feat: add thinking protocol events

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

ARCHITECTURE.md

@@ -214,6 +214,8 @@ The protocol also carries **turn-scoped lifecycle events** alongside output delt
 
 - **Sub-agent events**: started, completed, failed, selected, deselected — surfaced when custom agents are defined
 - **Skill invocation events**: emitted when an agent-side skill is triggered
+- **Message reclassification events**: let the sidecar retroactively mark a streamed assistant message as `thinking` once the SDK confirms that message requested tool work, so the UI can separate intermediate planning chatter from the final response without sacrificing live streaming
+- **Assistant intent and reasoning-delta events**: optional Copilot SDK metadata that exposes short "what I'm doing" labels plus incremental reasoning text for richer thinking-process surfaces
 - **Hook lifecycle events**: start and end of configured project hook commands discovered from `.github/hooks/*.json`; Aryx suppresses the SDK's built-in no-op hook chatter so the UI only sees meaningful hook activity
 - **Assistant usage events**: per-LLM-call tokens, cost, AIU, and quota snapshots from the Copilot SDK's `assistant.usage` stream
 - **Session compaction events**: start and complete, with token-reduction metrics when infinite sessions trigger context trimming

sidecar/src/Aryx.AgentHost/Contracts/ProtocolModels.cs

@@ -331,6 +331,13 @@ public sealed class TurnCompleteEventDto : SidecarEventDto
     public bool Cancelled { get; init; }
 }
 
+public sealed class MessageReclassifiedEventDto : SidecarEventDto
+{
+    public string SessionId { get; init; } = string.Empty;
+    public string MessageId { get; init; } = string.Empty;
+    public string NewKind { get; init; } = string.Empty;
+}
+
 public sealed class AgentActivityEventDto : SidecarEventDto
 {
     public string SessionId { get; init; } = string.Empty;
@@ -376,6 +383,23 @@ public sealed class SkillInvokedEventDto : SidecarEventDto
     public string? Description { get; init; }
 }
 
+public sealed class AssistantIntentEventDto : SidecarEventDto
+{
+    public string SessionId { get; init; } = string.Empty;
+    public string? AgentId { get; init; }
+    public string? AgentName { get; init; }
+    public string Intent { get; init; } = string.Empty;
+}
+
+public sealed class ReasoningDeltaEventDto : SidecarEventDto
+{
+    public string SessionId { get; init; } = string.Empty;
+    public string? AgentId { get; init; }
+    public string? AgentName { get; init; }
+    public string ReasoningId { get; init; } = string.Empty;
+    public string ContentDelta { get; init; } = string.Empty;
+}
+
 public sealed class HookLifecycleEventDto : SidecarEventDto
 {
     public string SessionId { get; init; } = string.Empty;

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

@@ -9,11 +9,13 @@ internal sealed class CopilotTurnExecutionState
 {
     private readonly RunTurnCommandDto _command;
     private readonly HashSet<string> _startedAgents = new(StringComparer.OrdinalIgnoreCase);
+    private readonly HashSet<string> _reclassifiedMessageIds = new(StringComparer.Ordinal);
     private readonly ConcurrentQueue<SidecarEventDto> _pendingEvents = new();
     private readonly ConcurrentQueue<McpOauthRequiredEventDto> _pendingMcpOauthRequests = new();
     private readonly ConcurrentDictionary<string, AgentIdentity> _observedAgentsByMessageId = new(StringComparer.Ordinal);
     private readonly StreamingTranscriptBuffer _transcriptBuffer = new();
     private int _fallbackMessageIndex;
+    private string? _lastObservedMessageId;
 
