diff --git a/lib/__tests__/jmap-lite-credentials.test.ts b/lib/__tests__/jmap-lite-credentials.test.ts
new file mode 100644
index 000000000..c14cc19ad
--- /dev/null
+++ b/lib/__tests__/jmap-lite-credentials.test.ts
@@ -0,0 +1,105 @@
+import { describe, it, expect, vi, beforeAll, beforeEach, afterEach } from 'vitest';
+
+/**
+ * #1396: in the Lite bundle served by Stalwart, JMAP requests share the page's
+ * origin, so a 401 with Stalwart's `WWW-Authenticate: Basic` challenge made
+ * Safari show its login dialog unless credentials are omitted. The regular
+ * build must keep sending cookies (cookie-gated proxies in front of JMAP).
+ */
+
+const session = {
+  capabilities: { 'urn:ietf:params:jmap:core': {} },
+  accounts: { 'acct-1': { name: 'test', isPersonal: true, accountCapabilities: {} } },
+  primaryAccounts: { 'urn:ietf:params:jmap:mail': 'acct-1' },
+  apiUrl: 'https://mail.example.com/jmap/api',
+  downloadUrl: 'https://mail.example.com/jmap/download/{accountId}/{blobId}/{name}',
+  uploadUrl: 'https://mail.example.com/jmap/upload/{accountId}/',
+  eventSourceUrl: 'https://mail.example.com/jmap/eventsource?types={types}&closeafter={closeafter}&ping={ping}',
+};
+
+function json(status: number, body?: unknown): Response {
+  return new Response(body ? JSON.stringify(body) : null, {
+    status,
+    headers: { 'Content-Type': 'application/json' },
+  });
+}
+
+type Client = typeof import('../jmap/client').JMAPClient;
+
+// The build target is read once at import, so each mode needs a fresh module.
+// A cold import of the client takes a few seconds, hence the hook timeout.
+const IMPORT_TIMEOUT = 30_000;
+const clients: Partial<Record<'lite' | 'regular', Client>> = {};
+
+async function loadClient(liteTarget: 'stalwart' | null): Promise<Client> {
+  vi.resetModules();
+  vi.stubEnv('NEXT_PUBLIC_BULWARK_LITE', liteTarget ? '1' : '');
+  vi.stubEnv('NEXT_PUBLIC_LITE_TARGET', liteTarget ?? '');
+  const { JMAPClient } = await import('../jmap/client');
+  vi.unstubAllEnvs();
+  return JMAPClient;
+}
+
+describe('JMAP request credentials mode', () => {
+  let fetchSpy: ReturnType<typeof vi.spyOn>;
+
+  beforeAll(async () => {
+    clients.lite = await loadClient('stalwart');
+    clients.regular = await loadClient(null);
+  }, IMPORT_TIMEOUT);
+
+  beforeEach(() => {
+    fetchSpy = vi.spyOn(globalThis, 'fetch').mockImplementation((url) =>
+      Promise.reject(new Error(`Unmocked fetch: ${String(url)}`)));
+  });
+
+  afterEach(() => {
+    fetchSpy.mockRestore();
+  });
+
+  it('omits credentials on the session fetch, the request and its refresh retry in Lite on Stalwart', async () => {
+    const JMAPClient = clients.lite!;
+    const refresh = vi.fn().mockResolvedValue('fresh-token');
+    fetchSpy.mockResolvedValueOnce(json(200, session));
+    const client = JMAPClient.withBearer('https://mail.example.com', 'expired', 'user@test.com', refresh);
+    await client.connect();
+
+    fetchSpy
+      .mockResolvedValueOnce(json(401))
+      .mockResolvedValueOnce(json(200, { methodResponses: [['Core/echo', {}, '0']] }));
+    await client.ping();
+    client.disconnect();
+
+    expect(refresh).toHaveBeenCalledOnce();
+    expect(fetchSpy).toHaveBeenCalledTimes(3);
+    for (const [, init] of fetchSpy.mock.calls) {
+      expect((init as RequestInit).credentials).toBe('omit');
+    }
+  });
+
+  it('keeps an explicit credentials mode chosen by the caller', async () => {
+    const JMAPClient = clients.lite!;
+    fetchSpy.mockResolvedValueOnce(json(200, session));
+    const client = JMAPClient.withBearer('https://mail.example.com', 'token', 'user@test.com');
+    await client.connect();
+
+    fetchSpy.mockResolvedValueOnce(json(200, {}));
+    // authenticatedFetch is private; reach it the way the class's own callers do.
+    await (client as unknown as {
+      authenticatedFetch(url: string, init: RequestInit): Promise<Response>;
+    }).authenticatedFetch('https://mail.example.com/jmap/api', { credentials: 'include' });
+    client.disconnect();
+
+    expect((fetchSpy.mock.calls[1][1] as RequestInit).credentials).toBe('include');
+  });
+
+  it('leaves the browser default in the regular build', async () => {
+    const JMAPClient = clients.regular!;
+    fetchSpy.mockResolvedValueOnce(json(200, session));
+    const client = JMAPClient.withBearer('https://mail.example.com', 'token', 'user@test.com');
+    await client.connect();
+    client.disconnect();
+
+    expect((fetchSpy.mock.calls[0][1] as RequestInit).credentials).toBeUndefined();
+  });
+});
diff --git a/lib/jmap/client.ts b/lib/jmap/client.ts
index 43aaaf67c..1ba980a10 100644
--- a/lib/jmap/client.ts
+++ b/lib/jmap/client.ts
@@ -40,6 +40,7 @@ import type { AccountInfo } from "./types";
 import { stalwartChangeId } from "./stalwart-state";
 import { downloadNameSegment } from "./download-name";
 import { toInertBlob } from "@/lib/file-preview";
+import { IS_LITE_STALWART } from "@/lib/lite";
 import { toStorageQuota, type JMAPQuota, type StorageQuota } from "@/lib/quota";
 import { messagePriorityHeaders, type MessagePriority } from "@/lib/message-priority";
 
