diff --git a/matrix/matrix.cc b/matrix/matrix.cc
index 8b72c75bd..28a706016 100644
--- a/matrix/matrix.cc
+++ b/matrix/matrix.cc
@@ -157,6 +157,7 @@ ircd::m::module_names
 	"client_sync_account_data",
 	"client_sync_device_lists",
 	"client_sync_device_one_time_keys_count",
+	"client_sync_device_unused_fallback_key_types",
 	"client_sync_groups",
 	"client_sync_presence",
 	"client_sync_to_device",
diff --git a/modules/Makefile.am b/modules/Makefile.am
index 00d80857b..6bf8d3226 100644
--- a/modules/Makefile.am
+++ b/modules/Makefile.am
@@ -443,6 +443,7 @@ client_client_sync_groups_la_SOURCES = client/sync/groups.cc
 client_client_sync_to_device_la_SOURCES = client/sync/to_device.cc
 client_client_sync_device_lists_la_SOURCES = client/sync/device_lists.cc
 client_client_sync_device_one_time_keys_count_la_SOURCES = client/sync/device_one_time_keys_count.cc
+client_client_sync_device_unused_fallback_key_types_la_SOURCES = client/sync/device_unused_fallback_key_types.cc
 
 client_module_LTLIBRARIES += \
 	client/client_sync_account_data.la \
@@ -452,6 +453,7 @@ client_module_LTLIBRARIES += \
 	client/client_sync_to_device.la \
 	client/client_sync_device_lists.la \
 	client/client_sync_device_one_time_keys_count.la \
+	client/client_sync_device_unused_fallback_key_types.la \
 	###
 
 # client/sync/rooms/
diff --git a/modules/client/sync/device_unused_fallback_key_types.cc b/modules/client/sync/device_unused_fallback_key_types.cc
new file mode 100644
index 000000000..1795cb1e5
--- /dev/null
+++ b/modules/client/sync/device_unused_fallback_key_types.cc
@@ -0,0 +1,64 @@
+// Matrix Construct
+//
+// Copyright (C) Matrix Construct Developers, Authors & Contributors
+// Copyright (C) 2016-2023 Jason Volk <jason@zemos.net>
+//
+// Permission to use, copy, modify, and/or distribute this software for any
+// purpose with or without fee is hereby granted, provided that the above
+// copyright notice and this permission notice is present in all copies. The
+// full license for this software is available in the LICENSE file.
+
+namespace ircd::m::sync
+{
+	static bool device_unused_fallback_key_types_polylog(data &);
+	static bool device_unused_fallback_key_types_linear(data &);
+
+	extern item device_unused_fallback_key_types;
+}
+
+ircd::mapi::header
+IRCD_MODULE
+{
+	"Client Sync :Device Unused Fallback Key Types"
+};
+
+decltype(ircd::m::sync::device_unused_fallback_key_types)
+ircd::m::sync::device_unused_fallback_key_types
+{
+	"device_unused_fallback_key_types",
+	device_unused_fallback_key_types_polylog,
+	device_unused_fallback_key_types_linear
+};
+
+bool
+ircd::m::sync::device_unused_fallback_key_types_linear(data &data)
+{
+	if(!data.device_id)
+		return false;
+
+	if(!data.event || !data.event->event_id)
+		return false;
+
+	if(!startswith(json::get<"type"_>(*data.event), "ircd.device"))
+		return false;
+
+	if(json::get<"room_id"_>(*data.event) != data.user_room)
+		return false;
+
+	json::stack::array array
+	{
+		*data.out, "device_unused_fallback_key_types"
+	};
+
+	array.append("signed_curve25519");
+	return true;
+}
+
+bool
+ircd::m::sync::device_unused_fallback_key_types_polylog(data &data)
+{
+	if(!data.device_id)
+		return false;
+
+	return false;
+}