diff --git a/packages/server/src/utils/federationOutbox.systemType.test.ts b/packages/server/src/utils/federationOutbox.systemType.test.ts
new file mode 100644
index 00000000..4add98c5
--- /dev/null
+++ b/packages/server/src/utils/federationOutbox.systemType.test.ts
@@ -0,0 +1,38 @@
+import { describe, it, expect, vi } from 'vitest';
+
+vi.mock('../utils/federationAuth.js', () => ({
+  getOurOrigin: () => 'https://test.example',
+  buildFederationHeaders: () => ({}),
+  generateHmacSecret: () => 'secret',
+}));
+
+describe('buildRelayPayload — type field', () => {
+  const user = { id: 'u1', homeUserId: 'u1', homeInstance: 'https://x.example' };
+
+  it('omits type for user messages (backward-compat with old peers)', async () => {
+    const { buildRelayPayload } = await import('./federationOutbox.js');
+    const payload = buildRelayPayload(
+      { id: 'm1', type: 'user', content: 'hi', createdAt: 1 },
+      user,
+    );
+    expect((payload as any).type).toBeUndefined();
+  });
+
+  it('omits type when not set (defaults to user)', async () => {
+    const { buildRelayPayload } = await import('./federationOutbox.js');
+    const payload = buildRelayPayload(
+      { id: 'm1', content: 'hi', createdAt: 1 },
+      user,
+    );
+    expect((payload as any).type).toBeUndefined();
+  });
+
+  it('emits type for system messages', async () => {
+    const { buildRelayPayload } = await import('./federationOutbox.js');
+    const payload = buildRelayPayload(
+      { id: 'm1', type: 'system', content: '{"event":"space_invite"}', createdAt: 1 },
+      user,
+    );
+    expect((payload as any).type).toBe('system');
+  });
+});