diff --git a/lib/__tests__/verify-jmap-auth.test.ts b/lib/__tests__/verify-jmap-auth.test.ts
index efc8dfe8e..665bb27cd 100644
--- a/lib/__tests__/verify-jmap-auth.test.ts
+++ b/lib/__tests__/verify-jmap-auth.test.ts
@@ -173,3 +173,31 @@ describe('verifyJmapAuth SSRF protection', () => {
     expect(fetchSpy).not.toHaveBeenCalled();
   });
 });
+
+describe('normalizeJmapServerUrl', () => {
+  async function load() {
+    return import('@/lib/auth/verify-jmap-auth');
+  }
+
+  it('accepts an app-relative URL (dev mock JMAP server)', async () => {
+    const { normalizeJmapServerUrl } = await load();
+    expect(normalizeJmapServerUrl('/api/dev-jmap')).toBe('/api/dev-jmap');
+  });
+
+  it('strips query, hash, and trailing slashes from an app-relative URL', async () => {
+    const { normalizeJmapServerUrl } = await load();
+    expect(normalizeJmapServerUrl('/api/dev-jmap/?x=1#frag')).toBe('/api/dev-jmap');
+  });
+
+  it('rejects protocol-relative URLs', async () => {
+    const { normalizeJmapServerUrl } = await load();
+    expect(() => normalizeJmapServerUrl('//evil.example.com/api')).toThrow('Invalid server URL');
+  });
+
+  it('normalizes absolute URLs unchanged', async () => {
+    const { normalizeJmapServerUrl } = await load();
+    expect(normalizeJmapServerUrl('https://mail.example.com/jmap/?a=b')).toBe(
+      'https://mail.example.com/jmap',
+    );
+  });
+});
diff --git a/lib/auth/verify-jmap-auth.ts b/lib/auth/verify-jmap-auth.ts
index 06a557081..f7391c355 100644
--- a/lib/auth/verify-jmap-auth.ts
+++ b/lib/auth/verify-jmap-auth.ts
@@ -18,6 +18,15 @@ function isSupportedProtocol(protocol: string): boolean {
 }
 
 export function normalizeJmapServerUrl(serverUrl: string): string {
+  // App-relative URLs (the dev mock JMAP server, e.g. /api/dev-jmap) have no
+  // host or protocol to validate; they can only ever target this app itself.
+  // Protocol-relative URLs (//host/...) are NOT app-relative and fall through
+  // to the absolute-URL parse below, which rejects them.
+  if (serverUrl.startsWith('/') && !serverUrl.startsWith('//')) {
+    const url = new URL(serverUrl, 'http://relative.invalid');
+    return url.pathname.replace(/\/+$/, '');
+  }
+
   let url: URL;
   try {
     url = new URL(serverUrl);