diff --git a/models/migrations/v1_26/v324.go b/models/migrations/v1_26/v324.go
index 5c67ed5998..5d96bfa3ca 100644
--- a/models/migrations/v1_26/v324.go
+++ b/models/migrations/v1_26/v324.go
@@ -3,18 +3,22 @@
 
 package v1_26
 
-import "xorm.io/xorm"
+import (
+	"fmt"
 
-func AddGroupColumnsToRepositoryTable(x *xorm.Engine) error {
-	type Repository struct {
-		LowerName      string `xorm:"UNIQUE(s) UNIQUE(g) INDEX NOT NULL"`
-		GroupID        int64  `xorm:"UNIQUE(g) INDEX DEFAULT 0"`
-		OwnerID        int64  `xorm:"UNIQUE(s) UNIQUE(g) index"`
-		GroupSortOrder int
+	"xorm.io/xorm"
+)
+
+func FixClosedMilestoneCompleteness(x *xorm.Engine) error {
+	// Update all milestones to recalculate completeness with the new logic:
+	// - Closed milestones with 0 issues should show 100%
+	// - All other milestones should calculate based on closed/total ratio
+	_, err := x.Exec("UPDATE `milestone` SET completeness=(CASE WHEN is_closed = ? AND num_issues = 0 THEN 100 ELSE 100*num_closed_issues/(CASE WHEN num_issues > 0 THEN num_issues ELSE 1 END) END)",
+		true,
+	)
+	if err != nil {
+		return fmt.Errorf("error updating milestone completeness: %w", err)
 	}
-	_, err := x.SyncWithOptions(xorm.SyncOptions{
-		IgnoreConstrains: false,
-		IgnoreIndices:    false,
-	}, new(Repository))
-	return err
+
+	return nil
 }
diff --git a/models/migrations/v1_26/v331.go b/models/migrations/v1_26/v331.go
new file mode 100644
index 0000000000..25b5b14c6a
--- /dev/null
+++ b/models/migrations/v1_26/v331.go
@@ -0,0 +1,20 @@
+// Copyright 2025 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package v1_26
+
+import "xorm.io/xorm"
+
+func AddGroupColumnsToRepositoryTable(x *xorm.Engine) error {
+	type Repository struct {
+		LowerName      string `xorm:"UNIQUE(s) UNIQUE(g) INDEX NOT NULL"`
+		GroupID        int64  `xorm:"UNIQUE(s) INDEX DEFAULT 0"`
+		OwnerID        int64  `xorm:"UNIQUE(s) UNIQUE(g) index"`
+		GroupSortOrder int
+	}
+	_, err := x.SyncWithOptions(xorm.SyncOptions{
+		IgnoreConstrains: false,
+		IgnoreIndices:    false,
+	}, new(Repository))
+	return err
+}