diff --git a/tests/functional/flakes/flake-in-submodule.sh b/tests/functional/flakes/flake-in-submodule.sh
index 643d729ff..f98c19aa8 100755
--- a/tests/functional/flakes/flake-in-submodule.sh
+++ b/tests/functional/flakes/flake-in-submodule.sh
@@ -76,3 +76,21 @@ git -C "$rootRepo" commit -m "Add flake.nix"
 
 storePath=$(nix flake metadata --json "$rootRepo?submodules=1" | jq -r .path)
 [[ -e "$storePath/submodule" ]]
+
+# The root repo may use the submodule repo as an input
+# through the relative path. This may change in the future;
+# see: https://discourse.nixos.org/t/57783 and #9708.
+cat > "$rootRepo"/flake.nix <<EOF
+{
+    inputs.subRepo.url = "git+file:./submodule";
+    outputs = { ... }: { };
+}
+EOF
+git -C "$rootRepo" add flake.nix
+git -C "$rootRepo" commit -m "Add subRepo input"
+(
+  cd "$rootRepo"
+  # The submodule must be locked to the relative path,
+  # _not_ the absolute path:
+  [[ $(nix flake metadata --json | jq -r .locks.nodes.subRepo.locked.url) = "file:./submodule" ]]
+)