diff --git a/apps/pwa/src/api/client.test.ts b/apps/pwa/src/api/client.test.ts
index c4cae79..c370188 100644
--- a/apps/pwa/src/api/client.test.ts
+++ b/apps/pwa/src/api/client.test.ts
@@ -684,3 +684,142 @@ describe('fetchAdminResetPassword — URL contract (Phase 19, AUTH-LOCAL-08)', (
     );
   });
 });
+
+// ── Phase 20 (Plan 20-02): updateMemberProfile + AdminMember.isAdmin ─────────
+// TDD RED: these tests MUST fail before the implementation is added to client.ts.
+
+describe('updateMemberProfile — URL + verb contract (Phase 20, Plan 20-02)', () => {
+  beforeEach(() => {
+    vi.stubGlobal('fetch', vi.fn());
+  });
+  afterEach(() => {
+    vi.unstubAllGlobals();
+  });
+
+  it('PATCHes /api/admin/members/:id (must match PATCH /members/:id in admin.ts)', async () => {
+    vi.mocked(fetch).mockResolvedValueOnce({
+      ok: true,
+      type: 'basic',
+      status: 200,
+    } as unknown as Response);
+
+    const { updateMemberProfile } = await import('./client.js');
+    await updateMemberProfile(7, { displayName: 'Alice' });
+
+    expect(fetch).toHaveBeenCalledWith(
+      '/api/admin/members/7',
+      expect.objectContaining({ method: 'PATCH' }),
+    );
+  });
+
+  it('sends credentials:include and redirect:manual', async () => {
+    vi.mocked(fetch).mockResolvedValueOnce({
+      ok: true,
+      type: 'basic',
+      status: 200,
+    } as unknown as Response);
+
+    const { updateMemberProfile } = await import('./client.js');
+    await updateMemberProfile(3, { isAdmin: true });
+
+    expect(fetch).toHaveBeenCalledWith(
+      '/api/admin/members/3',
+      expect.objectContaining({
+        credentials: 'include',
+        redirect: 'manual',
+      }),
+    );
+  });
+
+  it('resolves void on 200', async () => {
+    vi.mocked(fetch).mockResolvedValueOnce({
+      ok: true,
+      type: 'basic',
+      status: 200,
+    } as unknown as Response);
+
+    const { updateMemberProfile } = await import('./client.js');
+    const result = await updateMemberProfile(7, { displayName: 'Bob' });
+    expect(result).toBeUndefined();
+  });
+
+  it('throws SessionExpiredError on opaqueredirect', async () => {
+    vi.mocked(fetch).mockResolvedValueOnce({
+      ok: false,
+      type: 'opaqueredirect',
+      status: 0,
+    } as unknown as Response);
+
+    const { updateMemberProfile, SessionExpiredError } = await import('./client.js');
+    await expect(updateMemberProfile(7, { displayName: 'X' })).rejects.toBeInstanceOf(
+      SessionExpiredError,
+    );
+  });
+
+  it('throws SessionExpiredError on 401', async () => {
+    vi.mocked(fetch).mockResolvedValueOnce({
+      ok: false,
+      type: 'basic',
+      status: 401,
+    } as unknown as Response);
+
+    const { updateMemberProfile, SessionExpiredError } = await import('./client.js');
+    await expect(updateMemberProfile(7, { displayName: 'X' })).rejects.toBeInstanceOf(
+      SessionExpiredError,
+    );
+  });
+
+  it('throws Error("last-admin") on 409 (last-admin demotion sentinel)', async () => {
+    vi.mocked(fetch).mockResolvedValueOnce({
+      ok: false,
+      type: 'basic',
+      status: 409,
+    } as unknown as Response);
+
+    const { updateMemberProfile } = await import('./client.js');
+    await expect(updateMemberProfile(7, { isAdmin: false })).rejects.toThrow('last-admin');
+  });
+
+  it('throws Error("last-admin") on 422 as well (server may return either)', async () => {
+    vi.mocked(fetch).mockResolvedValueOnce({
+      ok: false,
+      type: 'basic',
+      status: 422,
+    } as unknown as Response);
+
+    const { updateMemberProfile } = await import('./client.js');
+    await expect(updateMemberProfile(7, { isAdmin: false })).rejects.toThrow('last-admin');
+  });
+
+  it('throws a generic error on any other non-ok status (not last-admin sentinel)', async () => {
+    vi.mocked(fetch).mockResolvedValueOnce({
+      ok: false,
+      type: 'basic',
+      status: 500,
+    } as unknown as Response);
+
+    const { updateMemberProfile, SessionExpiredError } = await import('./client.js');
+    await expect(updateMemberProfile(7, { displayName: 'X' })).rejects.toSatisfy(
+      (e: unknown) =>
+        e instanceof Error &&
+        !(e instanceof SessionExpiredError) &&
+        e.message !== 'last-admin',
+    );
+  });
+});
+
+describe('AdminMember.isAdmin field (Phase 20, Plan 20-02)', () => {
+  it('AdminMember interface has isAdmin: boolean (compile-time type check via runtime shape)', async () => {
+    // Construct a conforming object — TypeScript will error at compile time if
+    // isAdmin is missing from the AdminMember interface (caught by tsc --noEmit).
+    const member: import('./client.js').AdminMember = {
+      id: 1,
+      displayName: 'Test',
+      color: '#abc',
+      isAdmin: true,
+      hasCredential: false,
+      hasLocalCredential: false,
+    };
+    expect(member.isAdmin).toBe(true);
+  });
+});