diff --git a/tests/test_models.py b/tests/test_models.py
new file mode 100644
index 0000000..f68bbdf
--- /dev/null
+++ b/tests/test_models.py
@@ -0,0 +1,93 @@
+import pytest
+
+from leakix.client import HostResult
+from leakix.domain import L9Subdomain
+from leakix.plugin import APIResult
+
+
+class TestAPIResultSerialization:
+    def test_to_dict(self):
+        data = {"name": "GrafanaOpenPlugin", "description": "Grafana open instances"}
+        obj = APIResult.from_dict(data)
+        result = obj.to_dict()
+        assert result["name"] == "GrafanaOpenPlugin"
+        assert result["description"] == "Grafana open instances"
+
+    def test_round_trip(self):
+        data = {"name": "MongoOpenPlugin", "description": "MongoDB open instances"}
+        first = APIResult.from_dict(data).to_dict()
+        second = APIResult.from_dict(first).to_dict()
+        assert first == second
+
+    @pytest.mark.parametrize(
+        "data",
+        [
+            {"name": "PluginA", "description": "Description A"},
+            {"name": "PluginB", "description": ""},
+            {"name": "", "description": "Empty name plugin"},
+        ],
+        ids=["normal", "empty-description", "empty-name"],
+    )
+    def test_round_trip_parametrized(self, data):
+        first = APIResult.from_dict(data).to_dict()
+        second = APIResult.from_dict(first).to_dict()
+        assert first == second
+
+
+class TestL9SubdomainSerialization:
+    def test_to_dict(self):
+        data = {
+            "subdomain": "api.example.com",
+            "distinct_ips": 3,
+            "last_seen": "2024-01-15T10:30:00+00:00",
+        }
+        obj = L9Subdomain.from_dict(data)
+        result = obj.to_dict()
+        assert result["subdomain"] == "api.example.com"
+        assert result["distinct_ips"] == 3
+        assert "2024-01-15" in result["last_seen"]
+
+    def test_round_trip(self):
+        data = {
+            "subdomain": "www.example.com",
+            "distinct_ips": 1,
+            "last_seen": "2024-06-20T12:00:00+00:00",
+        }
+        first = L9Subdomain.from_dict(data).to_dict()
+        second = L9Subdomain.from_dict(first).to_dict()
+        assert first == second
+
+    @pytest.mark.parametrize(
+        "data",
+        [
+            {
+                "subdomain": "mail.example.com",
+                "distinct_ips": 0,
+                "last_seen": "2024-01-01T00:00:00+00:00",
+            },
+            {
+                "subdomain": "a.b.c.example.com",
+                "distinct_ips": 999,
+                "last_seen": "2025-12-31T23:59:59+00:00",
+            },
+        ],
+        ids=["zero-ips", "deep-subdomain"],
+    )
+    def test_round_trip_parametrized(self, data):
+        first = L9Subdomain.from_dict(data).to_dict()
+        second = L9Subdomain.from_dict(first).to_dict()
+        assert first == second
+
+
+class TestHostResultSerialization:
+    def test_to_dict_empty(self):
+        data = {"Services": None, "Leaks": None}
+        obj = HostResult.from_dict(data)
+        result = obj.to_dict()
+        assert isinstance(result, dict)
+
+    def test_round_trip_empty(self):
+        data = {"Services": None, "Leaks": None}
+        first = HostResult.from_dict(data).to_dict()
+        second = HostResult.from_dict(first).to_dict()
+        assert first == second