stops at a nested go.mod, so a module that lives whole in this repository would have its
-# unit tests run by nobody. None is a pass. Each runs in its own directory, outside the vendor directory this
-# repository's own packages are built from; how its dependencies are found offline is issue 1019's (the move of the
-# first two modules in), and it is settled there. A failing module fails the check; the others are still run.
+# Every Go module of its own below the root, vetted and tested (novox/hq ADR 0420 §5, issue 1025): `./...` stops at a
+# nested go.mod. Its own file, check-nested-modules.sh, which takes $cgo and $race from here.
 nested_modules() {
-	nested=0
-	for gomod in modules/*/go.mod; do
-		[ -e "$gomod" ] || continue
-		dir=$(dirname "$gomod")
-		echo "nested module $dir:"
-		if ! (cd "$dir" && GOFLAGS= CGO_ENABLED=0 go vet ./... &&
-			GOFLAGS= CGO_ENABLED="${cgo:-0}" go test ${race:-} -count=1 -timeout 30m ./...); then
-			echo "nested module $dir FAILED"
-			nested=1
-		fi
-	done
-	return $nested
+	cgo="$cgo" race="$race" sh ./check-nested-modules.sh
 }
-# `sh merge-check.sh nested-modules` runs only that, to try it on a tree.
-if [ "${1:-}" = nested-modules ]; then
-	nested_modules
-	exit
-fi
 # `sh merge-check.sh judge-store-tests <package> <status> <output> <name>…` runs only the judging, to try it on a
 # recorded `go test -v` output.
 if [ "${1:-}" = judge-store-tests ]; then