diff --git a/apps/api/tests/routes/admin.test.ts b/apps/api/tests/routes/admin.test.ts
index 9c4bfd9..9bd1062 100644
--- a/apps/api/tests/routes/admin.test.ts
+++ b/apps/api/tests/routes/admin.test.ts
@@ -1071,3 +1071,182 @@ describe('POST /api/admin/members', () => {
     expect(parsed.error).toBe('Invalid request');
   });
 });
+
+// ===========================================================================
+// PATCH /api/admin/members/:id — member-profile update + last-admin guard (Plan 20-01)
+// ===========================================================================
+
+describe('PATCH /api/admin/members/:id', () => {
+  // Test A: happy path — update displayName only
+  it('Test A (happy path displayName): PATCH with { displayName } as admin returns 200; GET reflects new name', async () => {
+    const adminId = await seedUser('admin-patch-name', true);
+    const memberId = await seedUser('member-patch-target', false);
+    currentDevUserId = adminId;
+    const app = await getApp();
+
+    const res = await app.fetch(
+      jsonRequest('PATCH', `/api/admin/members/${memberId}`, { displayName: 'New Name' }),
+    );
+    expect(res.status).toBe(200);
+    const body = (await res.json()) as { ok: boolean };
+    expect(body.ok).toBe(true);
+
+    // GET /members should reflect the updated displayName
+    const getRes = await app.fetch(jsonRequest('GET', '/api/admin/members'));
+    expect(getRes.status).toBe(200);
+    const getBody = (await getRes.json()) as { members: Array<{ id: number; displayName: string }> };
+    const updated = getBody.members.find((m) => m.id === memberId);
+    expect(updated).toBeDefined();
+    expect(updated!.displayName).toBe('New Name');
+  });
+
+  // Test B: happy path — promote non-admin to admin
+  it('Test B (happy path isAdmin promote): PATCH with { isAdmin: true } returns 200; GET shows isAdmin true', async () => {
+    const adminId = await seedUser('admin-patch-promote', true);
+    const memberId = await seedUser('member-patch-promote', false);
+    currentDevUserId = adminId;
+    const app = await getApp();
+
+    const res = await app.fetch(
+      jsonRequest('PATCH', `/api/admin/members/${memberId}`, { isAdmin: true }),
+    );
+    expect(res.status).toBe(200);
+
+    const getRes = await app.fetch(jsonRequest('GET', '/api/admin/members'));
+    expect(getRes.status).toBe(200);
+    const getBody = (await getRes.json()) as {
+      members: Array<{ id: number; isAdmin: boolean }>;
+    };
+    const promoted = getBody.members.find((m) => m.id === memberId);
+    expect(promoted).toBeDefined();
+    expect(promoted!.isAdmin).toBe(true);
+  });
+
+  // Test C: last-admin guard — only admin cannot demote themselves
+  it('Test C (last-admin guard): with exactly one admin, PATCH { isAdmin: false } returns 409; member stays admin', async () => {
+    const adminId = await seedUser('admin-last-admin', true);
+    currentDevUserId = adminId;
+    const app = await getApp();
+
+    const res = await app.fetch(
+      jsonRequest('PATCH', `/api/admin/members/${adminId}`, { isAdmin: false }),
+    );
+    expect(res.status).toBe(409);
+    const body = (await res.json()) as { error: string };
+    expect(typeof body.error).toBe('string');
+    expect(body.error.length).toBeGreaterThan(0);
+
+    // The admin flag must still be true after the rejected demotion
+    const getRes = await app.fetch(jsonRequest('GET', '/api/admin/members'));
+    expect(getRes.status).toBe(200);
+    const getBody = (await getRes.json()) as {
+      members: Array<{ id: number; isAdmin: boolean }>;
+    };
+    const adminRow = getBody.members.find((m) => m.id === adminId);
+    expect(adminRow).toBeDefined();
+    expect(adminRow!.isAdmin).toBe(true);
+  });
+
+  // Test D: self-demotion allowed when another admin exists
+  it('Test D (self-demotion allowed): with two admins, PATCH { isAdmin: false } returns 200; one admin remains', async () => {
+    const adminId1 = await seedUser('admin-demote-1', true);
