diff --git a/changelogs/fragments/inventory-fix-serialization.yml b/changelogs/fragments/inventory-fix-serialization.yml
new file mode 100644
index 00000000..cc91bf4e
--- /dev/null
+++ b/changelogs/fragments/inventory-fix-serialization.yml
@@ -0,0 +1,2 @@
+bugfixes:
+  - inventory - Ensure inventory host variables are serializable and can be cached.
diff --git a/plugins/inventory/hcloud.py b/plugins/inventory/hcloud.py
index 79e913da..d1050c92 100644
--- a/plugins/inventory/hcloud.py
+++ b/plugins/inventory/hcloud.py
@@ -261,7 +261,7 @@ def first_ipv6_address(network: str) -> str:
 
     :param network: IPv6 Network.
     """
-    return next(IPv6Network(network).hosts())
+    return str(next(IPv6Network(network).hosts()))
 
 
 class InventoryModule(BaseInventoryPlugin, Constructable, Cacheable):
diff --git a/tests/unit/inventory/test_hcloud.py b/tests/unit/inventory/test_hcloud.py
new file mode 100644
index 00000000..b5d3b777
--- /dev/null
+++ b/tests/unit/inventory/test_hcloud.py
@@ -0,0 +1,75 @@
+from __future__ import annotations
+
+import json
+from unittest.mock import MagicMock
+
+from plugins.inventory.hcloud import InventoryModule, first_ipv6_address
+from plugins.module_utils.vendor.hcloud.servers import BoundServer
+
+
+def test_first_ipv6_address():
+    found = first_ipv6_address("2001:db8::/64")
+    assert isinstance(found, str)
+    assert found == "2001:db8::1"
+
+
+def test_build_inventory_server():
+    client = MagicMock()
+    inventory = InventoryModule()
+    inventory.get_option = MagicMock()
+    inventory.get_option.return_value = None
+
+    server = BoundServer(
+        client,
+        {
+            "id": 45921624,
+            "name": "my-server",
+            "labels": {},
+            "status": "running",
+            "public_net": {
+                "ipv4": {
+                    "id": 56583278,
+                    "ip": "127.0.0.1",
+                    "blocked": False,
+                    "dns_ptr": "static.1.0.0.127.clients.your-server.de",
+                },
+                "ipv6": {"id": 56583279, "ip": "2001:db8::/64", "blocked": False, "dns_ptr": []},
+                "floating_ips": [],
+                "firewalls": [],
+            },
+            "private_net": [],
+            "server_type": {"id": 1, "name": "cx11", "architecture": "x86"},
+            "datacenter": {
+                "id": 3,
+                "name": "hel1-dc2",
+                "location": {"id": 3, "name": "hel1"},
+            },
+            "image": {"id": 114690387, "name": "debian-12", "os_flavor": "debian", "os_version": "12"},
+        },
+    )
+    # pylint: disable=protected-access
+    variables = inventory._build_inventory_server(server)
+
+    # Ensure the host_vars are json serializable
+    json.dumps(variables)
+
+    assert variables == {
+        "id": 45921624,
+        "name": "my-server",
+        "status": "running",
+        "type": "cx11",
+        "server_type": "cx11",
+        "architecture": "x86",
+        "location": "hel1",
+        "datacenter": "hel1-dc2",
+        "labels": {},
+        "ipv4": "127.0.0.1",
+        "ipv6": "2001:db8::1",
+        "ipv6_network": "2001:db8::",
+        "ipv6_network_mask": "64",
+        "private_networks": [],
+        "image_id": 114690387,
+        "image_name": "debian-12",
+        "image_os_flavor": "debian",
+        "ansible_host": None,
+    }