diff --git a/sw/host/rom_ext_image_tools/signer/rustfmt.toml b/sw/host/rom_ext_image_tools/signer/rustfmt.toml
new file mode 100644
index 0000000..30b7567
--- /dev/null
+++ b/sw/host/rom_ext_image_tools/signer/rustfmt.toml
@@ -0,0 +1,5 @@
+# Copyright lowRISC contributors.
+# Licensed under the Apache License, Version 2.0, see LICENSE for details.
+# SPDX-License-Identifier: Apache-2.0
+
+max_width = 80