refactor: finalize backend streaming ownership

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

sidecar/tests/Aryx.AgentHost.Tests/CopilotTurnExecutionStateTests.cs

@@ -847,6 +847,53 @@ public sealed class CopilotTurnExecutionStateTests
         Assert.Equal("agent-1", evt.AgentId);
     }
 
+    [Fact]
+    public void ObserveSessionEvent_AssistantMessage_FinalizesTranscriptAndSuppressesLateDeltas()
+    {
+        RunTurnCommandDto command = CreateCommand();
+        CopilotTurnExecutionState state = new(command);
+
+        Assert.True(state.TryAppendDelta("msg-final", "Primary", "Draft response", out TranscriptSegment streamed));
+        Assert.False(streamed.IsFinalized);
+
+        state.ObserveSessionEvent(
+            command.Workflow.GetAgentNodes()[0],
+            SessionEvent.FromJson(
+                """
+                {
+                  "type": "assistant.message",
+                  "data": {
+                    "messageId": "msg-final",
+                    "content": "Provider final response"
+                  },
+                  "id": "12121212-1212-1212-1212-121212121212",
+                  "timestamp": "2026-03-27T00:00:00Z"
+                }
+                """));
+
+        TurnDeltaEventDto correction = Assert.Single(state.DrainPendingEvents().OfType<TurnDeltaEventDto>());
+        Assert.Equal("msg-final", correction.MessageId);
+        Assert.Equal("Primary", correction.AuthorName);
+        Assert.Equal(string.Empty, correction.ContentDelta);
+        Assert.Equal("Provider final response", correction.Content);
+
+        Assert.False(state.TryAppendDelta("msg-final", "Primary", " late delta", out TranscriptSegment unchanged));
+        Assert.True(unchanged.IsFinalized);
+        Assert.Equal("Provider final response", unchanged.Content);
+
+        ChatMessage output = new(ChatRole.Assistant, "Workflow wording")
+        {
+            MessageId = "msg-final",
+            AuthorName = "assistant",
+        };
+
+        state.UpdateCompletedMessages([output], []);
+        ChatMessageDto completed = Assert.Single(state.FinalizeCompletedMessages());
+        Assert.Equal("msg-final", completed.Id);
+        Assert.Equal("Primary", completed.AuthorName);
+        Assert.Equal("Provider final response", completed.Content);
+    }
+
     private static SessionEvent CreateHookStartEvent()
     {
         return SessionEvent.FromJson(

sidecar/tests/Aryx.AgentHost.Tests/CopilotWorkflowRunnerTests.cs

@@ -451,6 +451,50 @@ public sealed class CopilotWorkflowRunnerTests
             });
     }
 
+    [Fact]
+    public void ProjectCompletedMessages_UsesExactMessageIdMatchBeforeFallbackHeuristics()
+    {
+        RunTurnCommandDto command = CreateCommand(
+            "group-chat",
+            CreateAgent(id: "agent-group-writer", name: "Writer"),
+            CreateAgent(id: "agent-group-reviewer", name: "Reviewer"));
+
+        IReadOnlyList<ChatMessageDto> messages = WorkflowTranscriptProjector.ProjectCompletedMessagesFromSegments(
+            command,
+            [
+                new ChatMessage(ChatRole.Assistant, "Revised draft with cleaner wording.")
+                {
+                    AuthorName = "assistant",
+                    MessageId = "msg-writer-2",
+                },
+                new ChatMessage(ChatRole.Assistant, "Review feedback with cleaner wording.")
+                {
+                    AuthorName = "assistant",
+                    MessageId = "msg-reviewer-1",
+                },
+            ],
+            [
+                new TranscriptSegment("msg-writer-1", "Writer", "Initial draft."),
+                new TranscriptSegment("msg-reviewer-1", "Reviewer", "Review feedback with draft wording."),
+                new TranscriptSegment("msg-writer-2", "Writer", "Revised draft with draft wording."),
+            ]);
+
+        Assert.Collection(
+            messages,
+            writer =>
+            {
+                Assert.Equal("msg-writer-2", writer.Id);
+                Assert.Equal("Writer", writer.AuthorName);
+                Assert.Equal("Revised draft with cleaner wording.", writer.Content);
+            },
+            reviewer =>
+            {
+                Assert.Equal("msg-reviewer-1", reviewer.Id);
+                Assert.Equal("Reviewer", reviewer.AuthorName);
+                Assert.Equal("Review feedback with cleaner wording.", reviewer.Content);
+            });
+    }
+
     [Fact]
     public void ProjectCompletedMessages_UsesFallbackAgentForGenericAssistantOutput()
     {
@@ -531,6 +575,36 @@ public sealed class CopilotWorkflowRunnerTests
         Assert.Equal("Done — GoogleClient.cs now contains the requested class with cleaned formatting.", message.Content);
     }
 
+    [Fact]
+    public void ProjectCompletedMessages_PrefersFinalizedSegmentContentOverWorkflowOutput()
+    {
+        RunTurnCommandDto command = CreateCommand(
+            "single",
+            CreateAgent(id: "agent-single-primary", name: "Primary Agent"));
+
+        IReadOnlyList<ChatMessageDto> messages = WorkflowTranscriptProjector.ProjectCompletedMessagesFromSegments(
+            command,
+            [
+                new ChatMessage(ChatRole.Assistant, "Workflow wording that arrived later.")
+                {
+                    AuthorName = "assistant",
+                    MessageId = "msg-1",
+                },
+            ],
+            [
+                new TranscriptSegment(
+                    "msg-1",
+                    "Primary Agent",
+                    "Provider final wording that should win.",
+                    IsFinalized: true),
+            ]);
+
+        ChatMessageDto message = Assert.Single(messages);
+        Assert.Equal("msg-1", message.Id);
+        Assert.Equal("Primary Agent", message.AuthorName);
+        Assert.Equal("Provider final wording that should win.", message.Content);
+    }
+
     [Fact]
     public void ProjectCompletedMessages_DropsBlankAssistantOutputMessages()
     {
@@ -602,7 +676,22 @@ public sealed class CopilotWorkflowRunnerTests
                  Assert.Equal("msg-2", second.MessageId);
                  Assert.Equal("Implementer", second.AuthorName);
                  Assert.Equal("B", second.Content);
-             });
+              });
+    }
+
+    [Fact]
+    public void StreamingTranscriptBuffer_IgnoresLateDeltasAfterFinalSnapshot()
+    {
+        StreamingTranscriptBuffer buffer = new();
+
+        buffer.AppendDelta("msg-1", "Architect", "Draft");
+        Assert.True(buffer.TryApplySnapshot("msg-1", "Architect", "Final", out TranscriptSegment finalized));
+        Assert.True(finalized.IsFinalized);
+        Assert.Equal("Final", finalized.Content);
+
+        Assert.False(buffer.TryAppendDelta("msg-1", "Architect", " late delta", out TranscriptSegment unchanged));
+        Assert.True(unchanged.IsFinalized);
+        Assert.Equal("Final", unchanged.Content);
     }
 
     [Fact]