diff --git a/src/db/repositories/plan-repository.ts b/src/db/repositories/plan-repository.ts
index 1cb3f2a..ea4e097 100644
--- a/src/db/repositories/plan-repository.ts
+++ b/src/db/repositories/plan-repository.ts
@@ -46,6 +46,12 @@ export interface PlanRepository {
    */
   findByPhaseId(phaseId: string): Promise<Plan[]>;
 
+  /**
+   * Get the next available plan number for a phase.
+   * Returns MAX(number) + 1, or 1 if no plans exist.
+   */
+  getNextNumber(phaseId: string): Promise<number>;
+
   /**
    * Update a plan.
    * Throws if plan not found.