Derive session event envelopes from schema (#1184)

* Generate C# session event envelope properties

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Remove unused session event initializer helper

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Derive session event envelopes across SDKs

Share schema-level session event envelope discovery across code generators and use it for Go and Python session-event wrappers so top-level envelope fields like agentId round-trip consistently.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: stephentoub

dotnet/src/Generated/SessionEvents.cs

@@ -20,6 +20,7 @@ public void FromJson_KnownEventType_DeserializesNormally()
             "id": "00000000-0000-0000-0000-000000000001",
             "timestamp": "2026-01-01T00:00:00Z",
             "parentId": null,
+            "agentId": "agent-1",
             "type": "user.message",
             "data": {
                 "content": "Hello"
@@ -31,6 +32,7 @@ public void FromJson_KnownEventType_DeserializesNormally()
 
         Assert.IsType<UserMessageEvent>(result);
         Assert.Equal("user.message", result.Type);
+        Assert.Equal("agent-1", result.AgentId);
     }
 
     [Fact]
@@ -41,6 +43,7 @@ public void FromJson_UnknownEventType_ReturnsBaseSessionEvent()
             "id": "12345678-1234-1234-1234-123456789abc",
             "timestamp": "2026-06-15T10:30:00Z",
             "parentId": "abcdefab-abcd-abcd-abcd-abcdefabcdef",
+            "agentId": "future-agent",
             "type": "future.feature_from_server",
             "data": { "key": "value" }
         }
@@ -50,6 +53,7 @@ public void FromJson_UnknownEventType_ReturnsBaseSessionEvent()
 
         Assert.IsType<SessionEvent>(result);
         Assert.Equal("unknown", result.Type);
+        Assert.Equal("future-agent", result.AgentId);
     }
 
     [Fact]

dotnet/test/ForwardCompatibilityTests.cs

@@ -18,6 +18,7 @@ public class SessionEventSerializationTests
                 Id = Guid.Parse("11111111-1111-1111-1111-111111111111"),
                 Timestamp = DateTimeOffset.Parse("2026-03-15T21:26:02.642Z"),
                 ParentId = Guid.Parse("22222222-2222-2222-2222-222222222222"),
+                AgentId = "agent-1",
                 Data = new AssistantMessageData
                 {
                     MessageId = "msg-1",
@@ -134,6 +135,7 @@ public void SessionEvent_ToJson_RoundTrips_JsonElementBackedPayloads(SessionEven
         switch (expectedType)
         {
             case "assistant.message":
+                Assert.Equal("agent-1", root.GetProperty("agentId").GetString());
                 Assert.Equal(
                     "README.md",
                     root.GetProperty("data")

dotnet/test/SessionEventSerializationTests.cs

@@ -0,0 +1,78 @@
+package copilot
+
+import (
+	"encoding/json"
+	"testing"
+)
+
+func TestSessionEventAgentIDRoundTripsKnownEvent(t *testing.T) {
+	event, err := UnmarshalSessionEvent([]byte(`{
+		"id": "00000000-0000-0000-0000-000000000001",
+		"timestamp": "2026-01-01T00:00:00Z",
+		"parentId": null,
+		"agentId": "agent-1",
+		"type": "user.message",
+		"data": {
+			"content": "Hello"
+		}
+	}`))
+	if err != nil {
+		t.Fatalf("failed to unmarshal session event: %v", err)
+	}
+
+	if event.AgentID == nil || *event.AgentID != "agent-1" {
+		t.Fatalf("expected agent ID to round-trip, got %v", event.AgentID)
+	}
+	if _, ok := event.Data.(*UserMessageData); !ok {
+		t.Fatalf("expected user message data, got %T", event.Data)
+	}
+
+	data, err := event.Marshal()
+	if err != nil {
+		t.Fatalf("failed to marshal session event: %v", err)
+	}
+
+	var serialized map[string]any
+	if err := json.Unmarshal(data, &serialized); err != nil {
+		t.Fatalf("failed to unmarshal serialized session event: %v", err)
+	}
+	if serialized["agentId"] != "agent-1" {
+		t.Fatalf("expected serialized agentId to round-trip, got %v", serialized["agentId"])
+	}
+}
+
+func TestSessionEventAgentIDRoundTripsUnknownEvent(t *testing.T) {
+	event, err := UnmarshalSessionEvent([]byte(`{
+		"id": "00000000-0000-0000-0000-000000000002",
+		"timestamp": "2026-01-01T00:00:00Z",
+		"parentId": null,
+		"agentId": "future-agent",
+		"type": "future.feature_from_server",
+		"data": {
+			"key": "value"
+		}
+	}`))
+	if err != nil {
+		t.Fatalf("failed to unmarshal session event: %v", err)
+	}
+
+	if event.AgentID == nil || *event.AgentID != "future-agent" {
+		t.Fatalf("expected agent ID to round-trip, got %v", event.AgentID)
+	}
+	if _, ok := event.Data.(*RawSessionEventData); !ok {
+		t.Fatalf("expected raw session event data, got %T", event.Data)
+	}
+
+	data, err := event.Marshal()
+	if err != nil {
+		t.Fatalf("failed to marshal session event: %v", err)
+	}
+
+	var serialized map[string]any
+	if err := json.Unmarshal(data, &serialized); err != nil {
+		t.Fatalf("failed to unmarshal serialized session event: %v", err)
+	}
+	if serialized["agentId"] != "future-agent" {
+		t.Fatalf("expected serialized agentId to round-trip, got %v", serialized["agentId"])
+	}
+}

go/generated_session_events.go

@@ -24,6 +24,7 @@
     UserMessageAgentMode,
     UserMessageAttachmentGithubReferenceType,
     session_event_from_dict,
+    session_event_to_dict,
 )
 
 
@@ -47,6 +48,39 @@ def test_unknown_event_type_maps_to_unknown(self):
         event = session_event_from_dict(unknown_event)
         assert event.type == SessionEventType.UNKNOWN, f"Expected UNKNOWN, got {event.type}"
 
+    def test_known_event_preserves_top_level_agent_id(self):
+        """Known events should preserve the top-level sub-agent envelope ID."""
+        known_event = {
+            "id": str(uuid4()),
+            "timestamp": datetime.now().isoformat(),
+            "parentId": None,
+            "agentId": "agent-1",
+            "type": "user.message",
+            "data": {"content": "Hello"},
+        }
+
+        event = session_event_from_dict(known_event)
+        assert event.agent_id == "agent-1"
+        assert session_event_to_dict(event)["agentId"] == "agent-1"
+
+    def test_unknown_event_preserves_top_level_agent_id(self):
+        """Unknown events should preserve the top-level sub-agent envelope ID."""
+        unknown_event = {
+            "id": str(uuid4()),
+            "timestamp": datetime.now().isoformat(),
+            "parentId": None,
+            "agentId": "future-agent",
+            "type": "session.future_feature_from_server",
+            "data": {"key": "value"},
+        }
+
+        event = session_event_from_dict(unknown_event)
+        assert event.type == SessionEventType.UNKNOWN
+        assert event.agent_id == "future-agent"
+        serialized = session_event_to_dict(event)
+        assert serialized["agentId"] == "future-agent"
+        assert serialized["type"] == "session.future_feature_from_server"
+
     def test_malformed_uuid_raises_error(self):
         """Malformed UUIDs should raise ValueError for visibility, not be suppressed."""
         malformed_event = {