diff --git a/.env.example b/.env.example
index 96487c8..5cf6e15 100644
--- a/.env.example
+++ b/.env.example
@@ -1,6 +1,9 @@
 # HTTP port (default: 3003)
 PORT=3003
 
+# Endorser API base URL (default: https://api.endorser.ch)
+# ENDORSER_URL=https://api.endorser.ch
+
 # Firebase Admin: inline service account JSON (one line).
 # If unset, uses Application Default Credentials (e.g. GOOGLE_APPLICATION_CREDENTIALS).
 # FIREBASE_SERVICE_ACCOUNT_JSON={"type":"service_account",...}
diff --git a/README.md b/README.md
index 03cde7e..5c956a9 100644
--- a/README.md
+++ b/README.md
@@ -6,9 +6,11 @@ A lightweight Express service that schedules and sends Firebase Cloud Messaging
 cp .env.example .env
 ```
 
-Edit .env — set FIREBASE_SERVICE_ACCOUNT_JSON
+Edit .env — set `FIREBASE_SERVICE_ACCOUNT_JSON`.
 Here is one way to generate the contents: `cat your-downloaded-key.json | jq -c .`
 
+Optionally set `ENDORSER_URL` if you are not using the default production Endorser API (`https://api.endorser.ch`).
+
 ```bash
 pnpm install
 pnpm run dev
@@ -16,7 +18,13 @@ pnpm run dev
 
 The server starts on `http://localhost:3003` (or the port in `PORT`). Hot-reloads on file changes.
 
-Set `NODE_ENV=test-local` in `.env` to bypass JWT expiry verification during local development.
+### Authentication
+
+`POST /notifications/register` and `POST /notifications/refresh` require a Bearer JWT. After local JWT verification, the service checks the token with Endorser (`GET /api/report/rateLimits` on `ENDORSER_URL`). Registration and refresh continue only if Endorser accepts the JWT.
+
+**Local notification test bypass:** send `testMode: true` in the JSON body and omit the `Authorization` header. The request skips JWT and Endorser checks and uses a synthetic local test user, same as before.
+
+Set `NODE_ENV=test-local` in `.env` to bypass ethr JWT *expiry* verification during local development (this is separate from the `testMode` bypass above).
 
 ## Production
 
@@ -40,4 +48,5 @@ Required environment variables:
 |---|---|
 | `FIREBASE_SERVICE_ACCOUNT_JSON` | Inline service account JSON (one line). If unset, falls back to Application Default Credentials. |
 | `PORT` | HTTP port (default: `3003`). |
+| `ENDORSER_URL` | Endorser API base URL used for auth checks on register/refresh (default: `https://api.endorser.ch`). |
 | `FCM_TOKEN_DATA_DIR` | Directory for persisting registered FCM tokens (default: `./data`). |
diff --git a/src/env.ts b/src/env.ts
index f4378ae..dc2d113 100644
--- a/src/env.ts
+++ b/src/env.ts
@@ -1,3 +1,7 @@
 import { config } from "dotenv";
 
 config();
+
+/** Base URL for the Endorser API server. */
+export const ENDORSER_URL =
+  process.env.ENDORSER_URL ?? "https://api.endorser.ch";
diff --git a/src/middleware/auth.ts b/src/middleware/auth.ts
index 919d099..db38442 100644
--- a/src/middleware/auth.ts
+++ b/src/middleware/auth.ts
@@ -1,4 +1,5 @@
 import type { NextFunction, Request, Response } from "express";
+import { checkAuth } from "../services/endorserClient.js";
 import { decodeAndVerifyJwt } from "../vc/index.js";
 
