diff --git a/modules/composition/composition.tests/commands/BtwCommandTests.cs b/modules/composition/composition.tests/commands/BtwCommandTests.cs
new file mode 100644
index 0000000000..cbf31f293b
--- /dev/null
+++ b/modules/composition/composition.tests/commands/BtwCommandTests.cs
@@ -0,0 +1,105 @@
+namespace Composition.Tests.Commands;
+
+/// <summary>
+/// /btw 隔离 fork 测试（对齐 TS runSideQuestion→runForkedAgent 语义）—
+/// 主历史副本做前缀 + 独立 AgentBase 单轮执行（MaxIterations=1）+ 不写回主上下文
+///（ContextManager 不注入天然隔离）；修复背景：原 ChatService 共享管道受主对话会话态
+/// 影响偶发空响应。IQueryEngine 假体直证 fork 请求消息形态。
+/// </summary>
+public sealed class BtwCommandTests {
+    /// <summary>假 IQueryEngine — 捕获收到的 chatHistory，返回固定单 content chunk。</summary>
+    private sealed class CaptureQueryEngine : IQueryEngine {
+        public MessageList? CapturedHistory { get; private set; }
+        public string CapturedPrompt { get; private set; } = string.Empty;
+
+        public Task<string> ExecuteQueryAsync(string query, CancellationToken cancellationToken = default)
+            => Task.FromResult("侧边回答");
+
+        public async IAsyncEnumerable<QueryStreamChunk> QueryAsync(
+            string userInput, MessageList chatHistory,
+            [System.Runtime.CompilerServices.EnumeratorCancellation] CancellationToken cancellationToken = default) {
+            CapturedPrompt = userInput;
+            CapturedHistory = chatHistory;
+            await Task.Yield();
+            yield return new QueryStreamChunk { Type = AgentStreamChunkType.Content, Content = "这是侧边回答。" };
+        }
+
+        public IAsyncEnumerable<QueryStreamChunk> QueryAsync(
+            string userInput, MessageList chatHistory, QueryOptions? options, CancellationToken cancellationToken = default)
+            => QueryAsync(userInput, chatHistory, cancellationToken);
+
+        public JoinCode.Abstractions.LLM.IQueryService GetChatCompletionService()
+            => throw new NotSupportedException("测试假体不提供查询服务");
+
+        public JoinCode.Abstractions.LLM.IChatClient GetKernel()
+            => throw new NotSupportedException("测试假体不提供 kernel");
+    }
+
+    private static ChatCommandContext CreateContext(IQueryEngine queryEngine, string arguments = "你好") {
+        var services = new ServiceCollection();
+        services.AddSingleton(queryEngine);
+        var commandServices = new CommandServices {
+            ChatService = Mock.Of<IChatService>(),
+            CodeService = Mock.Of<ICodeService>(),
+            PlanService = Mock.Of<IPlanService>(),
+            FileSystem = TestFileSystem.Current,
+            ServiceProvider = services.BuildServiceProvider(),
+        };
+        return new ChatCommandContext {
+            Arguments = arguments,
+            CancellationToken = CancellationToken.None,
+            Services = new CommandServiceProvider(commandServices),
+        };
+    }
+
+    /// <summary>fork 调用 — 返回 QueryEngine 内容 chunk 聚合文本。</summary>
+    [Fact]
+    public async Task RunSideQuestion_AggregatesContentChunks() {
+        var engine = new CaptureQueryEngine();
+        var context = CreateContext(engine);
+
+        var response = await BtwCommand.RunSideQuestionAsync(context, "你好");
+
+        response.Should().Be("这是侧边回答。");
+    }
+
+    /// <summary>fork 消息形态 — 历史含 system-reminder 包裹的问题（对齐 TS wrappedQuestion）。</summary>
+    [Fact]
+    public async Task RunSideQuestion_WrapsQuestionWithReminder() {
+        var engine = new CaptureQueryEngine();
+        var context = CreateContext(engine);
+
+        await BtwCommand.RunSideQuestionAsync(context, "什么是 SOLID?");
+
+        engine.CapturedHistory.Should().NotBeNull();
+        Assert.NotNull(engine.CapturedHistory);
+        var history = engine.CapturedHistory;
