diff --git a/generator/dawn_json_generator.py b/generator/dawn_json_generator.py
index f957f01..c2b36c6 100644
--- a/generator/dawn_json_generator.py
+++ b/generator/dawn_json_generator.py
@@ -83,14 +83,15 @@
         for value in self.values:
             self.full_mask = self.full_mask | value.value
 
+class CallbackType(Type):
+    def __init__(self, name, json_data):
+        Type.__init__(self, name, json_data)
+        self.arguments = []
+
 class NativeType(Type):
     def __init__(self, name, json_data):
         Type.__init__(self, name, json_data, native=True)
 
-class NativelyDefined(Type):
-    def __init__(self, name, json_data):
-        Type.__init__(self, name, json_data)
-
 # Methods and structures are both "records", so record members correspond to
 # method arguments or structure members.
 class RecordMember:
@@ -201,6 +202,9 @@
 def link_structure(struct, types):
     struct.members = linked_record_members(struct.json_data['members'], types)
 
+def link_callback(callback, types):
+    callback.arguments = linked_record_members(callback.json_data['args'], types)
+
 # Sort structures so that if struct A has struct B as a member, then B is listed before A
 # This is a form of topological sort where we try to keep the order reasonably similar to the
 # original order (though th sort isn't technically stable).
@@ -242,7 +246,7 @@
         'bitmask': BitmaskType,
         'enum': EnumType,
         'native': NativeType,
-        'natively defined': NativelyDefined,
+        'callback': CallbackType,
         'object': ObjectType,
         'structure': StructureType,
     }
@@ -267,6 +271,9 @@
     for struct in by_category['structure']:
         link_structure(struct, types)
 
+    for callback in by_category['callback']:
+        link_callback(callback, types)
+
     for category in by_category.keys():
         by_category[category] = sorted(by_category[category], key=lambda typ: typ.name.canonical_case())
 
@@ -292,6 +299,8 @@
     commands = []
     return_commands = []
 
+    wire_json['special items']['client_handwritten_commands'] += wire_json['special items']['client_side_commands']
+
     # Generate commands from object methods
     for api_object in wire_params['by_category']['object']:
         for method in api_object.methods:
@@ -477,6 +486,9 @@
             for method in c_native_methods(api_params['types'], typ) ]
     return [(typ, method) for (_, typ, method) in sorted(unsorted)]
 
+def has_callback_arguments(method):
+    return any(arg.type.category == 'callback' for arg in method.arguments)
+
 class MultiGeneratorFromDawnJSON(Generator):
     def get_description(self):
         return 'Generates code for various target from Dawn.json.'
@@ -543,8 +555,16 @@
             renders.append(FileRender('webgpu_cpp.cpp', 'src/dawn/webgpu_cpp.cpp', [base_params, api_params, cpp_params]))
 
         if 'mock_webgpu' in targets:
-            renders.append(FileRender('mock_webgpu.h', 'src/dawn/mock_webgpu.h', [base_params, api_params, c_params]))
-            renders.append(FileRender('mock_webgpu.cpp', 'src/dawn/mock_webgpu.cpp', [base_params, api_params, c_params]))
+            mock_params = [
+                base_params,
+                api_params,
+                c_params,
+                {
+                    'has_callback_arguments': has_callback_arguments
+                }
+            ]
+            renders.append(FileRender('mock_webgpu.h', 'src/dawn/mock_webgpu.h', mock_params))
+            renders.append(FileRender('mock_webgpu.cpp', 'src/dawn/mock_webgpu.cpp', mock_params))
 
         if 'dawn_native_utils' in targets:
             frontend_params = [