RealtimeSession: execute server-side tools at response.done

backend/Realtime/RealtimeSession.cs

@@ -25,6 +25,7 @@ public class RealtimeSession
     private Conversation? _conversation;
     private EndReason _endReason = EndReason.Unknown;
     private bool _endRequested;
+    private readonly List<(string CallId, string Name, string ArgsJson)> _pendingCalls = new();
 
     private readonly System.Threading.Channels.Channel<byte[]> _uplink =
         System.Threading.Channels.Channel.CreateBounded<byte[]>(
@@ -135,14 +136,77 @@ public class RealtimeSession
                         await _log.AppendAssistantAsync(_conversation.Id, text!, ct);
                     break;
                 }
+                case "response.function_call_arguments.done":
+                {
+                    var callId = (string?)evt["call_id"] ?? "";
+                    var name = (string?)evt["name"] ?? "";
+                    var args = (string?)evt["arguments"] ?? "{}";
+                    _pendingCalls.Add((callId, name, args));
+                    break;
+                }
                 case "response.done":
+                {
                     await _output.WriteEnvelopeAsync(
                         new JsonObject { ["type"] = "assistant_done" }, ct);
+
+                    if (_pendingCalls.Count == 0) break;
+                    var calls = _pendingCalls.ToList();
+                    _pendingCalls.Clear();
+
+                    if (calls.Any(c => c.Name == "end_session"))
+                    {
+                        _endReason = EndReason.Tool;
+                        _endRequested = true;
+                        break;
+                    }
+
+                    foreach (var c in calls)
+                    {
+                        await ExecuteToolAsync(c.CallId, c.Name, c.ArgsJson, ct);
+                    }
                     break;
+                }
             }
         }
     }
 
+    private async Task ExecuteToolAsync(string callId, string name, string argsJson, CancellationToken ct)
+    {
+        var tool = _registry.Get(name);
+        string outputJson;
+        if (tool is null)
+        {
+            outputJson = $$"""{"ok":false,"error":"unknown tool: {{name}}"}""";
+        }
+        else
+        {
+            try
+            {
+                using var argsDoc = System.Text.Json.JsonDocument.Parse(argsJson);
+                var ctx = new DeviceContext(_deviceId, _conversation!.Id, _channel);
+                var res = await tool.ExecuteAsync(
+                    new ToolInvocation(callId, argsDoc.RootElement.Clone()), ctx, ct);
+                outputJson = res.Output.GetRawText();
+            }
+            catch (Exception ex)
+            {
+                outputJson = System.Text.Json.JsonSerializer.Serialize(new
+                {
+                    ok = false,
+                    error = ex.Message,
+                });
+            }
+        }
+
+        if (_conversation is not null)
+        {
+            await _log.AppendToolAsync(_conversation.Id, name, argsJson, outputJson, ct);
+        }
+
+        await _upstream.SendJsonAsync(RealtimeEvents.FunctionCallOutput(callId, outputJson), ct);
+        await _upstream.SendJsonAsync(RealtimeEvents.ResponseCreate(), ct);
+    }
+
     private async Task ForwardAudioDeltaAsync(JsonObject evt, CancellationToken ct)
     {
         var b64 = (string?)evt["delta"];