diff --git a/bellows/ezsp/__init__.py b/bellows/ezsp/__init__.py
index c7aefeb9..30d1e0a7 100644
--- a/bellows/ezsp/__init__.py
+++ b/bellows/ezsp/__init__.py
@@ -642,12 +642,21 @@ async def get_supported_firmware_features(
         self,
     ) -> custom_commands.FirmwareFeatures:
         """Get supported firmware extensions."""
+        req = custom_commands.CustomCommand(
+            command_id=custom_commands.CustomCommandId.CMD_GET_SUPPORTED_FEATURES_REQ,
+            payload=custom_commands.GetSupportedFeaturesReq().serialize(),
+        )
+
         try:
-            status, rsp_data = await self.customFrame(
-                bytes([custom_commands.CustomCommand.CMD_GET_SUPPORTED_FEATURES]),
-            )
+            status, data = await self.customFrame(req.serialize())
         except InvalidCommandError:
             return custom_commands.FirmwareFeatures(0)
 
-        features, _ = custom_commands.FirmwareFeatures.deserialize(rsp_data)
-        return features
+        rsp_cmd, _ = custom_commands.CustomCommand.deserialize(data)
+        assert (
+            rsp_cmd.command_id
+            == custom_commands.CustomCommandId.CMD_GET_SUPPORTED_FEATURES_RSP
+        )
+
+        rsp, _ = custom_commands.GetSupportedFeaturesRsp.deserialize(rsp_cmd.payload)
+        return rsp.features
diff --git a/bellows/ezsp/custom_commands.py b/bellows/ezsp/custom_commands.py
index e03b0767..e5edb6e4 100644
--- a/bellows/ezsp/custom_commands.py
+++ b/bellows/ezsp/custom_commands.py
@@ -1,12 +1,48 @@
 """Custom EZSP commands."""
+from __future__ import annotations
 
 import zigpy.types as t
 
 
-class CustomCommand(t.enum8):
-    CMD_GET_SUPPORTED_FEATURES = 0x00
+class Bytes(bytes):
+    def serialize(self) -> Bytes:
+        return self
+
+    @classmethod
+    def deserialize(cls, data: bytes) -> tuple[Bytes, bytes]:
+        return cls(data), b""
+
+    def __repr__(self) -> str:
+        # Reading byte sequences like \x200\x21 is extremely annoying
+        # compared to \x20\x30\x21
+        escaped = "".join(f"\\x{b:02X}" for b in self)
+
+        return f"b'{escaped}'"
+
+    __str__ = __repr__
+
+
+class CustomCommandId(t.enum16):
+    CMD_GET_PROTOCOL_VERSION_REQ = 0x0000
+    CMD_GET_PROTOCOL_VERSION_RSP = 0x8000
+
+    CMD_GET_SUPPORTED_FEATURES_REQ = 0x0001
+    CMD_GET_SUPPORTED_FEATURES_RSP = 0x8001
+
+
+class CustomCommand(t.Struct):
+    command_id: CustomCommandId
+    payload: Bytes
 
 
 class FirmwareFeatures(t.bitmap32):
     # The firmware passes through all group traffic, regardless of group membership
     MEMBER_OF_ALL_GROUPS = 0b00000000_00000000_00000000_00000001
+
+
+class GetSupportedFeaturesReq(t.Struct):
+    pass
+
+
+class GetSupportedFeaturesRsp(t.Struct):
+    features: FirmwareFeatures