diff --git a/app/api/auth/token/__tests__/route.test.ts b/app/api/auth/token/__tests__/route.test.ts
new file mode 100644
index 0000000..fc3c3aa
--- /dev/null
+++ b/app/api/auth/token/__tests__/route.test.ts
@@ -0,0 +1,122 @@
+import { describe, it, expect, beforeEach, vi } from 'vitest';
+
+vi.mock('@/lib/logger', () => ({
+  logger: { error: vi.fn(), warn: vi.fn(), info: vi.fn() },
+}));
+
+const discoverOAuth = vi.fn();
+vi.mock('@/lib/oauth/discovery', () => ({
+  discoverOAuth: (...args: unknown[]) => discoverOAuth(...args),
+}));
+
+const cookieStore = {
+  jar: new Map<string, string>(),
+  get(name: string) {
+    const value = this.jar.get(name);
+    return value === undefined ? undefined : { name, value };
+  },
+  set: vi.fn(function (this: typeof cookieStore, name: string, value: string) {
+    this.jar.set(name, value);
+  }),
+  delete: vi.fn(function (this: typeof cookieStore, name: string) {
+    this.jar.delete(name);
+  }),
+};
+
+vi.mock('next/headers', () => ({
+  cookies: async () => cookieStore,
+}));
+
+const METADATA = {
+  token_endpoint: 'https://idp.example/token',
+  revocation_endpoint: 'https://idp.example/revoke',
+  end_session_endpoint: 'https://idp.example/logout',
+};
+
+async function loadRoute() {
+  vi.resetModules();
+  return import('../route');
+}
+
+describe('OIDC token route', () => {
+  beforeEach(() => {
+    process.env.OAUTH_CLIENT_ID = 'webmail-client';
+    process.env.JMAP_SERVER_URL = 'https://mail.example';
+    cookieStore.jar.clear();
+    cookieStore.set.mockClear();
+    cookieStore.delete.mockClear();
+    discoverOAuth.mockReset();
+    discoverOAuth.mockResolvedValue(METADATA);
+    vi.stubGlobal(
+      'fetch',
+      vi.fn(async () => ({
+        ok: true,
+        json: async () => ({
+          access_token: 'at-1',
+          refresh_token: 'rt-1',
+          id_token: 'idt-1',
+          expires_in: 3600,
+        }),
+        text: async () => '',
+      })),
+    );
+  });
+
+  it('stores the id_token in a cookie at code exchange', async () => {
+    const { POST } = await loadRoute();
+    const request = {
+      json: async () => ({ code: 'c', code_verifier: 'v', redirect_uri: 'https://app/cb' }),
+    };
+
+    const response = await POST(request as never);
+
+    expect(response.status).toBe(200);
+    expect(cookieStore.jar.get('jmap_idt')).toBe('idt-1');
+  });
+
+  it('sends id_token_hint and client_id on the end-session URL at logout', async () => {
+    cookieStore.jar.set('jmap_rt', 'rt-1');
+    cookieStore.jar.set('jmap_idt', 'idt-1');
+    const { DELETE } = await loadRoute();
+
+    const response = await DELETE();
+    const body = await response.json();
+
+    const url = new URL(body.end_session_url);
+    expect(url.origin + url.pathname).toBe('https://idp.example/logout');
+    expect(url.searchParams.get('id_token_hint')).toBe('idt-1');
+    expect(url.searchParams.get('client_id')).toBe('webmail-client');
+    expect(cookieStore.jar.has('jmap_idt')).toBe(false);
+  });
+
+  it('falls back to client_id alone when no id_token was stored', async () => {
+    cookieStore.jar.set('jmap_rt', 'rt-1');
+    const { DELETE } = await loadRoute();
+
+    const response = await DELETE();
+    const body = await response.json();
+
+    const url = new URL(body.end_session_url);
+    expect(url.searchParams.get('client_id')).toBe('webmail-client');
+    expect(url.searchParams.get('id_token_hint')).toBeNull();
+  });
+
+  it('rotates the stored id_token on refresh', async () => {
+    cookieStore.jar.set('jmap_rt', 'rt-1');
+    cookieStore.jar.set('jmap_idt', 'idt-old');
+    vi.stubGlobal(
+      'fetch',
+      vi.fn(async () => ({
+        ok: true,
+        json: async () => ({ access_token: 'at-2', id_token: 'idt-2', expires_in: 3600 }),
+        text: async () => '',
+      })),
+    );
+    const { PUT } = await loadRoute();
+
+    const response = await PUT();
+
+    expect(response.status).toBe(200);
+    expect(cookieStore.jar.get('jmap_idt')).toBe('idt-2');
+  });
+});
diff --git a/app/api/auth/token/route.ts b/app/api/auth/token/route.ts
index f67a536..75c3f5a 100644
--- a/app/api/auth/token/route.ts
+++ b/app/api/auth/token/route.ts
@@ -2,7 +2,7 @@ import { NextRequest, NextResponse } from 'next/server';
 import { cookies } from 'next/headers';
 import { logger } from '@/lib/logger';
 import { discoverOAuth } from '@/lib/oauth/discovery';
-import { REFRESH_TOKEN_COOKIE } from '@/lib/oauth/tokens';
+import { ID_TOKEN_COOKIE, REFRESH_TOKEN_COOKIE } from '@/lib/oauth/tokens';
 
