diff --git a/backend/Realtime/IRealtimeUpstream.cs b/backend/Realtime/IRealtimeUpstream.cs
new file mode 100644
index 0000000..9a5b11f
--- /dev/null
+++ b/backend/Realtime/IRealtimeUpstream.cs
@@ -0,0 +1,9 @@
+using System.Text.Json.Nodes;
+
+namespace backend.Realtime;
+
+public interface IRealtimeUpstream : IAsyncDisposable
+{
+    Task SendJsonAsync(JsonObject envelope, CancellationToken ct);
+    Task<JsonObject?> ReceiveJsonAsync(CancellationToken ct);
+}
diff --git a/backend/Realtime/RealtimeEvents.cs b/backend/Realtime/RealtimeEvents.cs
new file mode 100644
index 0000000..1c5bcad
--- /dev/null
+++ b/backend/Realtime/RealtimeEvents.cs
@@ -0,0 +1,71 @@
+using System.Text.Json;
+using System.Text.Json.Nodes;
+using backend.Tools;
+
+namespace backend.Realtime;
+
+public static class RealtimeEvents
+{
+    public static JsonObject SessionUpdate(RealtimeSettings cfg, IEnumerable<ITool> enabledTools)
+    {
+        var tools = new JsonArray();
+        foreach (var t in enabledTools)
+        {
+            tools.Add(new JsonObject
+            {
+                ["type"] = "function",
+                ["name"] = t.Name,
+                ["description"] = t.Description,
+                ["parameters"] = JsonNode.Parse(t.ParameterSchema.GetRawText())!,
+            });
+        }
+
+        return new JsonObject
+        {
+            ["type"] = "session.update",
+            ["session"] = new JsonObject
+            {
+                ["voice"] = cfg.Voice,
+                ["instructions"] = cfg.SystemPrompt,
+                ["modalities"] = new JsonArray("text", "audio"),
+                ["input_audio_format"] = "pcm16",
+                ["output_audio_format"] = "pcm16",
+                ["input_audio_transcription"] = new JsonObject { ["model"] = "whisper-1" },
+                ["turn_detection"] = new JsonObject
+                {
+                    ["type"] = "server_vad",
+                    ["threshold"] = 0.5,
+                    ["prefix_padding_ms"] = 300,
+                    ["silence_duration_ms"] = 500,
+                },
+                ["tools"] = tools,
+                ["tool_choice"] = "auto",
+            },
+        };
+    }
+
+    public static JsonObject InputAudioBufferAppend(ReadOnlySpan<byte> pcm16)
+    {
+        return new JsonObject
+        {
+            ["type"] = "input_audio_buffer.append",
+            ["audio"] = Convert.ToBase64String(pcm16),
+        };
+    }
+
+    public static JsonObject FunctionCallOutput(string callId, string outputJson)
+    {
+        return new JsonObject
+        {
+            ["type"] = "conversation.item.create",
+            ["item"] = new JsonObject
+            {
+                ["type"] = "function_call_output",
+                ["call_id"] = callId,
+                ["output"] = outputJson,
+            },
+        };
+    }
+
+    public static JsonObject ResponseCreate() => new() { ["type"] = "response.create" };
+}