diff --git a/tests/checks/deep-cmdsub.fish b/tests/checks/deep-cmdsub.fish
new file mode 100644
index 000000000..5a6ac7966
--- /dev/null
+++ b/tests/checks/deep-cmdsub.fish
@@ -0,0 +1,11 @@
+# RUN: %fish %s
+
+# Ensure we don't hang on deep command substitutions - see #6503.
+
+set s "echo hooray"
+for i in (seq 63)
+    set s "echo ($s)"
+end
+eval $s
+#CHECK: hooray
+