 const CLIENT_SECRET = process.env.OAUTH_CLIENT_SECRET || '';
 
@@ -92,9 +92,14 @@ export async function POST(request: NextRequest) {
       expires_in: tokens.expires_in || 3600,
     });
 
-    if (tokens.refresh_token) {
+    if (tokens.refresh_token || tokens.id_token) {
       const cookieStore = await cookies();
-      cookieStore.set(REFRESH_TOKEN_COOKIE, tokens.refresh_token, COOKIE_OPTIONS);
+      if (tokens.refresh_token) {
+        cookieStore.set(REFRESH_TOKEN_COOKIE, tokens.refresh_token, COOKIE_OPTIONS);
+      }
+      if (tokens.id_token) {
+        cookieStore.set(ID_TOKEN_COOKIE, tokens.id_token, COOKIE_OPTIONS);
+      }
     }
 
     return response;
@@ -144,6 +149,10 @@ export async function PUT() {
       cookieStore.set(REFRESH_TOKEN_COOKIE, tokens.refresh_token, COOKIE_OPTIONS);
     }
 
+    if (tokens.id_token) {
+      cookieStore.set(ID_TOKEN_COOKIE, tokens.id_token, COOKIE_OPTIONS);
+    }
+
     return NextResponse.json({
       access_token: tokens.access_token,
       expires_in: tokens.expires_in || 3600,
@@ -189,12 +198,23 @@ export async function DELETE() {
       cookieStore.delete(REFRESH_TOKEN_COOKIE);
     }
 
+    const idToken = cookieStore.get(ID_TOKEN_COOKIE)?.value;
+    if (idToken) {
+      cookieStore.delete(ID_TOKEN_COOKIE);
+    }
+
     let end_session_url: string | undefined;
     if (metadata?.end_session_endpoint) {
       try {
         const parsed = new URL(metadata.end_session_endpoint);
         if (parsed.protocol === 'https:') {
-          end_session_url = metadata.end_session_endpoint;
+          // RP-initiated logout: the OP requires id_token_hint or client_id
+          // whenever post_logout_redirect_uri is sent (Keycloak enforces this).
+          parsed.searchParams.set('client_id', getRequiredConfig().clientId);
+          if (idToken) {
+            parsed.searchParams.set('id_token_hint', idToken);
+          }
+          end_session_url = parsed.toString();
         } else {
           logger.warn('Ignoring non-HTTPS end_session_endpoint', { url: metadata.end_session_endpoint });
         }
diff --git a/lib/oauth/tokens.ts b/lib/oauth/tokens.ts
index f91df7c..fbf98d3 100644
--- a/lib/oauth/tokens.ts
+++ b/lib/oauth/tokens.ts
@@ -1,2 +1,3 @@
 export const OAUTH_SCOPES = 'openid email profile';
 export const REFRESH_TOKEN_COOKIE = 'jmap_rt';
+export const ID_TOKEN_COOKIE = 'jmap_idt';