diff --git a/stdlib/europa/dagger/engine/spec/engine/address.cue b/stdlib/europa/dagger/engine/spec/engine/address.cue
new file mode 100644
index 00000000..ee1802e8
--- /dev/null
+++ b/stdlib/europa/dagger/engine/spec/engine/address.cue
@@ -0,0 +1,4 @@
+package engine
+
+// A network service address
+#Address: string & =~"^(tcp://|unix://|udp://).*"
\ No newline at end of file