     public CopilotTurnExecutionState(RunTurnCommandDto command)
     {
@@ -79,15 +81,34 @@ internal sealed class CopilotTurnExecutionState
             case AssistantMessageEvent assistantMessage when !string.IsNullOrWhiteSpace(assistantMessage.Data?.MessageId):
                 RecordObservedAgentForMessage(agent, assistantMessage.Data!.MessageId);
                 QueueThinkingIfNeeded(agent);
+                if (assistantMessage.Data?.ToolRequests is { Length: > 0 })
+                {
+                    QueueMessageReclassifiedIfNeeded(assistantMessage.Data.MessageId);
+                }
                 break;
             case ToolExecutionStartEvent toolExecutionStart
                 when !string.IsNullOrWhiteSpace(toolExecutionStart.Data?.ToolCallId)
                     && !string.IsNullOrWhiteSpace(toolExecutionStart.Data?.ToolName):
                 ToolNamesByCallId[toolExecutionStart.Data.ToolCallId.Trim()] = toolExecutionStart.Data.ToolName.Trim();
+                QueueMessageReclassifiedIfNeeded(_lastObservedMessageId);
                 break;
-            case AssistantReasoningDeltaEvent:
+            case AssistantIntentEvent intentEvent:
                 ActiveAgent = agent;
                 QueueThinkingIfNeeded(agent);
+                AssistantIntentEventDto? assistantIntent = CreateAssistantIntentEvent(agent, intentEvent.Data);
+                if (assistantIntent is not null)
+                {
+                    _pendingEvents.Enqueue(assistantIntent);
+                }
+                break;
+            case AssistantReasoningDeltaEvent reasoningDelta:
+                ActiveAgent = agent;
+                QueueThinkingIfNeeded(agent);
+                ReasoningDeltaEventDto? reasoningDeltaEvent = CreateReasoningDeltaEvent(agent, reasoningDelta.Data);
+                if (reasoningDeltaEvent is not null)
+                {
+                    _pendingEvents.Enqueue(reasoningDeltaEvent);
+                }
                 break;
             case SubagentStartedEvent started:
                 ActiveAgent = agent;
@@ -218,6 +239,23 @@ internal sealed class CopilotTurnExecutionState
     {
         ActiveAgent = agent;
         _observedAgentsByMessageId[messageId] = agent;
+        _lastObservedMessageId = messageId;
+    }
+
+    private void QueueMessageReclassifiedIfNeeded(string? messageId)
+    {
+        if (string.IsNullOrWhiteSpace(messageId))
+        {
+            return;
+        }
+
+        string normalizedMessageId = messageId.Trim();
+        if (!_reclassifiedMessageIds.Add(normalizedMessageId))
+        {
+            return;
+        }
+
+        _pendingEvents.Enqueue(CreateMessageReclassifiedEvent(normalizedMessageId));
     }
 
     private AgentActivityEventDto? CreateThinkingActivityIfNeeded(AgentIdentity agent)
@@ -240,6 +278,18 @@ internal sealed class CopilotTurnExecutionState
         };
     }
 
+    private MessageReclassifiedEventDto CreateMessageReclassifiedEvent(string messageId)
+    {
+        return new MessageReclassifiedEventDto
+        {
+            Type = "message-reclassified",
+            RequestId = _command.RequestId,
+            SessionId = _command.SessionId,
+            MessageId = messageId,
+            NewKind = "thinking",
+        };
+    }
+
     public void UpdateCompletedMessages(
         IReadOnlyList<ChatMessage> allMessages,
         IReadOnlyList<ChatMessage> inputMessages)
@@ -354,6 +404,50 @@ internal sealed class CopilotTurnExecutionState
         };
     }
 
+    private AssistantIntentEventDto? CreateAssistantIntentEvent(
+        AgentIdentity agent,
+        AssistantIntentData? data)
+    {
+        string? intent = data?.Intent?.Trim();
+        if (string.IsNullOrWhiteSpace(intent))
+        {
+            return null;
+        }
+
+        return new AssistantIntentEventDto
+        {
+            Type = "assistant-intent",
+            RequestId = _command.RequestId,
+            SessionId = _command.SessionId,
+            AgentId = agent.AgentId,
+            AgentName = agent.AgentName,
+            Intent = intent,
+        };
+    }
+
+    private ReasoningDeltaEventDto? CreateReasoningDeltaEvent(
+        AgentIdentity agent,
+        AssistantReasoningDeltaData? data)
+    {
+        if (data is null
+            || string.IsNullOrWhiteSpace(data.ReasoningId)
+            || string.IsNullOrEmpty(data.DeltaContent))
+        {
+            return null;
+        }
+
+        return new ReasoningDeltaEventDto
+        {
+            Type = "reasoning-delta",
+            RequestId = _command.RequestId,
+            SessionId = _command.SessionId,
+            AgentId = agent.AgentId,
+            AgentName = agent.AgentName,
+            ReasoningId = data.ReasoningId,
+            ContentDelta = data.DeltaContent,
+        };
+    }
+
     private SkillInvokedEventDto CreateSkillInvokedEvent(
         AgentIdentity agent,
         SkillInvokedData? data)

