fix: preserve handoff agent identity

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

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

@@ -22,6 +22,7 @@ internal sealed class CopilotAgentBundle : IAsyncDisposable
     public static async Task<CopilotAgentBundle> CreateAsync(
         RunTurnCommandDto command,
         Func<PatternAgentDefinitionDto, PermissionRequest, PermissionInvocation, Task<PermissionRequestResult>> onPermissionRequest,
+        Action<PatternAgentDefinitionDto, SessionEvent>? onSessionEvent,
         CancellationToken cancellationToken)
     {
         List<IAsyncDisposable> disposables = [];
@@ -53,6 +54,7 @@ internal sealed class CopilotAgentBundle : IAsyncDisposable
                 },
                 WorkingDirectory = command.ProjectPath,
                 OnPermissionRequest = (request, invocation) => onPermissionRequest(definition, request, invocation),
+                OnEvent = evt => onSessionEvent?.Invoke(definition, evt),
                 Streaming = true,
             };
 

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

@@ -1,5 +1,6 @@
 using System.Collections.Concurrent;
 using Eryx.AgentHost.Contracts;
+using GitHub.Copilot.SDK;
 using Microsoft.Extensions.AI;
 
 namespace Eryx.AgentHost.Services;
@@ -8,6 +9,7 @@ internal sealed class CopilotTurnExecutionState
 {
     private readonly RunTurnCommandDto _command;
     private readonly HashSet<string> _startedAgents = new(StringComparer.OrdinalIgnoreCase);
+    private readonly ConcurrentDictionary<string, AgentIdentity> _observedAgentsByMessageId = new(StringComparer.Ordinal);
     private readonly StreamingTranscriptBuffer _transcriptBuffer = new();
     private int _fallbackMessageIndex;
 
@@ -54,6 +56,34 @@ internal sealed class CopilotTurnExecutionState
         }
     }
 
+    public void ObserveSessionEvent(PatternAgentDefinitionDto agentDefinition, SessionEvent sessionEvent)
+    {
+        AgentIdentity agent = AgentIdentityResolver.ResolveAgentIdentity(
+            _command.Pattern,
+            agentDefinition.Id,
+            agentDefinition.Name);
+
+        switch (sessionEvent)
+        {
+            case AssistantMessageDeltaEvent messageDelta when !string.IsNullOrWhiteSpace(messageDelta.Data?.MessageId):
+                RecordObservedAgentForMessage(agent, messageDelta.Data!.MessageId);
+                break;
+            case AssistantMessageEvent assistantMessage when !string.IsNullOrWhiteSpace(assistantMessage.Data?.MessageId):
+                RecordObservedAgentForMessage(agent, assistantMessage.Data!.MessageId);
+                break;
+            case AssistantReasoningDeltaEvent:
+                ActiveAgent = agent;
+                break;
+        }
+    }
+
+    public bool TryResolveObservedAgentForMessage(string? messageId, out AgentIdentity agent)
+    {
+        agent = default;
+        return !string.IsNullOrWhiteSpace(messageId)
+            && _observedAgentsByMessageId.TryGetValue(messageId, out agent);
+    }
+
     public string CreateMessageId(string? messageId)
     {
         return messageId ?? $"{_command.RequestId}-delta-{_fallbackMessageIndex++}";
@@ -76,6 +106,12 @@ internal sealed class CopilotTurnExecutionState
         }
     }
 
+    private void RecordObservedAgentForMessage(AgentIdentity agent, string messageId)
+    {
+        ActiveAgent = agent;
+        _observedAgentsByMessageId[messageId] = agent;
+    }
+
     public void UpdateCompletedMessages(
         IReadOnlyList<ChatMessage> allMessages,
         IReadOnlyList<ChatMessage> inputMessages)

