diff --git a/internal/projectinstance/store.go b/internal/projectinstance/store.go
index 0bfd5863..e3555219 100644
--- a/internal/projectinstance/store.go
+++ b/internal/projectinstance/store.go
@@ -214,13 +214,24 @@ func (s *pgStore) UpsertPlanningInstance(ctx context.Context, inst *WorkflowInst
 		SELECT id FROM project_workflow_instances WHERE project_id = $1 AND status = 'planning' LIMIT 1
 	`, inst.ProjectID).Scan(&existingID)
 	if errors.Is(err, pgx.ErrNoRows) {
+		// Allocate next instance_no so planning drafts after a completed main/rdev
+		// instance do not collide on (project_id, instance_no).
+		var nextNo int
+		if err := tx.QueryRow(ctx, `
+			SELECT COALESCE(MAX(instance_no), 0) + 1
+			FROM project_workflow_instances
+			WHERE project_id = $1
+		`, inst.ProjectID).Scan(&nextNo); err != nil {
+			return nil, err
+		}
+		inst.InstanceNo = nextNo
 		err = tx.QueryRow(ctx, `
 			INSERT INTO project_workflow_instances (
 				project_id, workflow_version_id, instance_no, status, config_revision, plan_hash,
 				config_snapshot, created_by, created_at, updated_at
-			) VALUES ($1, $2, 1, 'planning', $3, $4, $5, $6, $7, $8)
+			) VALUES ($1, $2, $3, 'planning', $4, $5, $6, $7, $8, $9)
 			RETURNING id
-		`, inst.ProjectID, inst.WorkflowVersionID, inst.ConfigRevision, inst.PlanHash, snapRaw,
+		`, inst.ProjectID, inst.WorkflowVersionID, nextNo, inst.ConfigRevision, inst.PlanHash, snapRaw,
 			inst.CreatedBy, inst.CreatedAt, inst.UpdatedAt).Scan(&inst.ID)
 		if err != nil {
 			return nil, err