diff --git a/scripts/py_matter_yamltests/matter_yamltests/fixes.py b/scripts/py_matter_yamltests/matter_yamltests/fixes.py
index c073b923e163c1..d2d4a5fc48ee71 100644
--- a/scripts/py_matter_yamltests/matter_yamltests/fixes.py
+++ b/scripts/py_matter_yamltests/matter_yamltests/fixes.py
@@ -59,6 +59,10 @@ def convert_yaml_octet_string_to_bytes(s: str) -> bytes:
 
     This handles any c-style hex escapes (e.g. \x5a) and hex: prefix
     '''
+    # Step 0: Check if this is already of type bytes
+    if isinstance(s, bytes):
+        return s
+
     # Step 1: handle explicit "hex:" prefix
     if s.startswith('hex:'):
         return binascii.unhexlify(s[4:])
diff --git a/scripts/py_matter_yamltests/matter_yamltests/parser.py b/scripts/py_matter_yamltests/matter_yamltests/parser.py
index 13dc7156dd1f78..5307d9daffd58b 100644
--- a/scripts/py_matter_yamltests/matter_yamltests/parser.py
+++ b/scripts/py_matter_yamltests/matter_yamltests/parser.py
@@ -246,12 +246,12 @@ def __init__(self, test: dict, config: dict, definitions: SpecDefinitions, pics_
                 response_mapping = self._as_mapping(
                     definitions, self.cluster, command.output_param)
 
-        self._update_with_definition(
-            self.arguments_with_placeholders, argument_mapping)
-        self._update_with_definition(
-            self.response_with_placeholders, response_mapping)
+        self.argument_mapping = argument_mapping
+        self.response_mapping = response_mapping
+        self.update_arguments(self.arguments_with_placeholders)
+        self.update_response(self.response_with_placeholders)
 
-        # This performs a very basic sanity parse time check of constrains. This parsing happens
+        # This performs a very basic sanity parse time check of constraints. This parsing happens
         # again inside post processing response since at that time we will have required variables
         # to substitute in. This parsing check here has value since some test can take a really
         # long time to run so knowing earlier on that the test step would have failed at parsing
@@ -299,6 +299,14 @@ def _as_mapping(self, definitions, cluster_name, target_name):
 
         return target_name
 
+    def update_arguments(self, arguments_with_placeholders):
+        self._update_with_definition(
+            arguments_with_placeholders, self.argument_mapping)
+
+    def update_response(self, response_with_placeholders):
+        self._update_with_definition(
+            response_with_placeholders, self.response_mapping)
+
     def _update_with_definition(self, container: dict, mapping_type):
         if not container or not mapping_type:
             return
@@ -387,6 +395,8 @@ def __init__(self, test: _TestStepWithPlaceholders, runtime_config_variable_stor
         self.response = copy.deepcopy(test.response_with_placeholders)
         self._update_placeholder_values(self.arguments)
         self._update_placeholder_values(self.response)
+        test.update_arguments(self.arguments)
+        test.update_response(self.response)
 
     @property
     def is_enabled(self):