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

@@ -38,6 +38,7 @@ public sealed class CopilotWorkflowRunner : ITurnWorkflowRunner
                 state.ToolNamesByCallId,
                 onApproval,
                 cancellationToken),
+            (agent, sessionEvent) => state.ObserveSessionEvent(agent, sessionEvent),
             cancellationToken);
         Workflow workflow = bundle.BuildWorkflow(command.Pattern);
         List<ChatMessage> inputMessages = command.Messages.Select(WorkflowTranscriptProjector.ToChatMessage).ToList();
@@ -136,7 +137,12 @@ public sealed class CopilotWorkflowRunner : ITurnWorkflowRunner
     {
         AgentIdentity? updateAgent = null;
         string authorName = update.ExecutorId;
-        if (AgentIdentityResolver.TryResolveObservedAgentIdentity(
+        if (state.TryResolveObservedAgentForMessage(update.Update.MessageId, out AgentIdentity observedMessageAgent))
+        {
+            updateAgent = observedMessageAgent;
+            authorName = observedMessageAgent.AgentName;
+        }
+        else if (AgentIdentityResolver.TryResolveObservedAgentIdentity(
             command.Pattern,
             update.ExecutorId,
             state.ActiveAgent,

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

@@ -451,11 +451,59 @@ public sealed class CopilotWorkflowRunnerTests
                 Assert.Equal("A plus", first.Content);
             },
             second =>
+             {
+                 Assert.Equal("msg-2", second.MessageId);
+                 Assert.Equal("Implementer", second.AuthorName);
+                 Assert.Equal("B", second.Content);
+             });
+    }
+
+    [Fact]
+    public void ObserveSessionEvent_TracksObservedAgentByMessageId()
+    {
+        CopilotTurnExecutionState state = new(CreateHandoffCommand());
+        SessionEvent sessionEvent = SessionEvent.FromJson(
+            """
             {
-                Assert.Equal("msg-2", second.MessageId);
-                Assert.Equal("Implementer", second.AuthorName);
-                Assert.Equal("B", second.Content);
-            });
+              "type": "assistant.message_delta",
+              "data": {
+                "messageId": "msg-7",
+                "deltaContent": "Done."
+              },
+              "id": "11111111-1111-1111-1111-111111111111",
+              "timestamp": "2026-03-24T00:00:00Z"
+            }
+            """);
+
+        state.ObserveSessionEvent(CreateAgent("agent-handoff-ux", "UX Specialist"), sessionEvent);
+
+        Assert.True(state.TryResolveObservedAgentForMessage("msg-7", out AgentIdentity observedAgent));
+        Assert.Equal("agent-handoff-ux", observedAgent.AgentId);
+        Assert.Equal("UX Specialist", observedAgent.AgentName);
+        Assert.Equal("agent-handoff-ux", state.ActiveAgent?.AgentId);
+    }
+
+    [Fact]
+    public void ObserveSessionEvent_UsesReasoningDeltasToTrackActiveAgent()
+    {
+        CopilotTurnExecutionState state = new(CreateHandoffCommand());
+        SessionEvent sessionEvent = SessionEvent.FromJson(
+            """
+            {
+              "type": "assistant.reasoning_delta",
+              "data": {
+                "reasoningId": "reasoning-1",
+                "deltaContent": "Planning."
+              },
+              "id": "22222222-2222-2222-2222-222222222222",
+              "timestamp": "2026-03-24T00:00:00Z"
+            }
+            """);
+
+        state.ObserveSessionEvent(CreateAgent("agent-handoff-ux", "UX Specialist"), sessionEvent);
+
+        Assert.Equal("agent-handoff-ux", state.ActiveAgent?.AgentId);
+        Assert.Equal("UX Specialist", state.ActiveAgent?.AgentName);
     }
 
     [Fact]
@@ -770,6 +818,27 @@ public sealed class CopilotWorkflowRunnerTests
         };
     }
 
+    private static RunTurnCommandDto CreateHandoffCommand()
+    {
+        return new RunTurnCommandDto
+        {
+            RequestId = "turn-1",
+            SessionId = "session-1",
+            Pattern = new PatternDefinitionDto
+            {
+                Id = "pattern-handoff",
+                Name = "Handoff Flow",
+                Mode = "handoff",
+                Availability = "available",
+                Agents =
+                [
+                    CreateAgent("agent-handoff-triage", "Triage"),
+                    CreateAgent("agent-handoff-ux", "UX Specialist"),
+                ],
+            },
+        };
+    }
+
     private static RunTurnCommandDto CreateApprovalCommand()
     {
         return new RunTurnCommandDto