diff --git a/src/middleware/idempotency.ts b/src/middleware/idempotency.ts
index 404d4f2b..66c8d537 100644
--- a/src/middleware/idempotency.ts
+++ b/src/middleware/idempotency.ts
@@ -1,4 +1,4 @@
-import type { NextFunction, Request, Response } from 'express';
+import type { NextFunction, Request, Response, RequestHandler } from 'express';
 import type { Pool } from 'pg';
 import { createHash } from 'crypto';
 import { config } from '../config/index.js';
@@ -342,3 +342,15 @@ export async function idempotencyMiddleware(
     next(error);
   }
 }
+
+/**
+ * Factory that returns a 3-parameter Express RequestHandler for idempotencyMiddleware.
+ * Express treats 4-parameter functions as error handlers, so this wrapper ensures
+ * Express dispatches the middleware during normal request processing.
+ */
+export function createIdempotencyMiddleware(opts?: IdempotencyConfig): RequestHandler {
+  return (req: Request, res: Response, next: NextFunction) => {
+    idempotencyMiddleware(req, res, next, opts);
+  };
+}
+
diff --git a/src/routes/refresh-token.ts b/src/routes/refresh-token.ts
index 1f1616a4..9f7301fa 100644
--- a/src/routes/refresh-token.ts
+++ b/src/routes/refresh-token.ts
@@ -14,9 +14,10 @@
  *   - Token hashes are never exposed in the response
  */
 
-import { Router } from 'express';
+import { Router, type Request } from 'express';
 import { requireAuth } from '../middleware/requireAuth.js';
 import { correlationMiddleware } from '../middleware/correlation.js';
+import { idempotencyMiddleware } from '../middleware/idempotency.js';
 import { getClientIp, DEFAULT_PROXY_HEADERS } from '../lib/clientIp.js';
 import { encodeCursor, parseCursor } from '../lib/cursorPagination.js';
 import {
@@ -45,6 +46,7 @@ export interface RefreshTokenRouterDeps {
   refreshTokenRepository?: RefreshTokenRepository;
   rateLimitMiddleware?: RequestHandler;
   rateLimiter?: TokenBucketRateLimiter;
+  idempotencyMiddleware?: RequestHandler;
 }
 
 export function createRefreshTokenRouter(deps: RefreshTokenRouterDeps = {}): Router {
@@ -58,6 +60,15 @@ export function createRefreshTokenRouter(deps: RefreshTokenRouterDeps = {}): Rou
       deps.rateLimiter,
     );
 
+  const idempotencyHandler: RequestHandler =
+    deps.idempotencyMiddleware ??
+    ((req, res, next) =>
+      idempotencyMiddleware(req, res, next, {
+        methods: ['POST', 'PATCH'],
+      }));
+
+  router.use(idempotencyHandler);
+
   /**
    * GET /api/refresh-token
    *