+        history.Count.Should().Be(1);
+        var last = history[history.Count - 1];
+        last.Role.Should().Be(MessageRole.User);
+        last.Content.Should().Contain("侧边问题");
+        last.Content.Should().Contain("什么是 SOLID?");
+        last.Content.Should().Contain("没有任何可用工具");
+    }
+
+    /// <summary>空参 — 用法提示不调引擎（输出捕获经 Console.SetOut，不经 TerminalHelper 门面 —
+    /// 测试项目无 cli 域引用，SetOut 与命令侧 TerminalHelper.WriteLine 底层同为 Console.Out）。</summary>
+    [Fact]
+    public async Task Execute_EmptyQuestion_ShowsUsageWithoutEngineCall() {
+        var engine = new CaptureQueryEngine();
+        var context = CreateContext(engine, arguments: "");
+        var output = new System.Text.StringBuilder();
+        var originalOut = System.Console.Out;
+        await using var writer = new StringWriter(output);
+        System.Console.SetOut(writer);
+        try {
+            var cmd = new BtwCommand();
+            await cmd.ExecuteAsync(context);
+        } finally {
+            System.Console.SetOut(originalOut);
+        }
+
+        engine.CapturedHistory.Should().BeNull("空参不应触发 fork 调用");
+        output.ToString().Should().Contain("用法: /btw");
+    }
+}
diff --git a/modules/composition/slash/GlobalUsings.cs b/modules/composition/slash/GlobalUsings.cs
index b66d602087..b5561d485c 100644
--- a/modules/composition/slash/GlobalUsings.cs
+++ b/modules/composition/slash/GlobalUsings.cs
@@ -1 +1,2 @@
 global using System.Diagnostics.CodeAnalysis;
+global using Core.Agents.Coordinator;
diff --git a/modules/composition/slash/system/social/BtwCommand.cs b/modules/composition/slash/system/social/BtwCommand.cs
index 0349b6a2d6..b8eb13fed1 100644
--- a/modules/composition/slash/system/social/BtwCommand.cs
+++ b/modules/composition/slash/system/social/BtwCommand.cs
@@ -1,16 +1,38 @@
 namespace JoinCode.ChatCommands;
 
 /// <summary>
-/// /btw 命令 — 对齐 TS btw.ts
-/// TS 使用 sideQuestion 模式发送独立请求，C# 使用 ChatService.SendMessageAsync
-/// 对齐内容：侧边提问不影响主对话上下文
-/// 架构差异：TS 有 React 侧边栏渲染，C# 为终端文本输出
+/// /btw 命令 — 对齐 TS btw.ts + sideQuestion.ts + forkedAgent.ts：
+/// TS 用 runForkedAgent（隔离轻量 fork 代理：父上下文消息做前缀共享缓存、独立实例、
+/// canUseTool 全拒、maxTurns=1、不写回父上下文）；C# 照抄同语义 — AgentBase +
+/// Options.InitialMessageList（主历史副本 + 问题）+ MaxIterations=1 + ContextManager 不注入
+///（天然隔离：不碰主上下文状态、不写回历史）。修复背景：原实现走 ChatService 共享管道，
+/// 受主对话会话态影响偶发空响应（CLI/探针实证链路本身健康，共享态是架构 gap）。
 /// </summary>
 [ChatCommand(Name = ChatCommandNameEnumConstants.Btw, Description = "快速向 AI 提一个侧边问题", Usage = "/btw <question>", Category = ChatCommandCategory.Social, ArgumentHint = "<question>")]
 [ChatCommandArg("question", Type = "string", Description = "侧边问题内容", Required = true)]
 public sealed class BtwCommand : ChatCommandBase {
+    /// <summary>侧边问题 system-reminder 模板（对齐 TS runSideQuestion wrappedQuestion：隔离代理身份 + 无工具约束 + 单次回复）。</summary>
+    private const string SideQuestionReminder = """
+此为用户的侧边问题，请直接在单次回复中回答。
+
+重要背景：
+- 你是为回答这一个问题而生成的独立轻量代理
+- 主对话代理未被中断，仍在后台独立运行
+- 你共享对话上下文，但是完全独立的实例
+- 不要提及被打断或"之前正在做什么"——此表述不正确
+
+关键约束：
+- 你没有任何可用工具——不能读文件、执行命令、搜索或采取任何行动
+- 这是一次性回复——没有后续轮次
+- 只能基于对话上下文中已有的信息作答
+- 绝不说"让我试试"、"我将检查"或承诺采取任何行动
+- 不知道答案就直说——不要提出去查证或调查
+
+请直接用你已有的信息回答问题。
+""";
+
     /// <summary>
-    /// 执行 /btw 命令 — 将侧边问题发送给 AI，不影响主对话上下文
+    /// 执行 /btw 命令 — 隔离 fork 代理回答侧边问题，不影响主对话上下文（对齐 TS runSideQuestion）
     /// </summary>
     /// <param name="context">命令执行上下文，包含问题文本与取消令牌</param>
     /// <returns>命令执行结果（始终为 Continue，表示不中断主对话流）</returns>
@@ -27,16 +49,10 @@ public override async Task<ChatCommandResult> ExecuteAsync(ChatCommandContext co
             return ChatCommandResult.Continue();
         }
 
-        var prompt = $"""
-This is a quick side question that should not disrupt the main conversation context. Answer concisely.
