diff --git a/toolchain/xpcg/backend/c-mpc/backend.c b/toolchain/xpcg/backend/c-mpc/backend.c
index 2064c0c..6fb4aa5 100644
--- a/toolchain/xpcg/backend/c-mpc/backend.c
+++ b/toolchain/xpcg/backend/c-mpc/backend.c
@@ -1098,7 +1098,7 @@ static int emit_interface(const struct interface_definition *iface)
 	b_result result = b_file_open(
 		NULL,
 		B_RV_PATH(path),
-		B_FILE_WRITE_ONLY | B_FILE_CREATE,
+		B_FILE_WRITE_ONLY | B_FILE_CREATE | B_FILE_TRUNCATE,
 		&file);
 	if (b_result_is_error(result)) {
 		b_throw(result);