diff --git a/test/towerops_web/graphql/resolvers/happy_path_test.exs b/test/towerops_web/graphql/resolvers/happy_path_test.exs
index cf71ae74..60c3b72d 100644
--- a/test/towerops_web/graphql/resolvers/happy_path_test.exs
+++ b/test/towerops_web/graphql/resolvers/happy_path_test.exs
@@ -16,6 +16,57 @@ defmodule ToweropsWeb.GraphQL.Resolvers.HappyPathTest do
     %{user: user, org: org, ctx: %{context: %{organization_id: org.id, user: user}}}
   end
 
+  describe "Device (additional success/error branches)" do
+    test "update/3 with invalid input returns formatted error", %{ctx: ctx, org: org} do
+      {:ok, device} =
+        Towerops.Devices.create_device(%{
+          name: "ToUpdate",
+          ip_address: "10.66.0.1",
+          organization_id: org.id
+        })
+
+      assert {:error, msg} =
+               Resolvers.Device.update(nil, %{id: device.id, input: %{ip_address: ""}}, ctx)
+
+      assert is_binary(msg)
+    end
+
+    test "metrics/3 returns metrics shape for a real device", %{ctx: ctx, org: org} do
+      {:ok, device} =
+        Towerops.Devices.create_device(%{
+          name: "MetricsDev",
+          ip_address: "10.77.0.1",
+          organization_id: org.id
+        })
+
+      assert {:ok, metrics} =
+               Resolvers.Device.metrics(nil, %{device_id: device.id, time_range: "24h"}, ctx)
+
+      assert is_list(metrics)
+    end
+
+    test "metrics/3 unauthenticated returns auth error" do
+      assert {:error, _} =
+               Resolvers.Device.metrics(nil, %{device_id: Ecto.UUID.generate()}, %{context: %{}})
+    end
+
+    test "interfaces/3 returns [] for device without SNMP record", %{ctx: ctx, org: org} do
+      {:ok, device} =
+        Towerops.Devices.create_device(%{
+          name: "NoSnmp",
+          ip_address: "10.88.0.1",
+          organization_id: org.id
+        })
+
+      assert {:ok, []} = Resolvers.Device.interfaces(nil, %{device_id: device.id}, ctx)
+    end
+
+    test "interfaces/3 unauthenticated returns auth error" do
+      assert {:error, _} =
+               Resolvers.Device.interfaces(nil, %{device_id: Ecto.UUID.generate()}, %{context: %{}})
+    end
+  end
+
   describe "Schedule (success/error branches)" do
     test "create/3 returns the new schedule on success", %{ctx: ctx} do
       input = %{name: "Primary On-Call", description: "24/7", timezone: "Etc/UTC"}