diff --git a/prisma/migrations/20260428135613_add_mentor_onboarding_sent_at/migration.sql b/prisma/migrations/20260428135613_add_mentor_onboarding_sent_at/migration.sql
new file mode 100644
index 0000000..bd19e07
--- /dev/null
+++ b/prisma/migrations/20260428135613_add_mentor_onboarding_sent_at/migration.sql
@@ -0,0 +1,2 @@
+-- AlterTable
+ALTER TABLE "User" ADD COLUMN     "mentorOnboardingSentAt" TIMESTAMP(3);
diff --git a/prisma/schema.prisma b/prisma/schema.prisma
index 0d2db3a..5cfe1ec 100644
--- a/prisma/schema.prisma
+++ b/prisma/schema.prisma
@@ -302,6 +302,9 @@ model User {
   institution    String? // User's institution/organization
   metadataJson   Json?      @db.JsonB
 
+  // Mentor onboarding email idempotency: stamped once when MENTOR role is first added.
+  mentorOnboardingSentAt DateTime?
+
   // Profile
   bio                  String? // User bio for matching with project descriptions
   profileImageKey      String? // Storage key (e.g., "avatars/user123/1234567890.jpg")