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

@@ -83,6 +83,132 @@ public sealed class CopilotTurnExecutionStateTests
         Assert.Equal("view", toolName);
     }
 
+    [Fact]
+    public void ObserveSessionEvent_AssistantMessageWithToolRequests_QueuesMessageReclassifiedEvent()
+    {
+        RunTurnCommandDto command = CreateCommand();
+        CopilotTurnExecutionState state = new(command);
+
+        state.ObserveSessionEvent(
+            command.Pattern.Agents[0],
+            SessionEvent.FromJson(
+                """
+                {
+                  "type": "assistant.message",
+                  "data": {
+                    "messageId": "msg-2",
+                    "content": "Let me search for that.",
+                    "toolRequests": [
+                      {
+                        "toolCallId": "tool-call-1",
+                        "name": "rg",
+                        "arguments": {
+                          "pattern": "identifierUri"
+                        }
+                      }
+                    ]
+                  },
+                  "id": "3f75988b-8e69-4c90-a203-6b01d1c1f90b",
+                  "timestamp": "2026-03-27T00:00:00Z"
+                }
+                """));
+
+        IReadOnlyList<SidecarEventDto> pending = state.DrainPendingEvents();
+
+        AgentActivityEventDto thinking = Assert.Single(pending.OfType<AgentActivityEventDto>());
+        Assert.Equal("thinking", thinking.ActivityType);
+
+        MessageReclassifiedEventDto reclassified = Assert.Single(pending.OfType<MessageReclassifiedEventDto>());
+        Assert.Equal("session-1", reclassified.SessionId);
+        Assert.Equal("msg-2", reclassified.MessageId);
+        Assert.Equal("thinking", reclassified.NewKind);
+    }
+
+    [Fact]
+    public void ObserveSessionEvent_ToolExecutionStart_ReclassifiesLastObservedMessageOnce()
+    {
+        RunTurnCommandDto command = CreateCommand();
+        CopilotTurnExecutionState state = new(command);
+
+        state.ObserveSessionEvent(
+            command.Pattern.Agents[0],
+            SessionEvent.FromJson(
+                """
+                {
+                  "type": "assistant.message_delta",
+                  "data": {
+                    "messageId": "msg-3",
+                    "deltaContent": "Searching"
+                  },
+                  "id": "0b65f0e9-d0fb-417e-ab5c-7a3343d8581b",
+                  "timestamp": "2026-03-27T00:00:00Z"
+                }
+                """));
+        _ = state.DrainPendingEvents();
+
+        state.ObserveSessionEvent(
+            command.Pattern.Agents[0],
+            SessionEvent.FromJson(
+                """
+                {
+                  "type": "tool.execution_start",
+                  "data": {
+                    "toolCallId": "tool-call-1",
+                    "toolName": "rg"
+                  },
+                  "id": "8f33240e-bd3f-475c-aeb6-a4b7908e47b0",
+                  "timestamp": "2026-03-27T00:00:01Z"
+                }
+                """));
+        state.ObserveSessionEvent(
+            command.Pattern.Agents[0],
+            SessionEvent.FromJson(
+                """
+                {
+                  "type": "tool.execution_start",
+                  "data": {
+                    "toolCallId": "tool-call-2",
+                    "toolName": "view"
+                  },
+                  "id": "a23f9c9a-f947-4282-866d-f599451c3899",
+                  "timestamp": "2026-03-27T00:00:02Z"
+                }
+                """));
+
+        IReadOnlyList<SidecarEventDto> pending = state.DrainPendingEvents();
+
+        MessageReclassifiedEventDto reclassified = Assert.Single(pending.OfType<MessageReclassifiedEventDto>());
+        Assert.Equal("msg-3", reclassified.MessageId);
+        Assert.True(state.ToolNamesByCallId.TryGetValue("tool-call-1", out string? firstToolName));
+        Assert.Equal("rg", firstToolName);
+        Assert.True(state.ToolNamesByCallId.TryGetValue("tool-call-2", out string? secondToolName));
+        Assert.Equal("view", secondToolName);
+    }
+
+    [Fact]
+    public void ObserveSessionEvent_AssistantMessageWithoutToolRequests_DoesNotQueueMessageReclassifiedEvent()
+    {
+        RunTurnCommandDto command = CreateCommand();
+        CopilotTurnExecutionState state = new(command);
+
+        state.ObserveSessionEvent(
+            command.Pattern.Agents[0],
+            SessionEvent.FromJson(
+                """
+                {
+                  "type": "assistant.message",
+                  "data": {
+                    "messageId": "msg-4",
+                    "content": "Final answer."
+                  },
+                  "id": "d07fe954-1258-4f6a-bf79-1550d6143ed0",
+                  "timestamp": "2026-03-27T00:00:00Z"
+                }
+                """));
+
+        Assert.Empty(state.DrainPendingEvents().OfType<MessageReclassifiedEventDto>());
+    }
+
     [Fact]
     public async Task EmitThinkingIfNeeded_DoesNotDuplicateQueuedThinkingActivity()
     {
@@ -119,6 +245,70 @@ public sealed class CopilotTurnExecutionStateTests
         Assert.Equal("agent-1", thinking.AgentId);
     }
 
