diff --git a/extensions/cpp_extension.py b/extensions/cpp_extension.py
index aaa43f964..a92195dda 100644
--- a/extensions/cpp_extension.py
+++ b/extensions/cpp_extension.py
@@ -79,7 +79,7 @@ class _CppExtension(_Extension):
 
         # check if the kernel has been built
         compiled_before = False
-        kernel_file_path = build_directory.joinpath(f"{self.name}.o")
+        kernel_file_path = build_directory.joinpath(f"{self.name}.so")
         if kernel_file_path.exists():
             compiled_before = True
 
diff --git a/extensions/cuda_extension.py b/extensions/cuda_extension.py
index da15bcd57..214b83ec8 100644
--- a/extensions/cuda_extension.py
+++ b/extensions/cuda_extension.py
@@ -74,7 +74,7 @@ class _CudaExtension(_CppExtension):
 
         # check if the kernel has been built
         compiled_before = False
-        kernel_file_path = build_directory.joinpath(f"{self.name}.o")
+        kernel_file_path = build_directory.joinpath(f"{self.name}.so")
         if kernel_file_path.exists():
             compiled_before = True