 export type AuthContext = {
@@ -115,3 +116,54 @@ export async function requireAuthOrNotificationLocalTest(
   }
   return requireAuth(req, res, next);
 }
+
+/**
+ * When req.jwt is set (normal auth), require Endorser acceptance.
+ * Local test bypass leaves req.jwt unset and is allowed through unchanged.
+ */
+export async function requireEndorserAuth(
+  req: Request,
+  res: Response,
+  next: NextFunction
+): Promise<void> {
+  const jwt = req.jwt;
+  if (jwt === undefined) {
+    next();
+    return;
+  }
+
+  const result = await checkAuth(jwt);
+  if (result.ok) {
+    next();
+    return;
+  }
+
+  const errorTime = new Date().toISOString();
+  const did = req.did ?? "(unknown)";
+
+  if (result.reason === "unavailable") {
+    console.log("[Auth] Endorser unavailable");
+    console.error(
+      "[Auth] Endorser auth check unavailable at",
+      errorTime + ", did:",
+      did
+    );
+    res.status(503).json({
+      success: false,
+      message:
+        "Authentication service unavailable. See server logs at " + errorTime,
+    });
+    return;
+  }
+
+  console.log("[Auth] Endorser verification failed");
+  console.error(
+    "[Auth] Endorser rejected JWT at",
+    errorTime + ", did:",
+    did
+  );
+  res.status(401).json({
+    success: false,
+    message: "Unauthorized. See server logs at " + errorTime,
+  });
+}
diff --git a/src/routes/notifications.ts b/src/routes/notifications.ts
index d95bf19..a2f3847 100644
--- a/src/routes/notifications.ts
+++ b/src/routes/notifications.ts
@@ -1,6 +1,9 @@
 import express, { Router } from "express";
 import { db } from "../db/fcmTokens.js";
-import { requireAuthOrNotificationLocalTest } from "../middleware/auth.js";
+import {
+  requireAuthOrNotificationLocalTest,
+  requireEndorserAuth,
+} from "../middleware/auth.js";
 import { errorMessage, formatElapsedMs } from "../util/formatElapsed.js";
 import { maskToken } from "../util/maskToken.js";
 
@@ -13,6 +16,7 @@ notificationsRouter.get("/", (_req, res) => {
 notificationsRouter.post(
   "/refresh",
   requireAuthOrNotificationLocalTest,
+  requireEndorserAuth,
   async (req, res) => {
     const started = Date.now();
     const userId = req.did;
@@ -87,6 +91,7 @@ notificationsRouter.post(
 notificationsRouter.post(
   "/register",
   requireAuthOrNotificationLocalTest,
+  requireEndorserAuth,
   async (req, res) => {
     const started = Date.now();
     const userId = req.did;
diff --git a/src/services/endorserClient.ts b/src/services/endorserClient.ts
new file mode 100644
index 0000000..2179bb0
--- /dev/null
+++ b/src/services/endorserClient.ts
@@ -0,0 +1,63 @@
+import { ENDORSER_URL } from "../env.js";
+import { errorMessage } from "../util/formatElapsed.js";
+
+const RATE_LIMITS_PATH = "/api/report/rateLimits";
+
+export type EndorserAuthResult =
+  | { ok: true }
+  | { ok: false; reason: "unauthorized" | "unavailable" };
+
+function rateLimitsUrl(): string {
+  const base = ENDORSER_URL.replace(/\/+$/, "");
+  return `${base}${RATE_LIMITS_PATH}`;
+}
+
+/**
+ * Confirms a JWT is accepted by the Endorser server.
+ *
+ * Calls GET /api/report/rateLimits with the JWT as a Bearer token.
+ * Distinguishes auth rejection from Endorser unavailability.
+ * Does not expose HTTP status or response bodies to callers.
+ */
+export async function checkAuth(jwt: string): Promise<EndorserAuthResult> {
+  const url = rateLimitsUrl();
+
+  let response: Response;
+  try {
+    response = await fetch(url, {
+      method: "GET",
+      headers: {
+        Authorization: `Bearer ${jwt}`,
+      },
+    });
+  } catch (err) {
+    console.error(
+      "[Endorser] Auth check request failed for",
+      url + ":",
+      errorMessage(err)
+    );
+    return { ok: false, reason: "unavailable" };
+  }
+
+  if (response.ok) {
+    return { ok: true };
+  }
+
+  // 5xx: Endorser is up but unhealthy; treat as unavailable.
+  if (response.status >= 500) {
+    console.error(
+      "[Endorser] Auth check unavailable for",
+      url + ", status",
+      response.status
+    );
+    return { ok: false, reason: "unavailable" };
+  }
+
+  // 4xx: JWT rejected or user not registered on Endorser.
+  console.error(
+    "[Endorser] Auth check rejected for",
+    url + ", status",
+    response.status
+  );
+  return { ok: false, reason: "unauthorized" };
+}