diff --git a/go.mod b/go.mod
index 18ef18f..920ebc4 100644
--- a/go.mod
+++ b/go.mod
@@ -1,7 +1,14 @@
 module github.com/chatwork/kibertas
 
+// The minimum Go version required to build this project
 go 1.25.0
 
+// A hint for setup-go action on CI to pick the Go version used to build
+// not only this project but also the karpenter's ko tool used in tests via setup-ko action.
+// Ensure this is greater than or equal to the 'go' version in
+// https://github.com/kubernetes-sigs/karpenter/blob/main/go.mod#L3
+toolchain go1.25.3
+
 require (
 	github.com/DataDog/datadog-api-client-go/v2 v2.30.0
 	github.com/aws/aws-sdk-go-v2 v1.39.2