diff --git a/clientapi/routing/routing.go b/clientapi/routing/routing.go
index cfc6c47fb..0a2efd6cc 100644
--- a/clientapi/routing/routing.go
+++ b/clientapi/routing/routing.go
@@ -117,6 +117,7 @@ func Setup(
 
 	r0mux := publicAPIMux.PathPrefix("/r0").Subrouter()
 	unstableMux := publicAPIMux.PathPrefix("/unstable").Subrouter()
+	unstableMux.NotFoundHandler = r0mux // serve r0 endpoints by default unless overridden
 
 	r0mux.Handle("/createRoom",
 		httputil.MakeAuthAPI("createRoom", userAPI, func(req *http.Request, device *userapi.Device) util.JSONResponse {
diff --git a/federationapi/routing/routing.go b/federationapi/routing/routing.go
index 8f33c7660..c479e3d93 100644
--- a/federationapi/routing/routing.go
+++ b/federationapi/routing/routing.go
@@ -57,6 +57,12 @@ func Setup(
 	v1fedmux := fedMux.PathPrefix("/v1").Subrouter()
 	v2fedmux := fedMux.PathPrefix("/v2").Subrouter()
 
+	unstableFedMux := fedMux.PathPrefix("/unstable").Subrouter()
+	unstableFedMux.NotFoundHandler = v2fedmux // serve v2 endpoints by default unless overridden
+
+	unstableKeysMux := keyMux.PathPrefix("/unstable").Subrouter()
+	unstableKeysMux.NotFoundHandler = v2keysmux // serve v2 endpoints by default unless overridden
+
 	wakeup := &httputil.FederationWakeups{
 		FsAPI: fsAPI,
 	}
diff --git a/syncapi/routing/routing.go b/syncapi/routing/routing.go
index e2ff27395..1ffd3cb26 100644
--- a/syncapi/routing/routing.go
+++ b/syncapi/routing/routing.go
@@ -40,6 +40,8 @@ func Setup(
 	cfg *config.SyncAPI,
 ) {
 	r0mux := csMux.PathPrefix("/r0").Subrouter()
+	unstableMux := csMux.PathPrefix("/unstable").Subrouter()
+	unstableMux.NotFoundHandler = r0mux // serve r0 endpoints by default unless overridden
 
 	// TODO: Add AS support for all handlers below.
 	r0mux.Handle("/sync", httputil.MakeAuthAPI("sync", userAPI, func(req *http.Request, device *userapi.Device) util.JSONResponse {