+    const adminId2 = await seedUser('admin-demote-2', true);
+    currentDevUserId = adminId1;
+    const app = await getApp();
+
+    const res = await app.fetch(
+      jsonRequest('PATCH', `/api/admin/members/${adminId1}`, { isAdmin: false }),
+    );
+    expect(res.status).toBe(200);
+
+    // Only adminId2 should remain as admin
+    const getRes = await app.fetch(jsonRequest('GET', '/api/admin/members'));
+    expect(getRes.status).toBe(200);
+    const getBody = (await getRes.json()) as {
+      members: Array<{ id: number; isAdmin: boolean }>;
+    };
+    const row1 = getBody.members.find((m) => m.id === adminId1);
+    const row2 = getBody.members.find((m) => m.id === adminId2);
+    expect(row1!.isAdmin).toBe(false);
+    expect(row2!.isAdmin).toBe(true);
+  });
+
+  // Test E: auth boundary — non-admin gets 403
+  it('Test E (auth boundary): non-admin PATCH returns 403', async () => {
+    const adminId = await seedUser('admin-patch-auth', true);
+    const nonAdminId = await seedUser('non-admin-patch', false);
+    currentDevUserId = nonAdminId;
+    const app = await getApp();
+
+    const res = await app.fetch(
+      jsonRequest('PATCH', `/api/admin/members/${adminId}`, { displayName: 'Hacked' }),
+    );
+    expect(res.status).toBe(403);
+  });
+
+  // Test F: validation — wrong type and malformed id
+  it('Test F (validation): PATCH with { isAdmin: "yes" } returns 400 { error: "Invalid request" }', async () => {
+    const adminId = await seedUser('admin-patch-validation', true);
+    const memberId = await seedUser('member-patch-validation', false);
+    currentDevUserId = adminId;
+    const app = await getApp();
+
+    const res = await app.fetch(
+      jsonRequest('PATCH', `/api/admin/members/${memberId}`, { isAdmin: 'yes' }),
+    );
+    expect(res.status).toBe(400);
+    const body = (await res.json()) as { error: string };
+    expect(body.error).toBe('Invalid request');
+  });
+
+  it('Test F (malformed id): PATCH with malformed :id (e.g. "1abc") returns 400', async () => {
+    const adminId = await seedUser('admin-patch-badid', true);
+    currentDevUserId = adminId;
+    const app = await getApp();
+
+    const res = await app.fetch(
+      jsonRequest('PATCH', '/api/admin/members/1abc', { displayName: 'Test' }),
+    );
+    expect(res.status).toBe(400);
+  });
+
+  // Test G: not found — non-existent member id
+  it('Test G (not found): PATCH non-existent member id returns 404', async () => {
+    const adminId = await seedUser('admin-patch-notfound', true);
+    currentDevUserId = adminId;
+    const app = await getApp();
+
+    const res = await app.fetch(
+      jsonRequest('PATCH', '/api/admin/members/99999999', { displayName: 'Ghost' }),
+    );
+    expect(res.status).toBe(404);
+  });
+});
+
+// ===========================================================================
+// GET /api/admin/members — isAdmin field (Plan 20-01)
+// ===========================================================================
+
+describe('GET /api/admin/members — isAdmin field', () => {
+  it('Test H (GET isAdmin field): each member object includes a boolean isAdmin field', async () => {
+    const adminId = await seedUser('admin-isadmin-field', true);
+    const memberId = await seedUser('member-isadmin-field', false);
+    currentDevUserId = adminId;
+    const app = await getApp();
+
+    const res = await app.fetch(jsonRequest('GET', '/api/admin/members'));
+    expect(res.status).toBe(200);
+    const body = (await res.json()) as {
+      members: Array<{ id: number; isAdmin: boolean }>;
+    };
+    // Both seeded users should have a boolean isAdmin field
+    const adminRow = body.members.find((m) => m.id === adminId);
+    const memberRow = body.members.find((m) => m.id === memberId);
+    expect(adminRow).toBeDefined();
+    expect(typeof adminRow!.isAdmin).toBe('boolean');
+    expect(adminRow!.isAdmin).toBe(true);
+    expect(memberRow).toBeDefined();
+    expect(typeof memberRow!.isAdmin).toBe('boolean');
+    expect(memberRow!.isAdmin).toBe(false);
+  });
+});