diff --git a/script/api_protobuf/api_protobuf.py b/script/api_protobuf/api_protobuf.py
index 23d8a53b70..4df7692167 100755
--- a/script/api_protobuf/api_protobuf.py
+++ b/script/api_protobuf/api_protobuf.py
@@ -1495,6 +1495,7 @@ def build_base_class(
     base_class_name: str,
     common_fields: list[descriptor.FieldDescriptorProto],
     messages: list[descriptor.DescriptorProto],
+    message_source_map: dict[str, int],
 ) -> tuple[str, str, str]:
     """Build the base class definition and implementation."""
     public_content = []
@@ -1511,7 +1512,7 @@ def build_base_class(
 
     # Determine if any message using this base class needs decoding
     needs_decode = any(
-        get_opt(msg, pb.source, SOURCE_BOTH) in (SOURCE_BOTH, SOURCE_CLIENT)
+        message_source_map.get(msg.name, SOURCE_BOTH) in (SOURCE_BOTH, SOURCE_CLIENT)
         for msg in messages
     )
 
@@ -1543,6 +1544,7 @@ def build_base_class(
 
 def generate_base_classes(
     base_class_groups: dict[str, list[descriptor.DescriptorProto]],
+    message_source_map: dict[str, int],
 ) -> tuple[str, str, str]:
     """Generate all base classes."""
     all_headers = []
@@ -1556,7 +1558,7 @@ def generate_base_classes(
         if common_fields:
             # Generate base class
             header, cpp, dump_cpp = build_base_class(
-                base_class_name, common_fields, messages
+                base_class_name, common_fields, messages, message_source_map
             )
             all_headers.append(header)
             all_cpp.append(cpp)
@@ -1567,6 +1569,7 @@ def generate_base_classes(
 
 def build_service_message_type(
     mt: descriptor.DescriptorProto,
+    message_source_map: dict[str, int],
 ) -> tuple[str, str] | None:
     """Builds the service message type."""
     snake = camel_to_snake(mt.name)
@@ -1574,7 +1577,7 @@ def build_service_message_type(
     if id_ is None:
         return None
 
-    source: int = get_opt(mt, pb.source, 0)
+    source: int = message_source_map.get(mt.name, SOURCE_BOTH)
 
     ifdef: str | None = get_opt(mt, pb.ifdef)
     log: bool = get_opt(mt, pb.log, True)
@@ -1714,7 +1717,9 @@ namespace api {
 
     # Generate base classes
     if base_class_fields:
-        base_headers, base_cpp, base_dump_cpp = generate_base_classes(base_class_groups)
+        base_headers, base_cpp, base_dump_cpp = generate_base_classes(
+            base_class_groups, message_source_map
+        )
         content += base_headers
         cpp += base_cpp
         dump_cpp += base_dump_cpp
@@ -1832,7 +1837,7 @@ static const char *const TAG = "api.service";
     cpp += "#endif\n\n"
 
     for mt in file.message_type:
-        obj = build_service_message_type(mt)
+        obj = build_service_message_type(mt, message_source_map)
         if obj is None:
             continue
         hout, cout = obj