+    [Fact]
+    public void ObserveSessionEvent_AssistantIntent_QueuesIntentEvent()
+    {
+        RunTurnCommandDto command = CreateCommand();
+        CopilotTurnExecutionState state = new(command);
+
+        state.ObserveSessionEvent(
+            command.Pattern.Agents[0],
+            SessionEvent.FromJson(
+                """
+                {
+                  "type": "assistant.intent",
+                  "data": {
+                    "intent": "Searching incident playbooks"
+                  },
+                  "id": "64cf59fe-63f0-4217-adf4-9bd6b3a80452",
+                  "timestamp": "2026-03-27T00:00:00Z"
+                }
+                """));
+
+        IReadOnlyList<SidecarEventDto> pending = state.DrainPendingEvents();
+
+        AgentActivityEventDto thinking = Assert.Single(pending.OfType<AgentActivityEventDto>());
+        Assert.Equal("thinking", thinking.ActivityType);
+
+        AssistantIntentEventDto intent = Assert.Single(pending.OfType<AssistantIntentEventDto>());
+        Assert.Equal("session-1", intent.SessionId);
+        Assert.Equal("agent-1", intent.AgentId);
+        Assert.Equal("Searching incident playbooks", intent.Intent);
+    }
+
+    [Fact]
+    public void ObserveSessionEvent_AssistantReasoningDelta_QueuesReasoningDeltaEvent()
+    {
+        RunTurnCommandDto command = CreateCommand();
+        CopilotTurnExecutionState state = new(command);
+
+        state.ObserveSessionEvent(
+            command.Pattern.Agents[0],
+            SessionEvent.FromJson(
+                """
+                {
+                  "type": "assistant.reasoning_delta",
+                  "data": {
+                    "reasoningId": "reasoning-2",
+                    "deltaContent": "Searching logs."
+                  },
+                  "id": "bd269258-5e5d-46b6-bf3f-bd8cba793b1a",
+                  "timestamp": "2026-03-27T00:00:00Z"
+                }
+                """));
+
+        IReadOnlyList<SidecarEventDto> pending = state.DrainPendingEvents();
+
+        AgentActivityEventDto thinking = Assert.Single(pending.OfType<AgentActivityEventDto>());
+        Assert.Equal("thinking", thinking.ActivityType);
+
+        ReasoningDeltaEventDto reasoning = Assert.Single(pending.OfType<ReasoningDeltaEventDto>());
+        Assert.Equal("session-1", reasoning.SessionId);
+        Assert.Equal("agent-1", reasoning.AgentId);
+        Assert.Equal("reasoning-2", reasoning.ReasoningId);
+        Assert.Equal("Searching logs.", reasoning.ContentDelta);
+    }
+
     [Fact]
     public void DrainPendingMcpOauthRequests_ReturnsQueuedRequestsAndClearsQueue()
     {