@@ -212,6 +213,25 @@ export class RequestTimeoutError extends Error {
   }
 }
 
+/**
+ * Credentials mode for the client's own JMAP requests, which always carry
+ * their own Authorization header.
+ *
+ * In the Lite bundle served by Stalwart the page and JMAP share an origin, so
+ * the default (`same-origin`) lets the browser act on Stalwart's
+ * `WWW-Authenticate: Basic` challenge: an expired access token makes Safari
+ * show its native login dialog before the 401 reaches the refresh-and-retry
+ * code. With credentials omitted the Fetch standard forbids that prompt
+ * (#1396). The regular build keeps the default: a same-origin JMAP server
+ * behind a cookie-gated proxy (Pangolin, Authelia) needs the cookie sent.
+ */
+const JMAP_FETCH_CREDENTIALS: RequestCredentials | undefined = IS_LITE_STALWART ? 'omit' : undefined;
+
+function withJmapCredentials(init: RequestInit | undefined): RequestInit {
+  if (!JMAP_FETCH_CREDENTIALS || init?.credentials) return { ...init };
+  return { ...init, credentials: JMAP_FETCH_CREDENTIALS };
+}
+
 const READ_ONLY_METHOD = /\/(?:get|query|changes|queryChanges|parse)$|^Core\/echo$/;
 
 /**
@@ -1684,7 +1704,7 @@ export class JMAPClient implements IJMAPClient {
     }, timeoutMs);
 
     try {
-      return await fetch(url, { ...init, headers, signal: controller.signal });
+      return await fetch(url, { ...withJmapCredentials(init), headers, signal: controller.signal });
     } catch (error) {
       if (timedOut) throw new RequestTimeoutError(timeoutMs);
       throw error;
@@ -1797,10 +1817,10 @@ export class JMAPClient implements IJMAPClient {
     // request with 401 instead of Stalwart's silent 200 - the credentials are
     // fine, Safari just dropped them. Retry the final URL directly. (#892)
     if (response.redirected && response.status === 401) {
-      return fetch(response.url, {
+      return fetch(response.url, withJmapCredentials({
         method: 'GET',
         headers: { 'Authorization': this.authHeader },
-      });
+      }));
     }
     if (!response.ok || !response.redirected) return response;
 
@@ -1809,10 +1829,10 @@ export class JMAPClient implements IJMAPClient {
     const hasUsername = typeof peek?.username === 'string' && peek.username.length > 0;
     if (hasAccounts || hasUsername) return response;
 
-    return fetch(response.url, {
+    return fetch(response.url, withJmapCredentials({
       method: 'GET',
       headers: { 'Authorization': this.authHeader },
-    });
+    }));
   }
 
   private async refreshSession(): Promise<void> {