diff --git a/backend/src/diagnostics.test.ts b/backend/src/diagnostics.test.ts
index 23314a7..4e7be03 100644
--- a/backend/src/diagnostics.test.ts
+++ b/backend/src/diagnostics.test.ts
@@ -1,27 +1,16 @@
 import { describe, it, expect, beforeAll, afterAll } from 'vitest';
 import Fastify from 'fastify';
 import type { FastifyInstance } from 'fastify';
-import { getAllFlags } from './lib/feature-flags.js';
-import { getBufferedEvents, flushEvents } from './lib/telemetry.js';
+import { SignJWT } from 'jose';
 import { config } from './lib/config.js';
-import { PRODUCT_ID, productConfig } from './lib/product-config.js';
+import { productConfig } from './lib/product-config.js';
+import { assertDiagnosticsAccess, diagnosticsRoutes } from './lib/diagnostics-routes.js';
 
 let app: FastifyInstance;
 
 beforeAll(async () => {
   app = Fastify({ logger: false });
-  app.get('/api/diagnostics/flags', async () => getAllFlags());
-  app.get('/api/diagnostics/telemetry', async () => ({ events: getBufferedEvents() }));
-  app.post('/api/diagnostics/telemetry/flush', async () => ({ flushed: flushEvents().length }));
-  app.get('/api/diagnostics/config', async () => ({
-    productId: PRODUCT_ID,
-    serviceName: config.SERVICE_NAME,
-    port: config.PORT,
-    nodeEnv: config.NODE_ENV,
-    dbProvider: config.DB_PROVIDER,
-    telemetryEnabled: config.TELEMETRY_ENABLED,
-    featureFlagsEnabled: config.FEATURE_FLAGS_ENABLED,
-  }));
+  await diagnosticsRoutes(app);
   app.get('/health', async (req) => ({
     status: 'ok',
     service: config.SERVICE_NAME,
@@ -41,6 +30,18 @@ afterAll(async () => {
   await app.close();
 });
 
+function makeToken(role: string) {
+  return new SignJWT({
+    sub: 'diagnostics-user',
+    role,
+    type: 'access',
+  })
+    .setProtectedHeader({ alg: 'HS256' })
+    .setIssuedAt()
+    .setExpirationTime('5m')
+    .sign(new TextEncoder().encode(config.JWT_SECRET));
+}
+
 describe('diagnostics routes', () => {
   it('GET /api/diagnostics/flags returns feature flags', async () => {
     const res = await app.inject({ method: 'GET', url: '/api/diagnostics/flags' });
@@ -101,4 +102,29 @@ describe('diagnostics routes', () => {
     expect(typeof body.displayName).toBe('string');
     expect(typeof body.backendPort).toBe('number');
   });
+
+  it('keeps diagnostics open outside production for local smoke ergonomics', async () => {
+    await expect(assertDiagnosticsAccess({ headers: {} }, 'development')).resolves.toBeUndefined();
+    await expect(assertDiagnosticsAccess({ headers: {} }, 'test')).resolves.toBeUndefined();
+  });
+
+  it('requires auth for diagnostics in production', async () => {
+    await expect(assertDiagnosticsAccess({ headers: {} }, 'production')).rejects.toThrow('Unauthorized');
+  });
+
+  it('allows production diagnostics for admin or owner roles only', async () => {
+    const adminToken = await makeToken('admin');
+    const ownerToken = await makeToken('owner');
+    const viewerToken = await makeToken('viewer');
+
+    await expect(
+      assertDiagnosticsAccess({ headers: { authorization: `Bearer ${adminToken}` } }, 'production')
+    ).resolves.toBeUndefined();
+    await expect(
+      assertDiagnosticsAccess({ headers: { authorization: `Bearer ${ownerToken}` } }, 'production')
+    ).resolves.toBeUndefined();
+    await expect(
+      assertDiagnosticsAccess({ headers: { authorization: `Bearer ${viewerToken}` } }, 'production')
+    ).rejects.toThrow('Insufficient permissions');
+  });
 });
diff --git a/backend/src/lib/diagnostics-routes.ts b/backend/src/lib/diagnostics-routes.ts
new file mode 100644
index 0000000..2706208
--- /dev/null
+++ b/backend/src/lib/diagnostics-routes.ts
@@ -0,0 +1,53 @@
+import { requireRole } from './auth.js';
+import { config } from './config.js';
+import { getAllFlags } from './feature-flags.js';
+import { PRODUCT_ID } from './product-config.js';
+import { flushEvents, getBufferedEvents } from './telemetry.js';
+
+type AuthLikeRequest = { headers: { authorization?: string } };
+type DiagnosticsHandler = (req: AuthLikeRequest) => Promise<unknown>;
+type DiagnosticsRouteApp = {
+  get: (path: string, handler: DiagnosticsHandler) => unknown;
+  post: (path: string, handler: DiagnosticsHandler) => unknown;
+};
+
+export async function assertDiagnosticsAccess(
+  req: AuthLikeRequest,
+  nodeEnv: string = config.NODE_ENV,
+): Promise<void> {
+  if (nodeEnv !== 'production') {
+    return;
+  }
+
+  await requireRole(req, 'admin', 'owner');
+}
+
+export async function diagnosticsRoutes(app: DiagnosticsRouteApp) {
+  app.get('/api/diagnostics/flags', async req => {
+    await assertDiagnosticsAccess(req);
+    return getAllFlags();
+  });
+
+  app.get('/api/diagnostics/telemetry', async req => {
+    await assertDiagnosticsAccess(req);
+    return { events: getBufferedEvents() };
+  });
+
+  app.post('/api/diagnostics/telemetry/flush', async req => {
+    await assertDiagnosticsAccess(req);
+    return { flushed: flushEvents().length };
+  });
+
+  app.get('/api/diagnostics/config', async req => {
+    await assertDiagnosticsAccess(req);
+    return {
+      productId: PRODUCT_ID,
+      serviceName: config.SERVICE_NAME,
+      port: config.PORT,
+      nodeEnv: config.NODE_ENV,
+      dbProvider: config.DB_PROVIDER,
+      telemetryEnabled: config.TELEMETRY_ENABLED,
+      featureFlagsEnabled: config.FEATURE_FLAGS_ENABLED,
+    };
+  });
+}
diff --git a/backend/src/server.test.ts b/backend/src/server.test.ts
index cc4c615..eeed50f 100644
--- a/backend/src/server.test.ts
+++ b/backend/src/server.test.ts
@@ -5,6 +5,7 @@ const registerOptionalJwtContextMock = vi.fn(async () => undefined);
 const startServiceMock = vi.fn(async () => undefined);
 const initCosmosIfNeededMock = vi.fn(async () => undefined);
 const initDatastoreMock = vi.fn(() => undefined);
+const diagnosticsRoutesMock = vi.fn(async () => undefined);
 
