diff --git a/src/lettermint/types.py b/src/lettermint/types.py
index 153f3c1..0353a67 100644
--- a/src/lettermint/types.py
+++ b/src/lettermint/types.py
@@ -296,6 +296,7 @@
         "is_default": "Required[bool]",
         "inbound_address": "NotRequired[str | None]",
         "inbound_mx_hostname": "NotRequired[str]",
+        "inbound_route_domain": "NotRequired[str | None]",
         "inbound_domain": "NotRequired[str | None]",
         "inbound_domain_verified_at": "NotRequired[str | None]",
         "inbound_spam_threshold": "NotRequired[float | None]",
diff --git a/tests/test_route_contract.py b/tests/test_route_contract.py
new file mode 100644
index 0000000..56694b8
--- /dev/null
+++ b/tests/test_route_contract.py
@@ -0,0 +1,39 @@
+"""Check the optional, nullable inbound route domain."""
+
+from typing import Any
+
+import pytest
+import respx
+from httpx import Response
+
+from lettermint import Lettermint
+from lettermint.types import RouteData
+
+
+@pytest.mark.parametrize("domain", ["incoming.example.com", None, ...])
+@respx.mock
+def test_inbound_route_domain(domain: Any) -> None:
+    payload: 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",
+    }
+    if domain is not ...:
+        payload["inbound_route_domain"] = domain
+    respx.get("https://api.lettermint.co/v1/routes/route_1").mock(
+        return_value=Response(200, json=payload)
+    )
+    with Lettermint.api("team-token") as api:
+        result = api.routes.retrieve("route_1")
+    assert result == payload
+    assert result.get("inbound_route_domain") == (None if domain is ... else domain)
+
+
+def test_inbound_route_domain_is_declared() -> None:
+    assert "inbound_route_domain" in RouteData.__annotations__
+    assert "NotRequired[str | None]" in str(RouteData.__annotations__["inbound_route_domain"])