src/shared/contracts/sidecar.ts

@@ -239,6 +239,14 @@ export interface TurnCompleteEvent {
   cancelled?: boolean;
 }
 
+export interface MessageReclassifiedEvent {
+  type: 'message-reclassified';
+  requestId: string;
+  sessionId: string;
+  messageId: string;
+  newKind: 'thinking';
+}
+
 export type AgentActivityType = 'thinking' | 'tool-calling' | 'handoff' | 'completed';
 
 export interface ToolCallFileChangePreview {
@@ -297,6 +305,25 @@ export interface SkillInvokedEvent {
   description?: string;
 }
 
+export interface AssistantIntentEvent {
+  type: 'assistant-intent';
+  requestId: string;
+  sessionId: string;
+  agentId?: string;
+  agentName?: string;
+  intent: string;
+}
+
+export interface ReasoningDeltaEvent {
+  type: 'reasoning-delta';
+  requestId: string;
+  sessionId: string;
+  agentId?: string;
+  agentName?: string;
+  reasoningId: string;
+  contentDelta: string;
+}
+
 export interface HookLifecycleEvent {
   type: 'hook-lifecycle';
   requestId: string;
@@ -526,9 +553,12 @@ export type SidecarEvent =
   | PatternValidationEvent
   | TurnDeltaEvent
   | TurnCompleteEvent
+  | MessageReclassifiedEvent
   | AgentActivityEvent
   | SubagentEvent
   | SkillInvokedEvent
+  | AssistantIntentEvent
+  | ReasoningDeltaEvent
   | HookLifecycleEvent
   | SessionUsageEvent
   | SessionCompactionEvent

src/shared/domain/event.ts

@@ -1,4 +1,5 @@
 import type { SessionRunRecord } from '@shared/domain/runTimeline';
+import type { ChatMessageKind } from '@shared/domain/session';
 
 import type { QuotaSnapshot, ToolCallFileChangePreview } from '@shared/contracts/sidecar';
 
@@ -8,6 +9,7 @@ export type SessionEventKind =
   | 'status'
   | 'message-delta'
   | 'message-complete'
+  | 'message-reclassified'
   | 'agent-activity'
   | 'run-updated'
   | 'error'
@@ -27,6 +29,7 @@ export interface SessionEventRecord {
   occurredAt: string;
   status?: 'idle' | 'running' | 'error';
   messageId?: string;
+  messageKind?: ChatMessageKind;
   authorName?: string;
   contentDelta?: string;
   content?: string;

src/shared/domain/session.ts

@@ -18,6 +18,7 @@ import type { ChatMessageAttachment } from '@shared/domain/attachment';
 import type { InteractionMode } from '@shared/contracts/sidecar';
 
 export type ChatRole = 'system' | 'user' | 'assistant';
+export type ChatMessageKind = 'response' | 'thinking';
 export type SessionStatus = 'idle' | 'running' | 'error';
 export type SessionTitleSource = 'auto' | 'manual';
 export type SessionBranchOriginAction = 'branch' | 'regenerate' | 'edit-and-resend';
@@ -33,6 +34,7 @@ export interface ChatMessageRecord {
   authorName: string;
   content: string;
   createdAt: string;
+  messageKind?: ChatMessageKind;
   isPinned?: boolean;
   pending?: boolean;
   attachments?: ChatMessageAttachment[];