 const appMock = {
   register: vi.fn(async () => undefined),
@@ -61,6 +62,7 @@ vi.mock('./lib/field-encrypt.js', () => ({ initEncryption: vi.fn(async () => und
 vi.mock('./lib/request-context.js', () => ({ getUserId: vi.fn(), getRequestProductId: vi.fn() }));
 vi.mock('./lib/feature-flags.js', () => ({ getAllFlags: vi.fn(() => ({})) }));
 vi.mock('./lib/telemetry.js', () => ({ getBufferedEvents: vi.fn(() => []), flushEvents: vi.fn(() => []) }));
+vi.mock('./lib/diagnostics-routes.js', () => ({ diagnosticsRoutes: diagnosticsRoutesMock }));
 vi.mock('./modules/note-shares/repository.js', () => ({ findShareByToken: vi.fn(async () => null) }));
 vi.mock('./modules/notes/repository.js', () => ({ getNote: vi.fn(async () => null) }));
 
@@ -81,6 +83,7 @@ describe('server bootstrap', () => {
     expect(createServiceAppMock).toHaveBeenCalledOnce();
     expect(registerOptionalJwtContextMock).toHaveBeenCalledOnce();
     expect(appMock.register).toHaveBeenCalledTimes(14);
+    expect(diagnosticsRoutesMock).toHaveBeenCalledWith(appMock);
     expect(startServiceMock).toHaveBeenCalledWith(appMock, { port: 4016, host: '0.0.0.0' });
   });
 });
diff --git a/backend/src/server.ts b/backend/src/server.ts
index bcfea1a..bb30f6e 100644
--- a/backend/src/server.ts
+++ b/backend/src/server.ts
@@ -19,9 +19,8 @@ import { initEncryption } from './lib/field-encrypt.js';
 import { initWebhookSubscriber, stopWebhookSubscriber } from './lib/webhook-subscriber.js';
 import { initDatastore } from './lib/datastore.js';
 import { config } from './lib/config.js';
-import { getAllFlags } from './lib/feature-flags.js';
-import { getBufferedEvents, flushEvents } from './lib/telemetry.js';
 import { DISPLAY_NAME, PRODUCT_ID, productConfig } from './lib/product-config.js';
+import { diagnosticsRoutes } from './lib/diagnostics-routes.js';
 import type { JwtPayload } from './lib/request-context.js';
 import { findShareByToken } from './modules/note-shares/repository.js';
 import * as noteRepo from './modules/notes/repository.js';
@@ -108,19 +107,8 @@ app.get('/api/bootstrap', async () => ({
   backendPort: config.PORT,
 }));
 
-// ── Diagnostics routes (no auth) ────────────────────────────────
-app.get('/api/diagnostics/flags', async () => getAllFlags());
-app.get('/api/diagnostics/telemetry', async () => ({ events: getBufferedEvents() }));
-app.post('/api/diagnostics/telemetry/flush', async () => ({ flushed: flushEvents().length }));
-app.get('/api/diagnostics/config', async () => ({
-  productId: PRODUCT_ID,
-  serviceName: config.SERVICE_NAME,
-  port: config.PORT,
-  nodeEnv: config.NODE_ENV,
-  dbProvider: config.DB_PROVIDER,
-  telemetryEnabled: config.TELEMETRY_ENABLED,
-  featureFlagsEnabled: config.FEATURE_FLAGS_ENABLED,
-}));
+// ── Diagnostics routes (dev/test open, production admin/owner gated) ───────
+await diagnosticsRoutes(app);
 
 await initEncryption(PRODUCT_ID, app.log);