diff --git a/src/route-contract.spec.ts b/src/route-contract.spec.ts
new file mode 100644
index 0000000..1825dac
--- /dev/null
+++ b/src/route-contract.spec.ts
@@ -0,0 +1,32 @@
+import { Lettermint } from './lettermint';
+import type { RouteData } from './types';
+
+const base: RouteData = {
+  id: 'route_1',
+  project_id: 'project_1',
+  slug: 'incoming',
+  name: 'Incoming',
+  route_type: 'inbound',
+  is_default: false,
+  created_at: '2026-10-01T12:00:00Z',
+  updated_at: '2026-10-01T12:00:00Z',
+};
+
+describe('inbound route domain contract', () => {
+  it.each(['incoming.example.com', null, undefined])(
+    'keeps a %s domain on the route client',
+    async (domain) => {
+      const payload: RouteData = {
+        ...base,
+        ...(domain === undefined ? {} : { inbound_route_domain: domain }),
+      };
+      global.fetch = jest
+        .fn()
+        .mockResolvedValue({ ok: true, json: async () => payload } as Response);
+      const result = await Lettermint.api('team-token').routes.retrieve('route_1');
+      const typedDomain: string | null | undefined = result.inbound_route_domain;
+      expect(typedDomain).toBe(domain);
+      expect(JSON.parse(JSON.stringify(result))).toEqual(payload);
+    }
+  );
+});
diff --git a/src/types.ts b/src/types.ts
index d3c1d48..9257b4f 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -268,6 +268,7 @@ export interface RouteData {
   "is_default": boolean;
   "inbound_address"?: string | null;
   "inbound_mx_hostname"?: string;
+  "inbound_route_domain"?: string | null;
   "inbound_domain"?: string | null;
   "inbound_domain_verified_at"?: string | null;
   "inbound_spam_threshold"?: number | null;