fix: preserve handoff final authorship

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

sidecar/src/Eryx.AgentHost/Services/WorkflowTranscriptProjector.cs

@@ -159,6 +159,24 @@ internal static class WorkflowTranscriptProjector
             }
         }
 
+        if (remainingMessageCount == 1)
+        {
+            if (fallbackAgent.HasValue
+                && AgentIdentityResolver.IsGenericAssistantIdentifier(message.AuthorName)
+                && TryFindLastSegment(
+                    remainingSegments,
+                    segment => string.Equals(
+                        AgentIdentityResolver.ResolveDisplayAuthorName(pattern, segment.AuthorName),
+                        fallbackAgent.Value.AgentName,
+                        StringComparison.Ordinal),
+                    out (string MessageId, string AuthorName, string Content) fallbackMatchedSegment))
+            {
+                return fallbackMatchedSegment;
+            }
+
+            return remainingSegments[^1];
+        }
+
         return remainingSegments.Count == remainingMessageCount
             ? remainingSegments[0]
             : null;
@@ -182,6 +200,25 @@ internal static class WorkflowTranscriptProjector
         return false;
     }
 
+    private static bool TryFindLastSegment(
+        IReadOnlyList<(string MessageId, string AuthorName, string Content)> segments,
+        Func<(string MessageId, string AuthorName, string Content), bool> predicate,
+        out (string MessageId, string AuthorName, string Content) matchedSegment)
+    {
+        for (int index = segments.Count - 1; index >= 0; index--)
+        {
+            (string MessageId, string AuthorName, string Content) segment = segments[index];
+            if (predicate(segment))
+            {
+                matchedSegment = segment;
+                return true;
+            }
+        }
+
+        matchedSegment = default;
+        return false;
+    }
+
     public static List<ChatMessage> SelectNewOutputMessages(
         IReadOnlyList<ChatMessage> outputMessages,
         IReadOnlyList<ChatMessage> inputMessages)

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

@@ -494,6 +494,47 @@ public sealed class CopilotWorkflowRunnerTests
         Assert.Equal("Done — GoogleClient.cs now contains the requested class.", message.Content);
     }
 
+    [Fact]
+    public void ProjectCompletedMessages_UsesFallbackAgentForSingleGenericAssistantOutputWithMultipleSegments()
+    {
+        RunTurnCommandDto command = new()
+        {
+            RequestId = "turn-1",
+            SessionId = "session-1",
+            Pattern = new PatternDefinitionDto
+            {
+                Id = "pattern-handoff",
+                Name = "Handoff Support Flow",
+                Mode = "handoff",
+                Availability = "available",
+                Agents =
+                [
+                    CreateAgent(id: "agent-handoff-triage", name: "Triage"),
+                    CreateAgent(id: "agent-handoff-runtime", name: "Runtime Specialist"),
+                ],
+            },
+        };
+
+        IReadOnlyList<ChatMessageDto> messages = WorkflowTranscriptProjector.ProjectCompletedMessages(
+            command,
+            [
+                new ChatMessage(ChatRole.Assistant, "Done — GoogleClient.cs now contains the requested class with cleaned formatting.")
+                {
+                    AuthorName = "assistant",
+                },
+            ],
+            [
+                ("msg-1", "Triage", "I'll hand this to a coding specialist."),
+                ("msg-2", "Runtime Specialist", "Done — GoogleClient.cs now contains the requested class with draft formatting."),
+            ],
+            new AgentIdentity("agent-handoff-runtime", "Runtime Specialist"));
+
+        ChatMessageDto message = Assert.Single(messages);
+        Assert.Equal("msg-2", message.Id);
+        Assert.Equal("Runtime Specialist", message.AuthorName);
+        Assert.Equal("Done — GoogleClient.cs now contains the requested class with cleaned formatting.", message.Content);
+    }
+
     [Fact]
     public void ProjectCompletedMessages_DropsBlankAssistantOutputMessages()
     {