diff --git a/backend.tests/GetCurrentTimeToolTests.cs b/backend.tests/GetCurrentTimeToolTests.cs
new file mode 100644
index 0000000..beb0134
--- /dev/null
+++ b/backend.tests/GetCurrentTimeToolTests.cs
@@ -0,0 +1,29 @@
+using System.Text.Json;
+using backend.Tools;
+using FluentAssertions;
+using Xunit;
+
+namespace backend.tests;
+
+public class GetCurrentTimeToolTests
+{
+    [Fact]
+    public async Task Returns_now_field_with_iso_utc_timestamp()
+    {
+        var tool = new GetCurrentTimeTool();
+        var args = JsonDocument.Parse("{}").RootElement;
+        var ctx = new DeviceContext(Guid.NewGuid(), Guid.NewGuid(), new NoopChannel());
+
+        var res = await tool.ExecuteAsync(new ToolInvocation("call_1", args), ctx, default);
+
+        res.Output.TryGetProperty("now", out var now).Should().BeTrue();
+        now.GetString().Should().MatchRegex(@"^\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}");
+        now.GetString()!.Should().EndWith("Z");
+    }
+
+    private class NoopChannel : IDeviceChannel
+    {
+        public Task<JsonElement> CallPiToolAsync(string name, JsonElement args, CancellationToken ct)
+            => throw new NotSupportedException();
+    }
+}
diff --git a/backend.tests/ToolRegistryTests.cs b/backend.tests/ToolRegistryTests.cs
new file mode 100644
index 0000000..aea942f
--- /dev/null
+++ b/backend.tests/ToolRegistryTests.cs
@@ -0,0 +1,29 @@
+using backend.Tools;
+using FluentAssertions;
+using Xunit;
+
+namespace backend.tests;
+
+public class ToolRegistryTests
+{
+    private readonly ITool[] _all = new ITool[]
+    {
+        new GetCurrentTimeTool(),
+    };
+
+    [Fact]
+    public void Get_returns_tool_by_exact_name()
+    {
+        var reg = new ToolRegistry(_all);
+        reg.Get("get_current_time").Should().NotBeNull();
+        reg.Get("does_not_exist").Should().BeNull();
+    }
+
+    [Fact]
+    public void Filter_returns_only_enabled_tools_preserving_order()
+    {
+        var reg = new ToolRegistry(_all);
+        var enabled = new HashSet<string> { "get_current_time", "ghost" };
+        reg.EnabledFor(enabled).Select(t => t.Name).Should().Equal("get_current_time");
+    }
+}
diff --git a/backend/Tools/GetCurrentTimeTool.cs b/backend/Tools/GetCurrentTimeTool.cs
new file mode 100644
index 0000000..e603082
--- /dev/null
+++ b/backend/Tools/GetCurrentTimeTool.cs
@@ -0,0 +1,20 @@
+using System.Text.Json;
+
+namespace backend.Tools;
+
+public class GetCurrentTimeTool : ITool
+{
+    public string Name => "get_current_time";
+    public string Description => "Returns the current UTC time as an ISO-8601 string.";
+    public bool RunsDuringResponse => false;
+
+    public JsonElement ParameterSchema { get; } =
+        JsonDocument.Parse("""{"type":"object","properties":{},"required":[]}""").RootElement;
+
+    public Task<ToolResult> ExecuteAsync(ToolInvocation invocation, DeviceContext device, CancellationToken ct)
+    {
+        var now = DateTimeOffset.UtcNow.ToString("yyyy-MM-ddTHH:mm:ssZ");
+        var output = JsonDocument.Parse($$"""{"now":"{{now}}"}""").RootElement;
+        return Task.FromResult(new ToolResult(output));
+    }
+}
diff --git a/backend/Tools/IDeviceChannel.cs b/backend/Tools/IDeviceChannel.cs
new file mode 100644
index 0000000..1658bc4
--- /dev/null
+++ b/backend/Tools/IDeviceChannel.cs
@@ -0,0 +1,8 @@
+using System.Text.Json;
+
+namespace backend.Tools;
+
+public interface IDeviceChannel
+{
+    Task<JsonElement> CallPiToolAsync(string name, JsonElement args, CancellationToken ct);
+}
diff --git a/backend/Tools/ITool.cs b/backend/Tools/ITool.cs
new file mode 100644
index 0000000..a0e1dbc
--- /dev/null
+++ b/backend/Tools/ITool.cs
@@ -0,0 +1,19 @@
+using System.Text.Json;
+
+namespace backend.Tools;
+
+public record DeviceContext(Guid DeviceId, Guid ConversationId, IDeviceChannel Channel);
+public record ToolInvocation(string CallId, JsonElement Arguments);
+public record ToolResult(JsonElement Output);
+
+public interface ITool
+{
+    string Name { get; }
+    string Description { get; }
+    JsonElement ParameterSchema { get; }
+    bool RunsDuringResponse { get; }
+    Task<ToolResult> ExecuteAsync(
+        ToolInvocation invocation,
+        DeviceContext device,
+        CancellationToken ct);
+}
diff --git a/backend/Tools/ToolRegistry.cs b/backend/Tools/ToolRegistry.cs
new file mode 100644
index 0000000..dfc21bc
--- /dev/null
+++ b/backend/Tools/ToolRegistry.cs
@@ -0,0 +1,20 @@
+namespace backend.Tools;
+
+public class ToolRegistry
+{
+    private readonly Dictionary<string, ITool> _byName;
+    private readonly List<ITool> _all;
+
+    public ToolRegistry(IEnumerable<ITool> tools)
+    {
+        _all = tools.ToList();
+        _byName = _all.ToDictionary(t => t.Name);
+    }
+
+    public IReadOnlyList<ITool> All => _all;
+
+    public ITool? Get(string name) => _byName.GetValueOrDefault(name);
+
+    public IEnumerable<ITool> EnabledFor(IReadOnlySet<string> enabled)
+        => _all.Where(t => enabled.Contains(t.Name));
+}