-
-Question: {question}
-""";
-
         try {
             TerminalHelper.WriteLine($"{TerminalColors.Muted}── 侧边问题 ──{AnsiStyleEnumConstants.Reset}");
-            var result = await context.GetCommandServices().ChatService.SendMessageAsync(prompt, context.CancellationToken).ConfigureAwait(false);
-            TerminalHelper.WriteLine(result);
+            var response = await RunSideQuestionAsync(context, question).ConfigureAwait(false);
+            TerminalHelper.WriteLine(response);
             TerminalHelper.WriteLine($"{TerminalColors.Muted}── 侧边回答结束 ──{AnsiStyleEnumConstants.Reset}");
         } catch (Exception ex) {
             ChatCommandBase.HandleError("侧边提问", ex);
@@ -44,4 +60,44 @@ This is a quick side question that should not disrupt the main conversation cont
 
         return ChatCommandResult.Continue();
     }
-}
\ No newline at end of file
+
+    /// <summary>隔离 fork 侧边问题（对齐 TS runSideQuestion→runForkedAgent）：
+    /// 主历史副本做前缀（共享上下文）+ system-reminder 包裹问题 + 独立 AgentBase 单轮执行
+    ///（MaxIterations=1 对齐 maxTurns:1；ContextManager 不注入 = 不写回主上下文天然隔离）。</summary>
+    internal static async Task<string> RunSideQuestionAsync(ChatCommandContext context, string question) {
+        var provider = context.GetCommandServices().ServiceProvider;
+        var queryEngine = provider?.GetService<IQueryEngine>()
+            ?? throw new InvalidOperationException("IQueryEngine 未注册 — 无法执行侧边问题 fork 调用");
+
+        // fork 前缀 — 主上下文消息副本（隔离：追加问题不写回主历史）
+        var forkHistory = new MessageList();
+        var contextManager = provider?.GetService<IChatContextManager>();
+        if (contextManager is not null) {
+            var parent = await contextManager.GetMessageListAsync(context.CancellationToken).ConfigureAwait(false);
+            foreach (var message in parent) {
+                forkHistory.Add(message);
+            }
+        }
+
+        forkHistory.AddUserMessage($"{SideQuestionReminder}\n\n{question}");
+
+        await using var agent = new AgentBase(
+            task: question,
+            options: new SubAgentOptions {
+                InitialMessageList = forkHistory,
+                MaxIterations = 1,
+            },
+            queryEngine: queryEngine,
+            logger: provider?.GetService<ILogger<AgentBase>>(),
+            name: "side-question");
+
+        var response = new StringBuilder();
+        await foreach (var chunk in agent.ExecuteStreamAsync(context.CancellationToken).ConfigureAwait(false)) {
+            if (chunk.Type == AgentStreamChunkType.Content && chunk.Content is { Length: > 0 }) {
+                response.Append(chunk.Content);
+            }
+        }
+
+        return response.ToString();
+    }
+}
diff --git a/native_tui/apps/NativeTui.Host/Program.cs b/native_tui/apps/NativeTui.Host/Program.cs
index ec05805fd9..1d54608ff7 100644
--- a/native_tui/apps/NativeTui.Host/Program.cs
+++ b/native_tui/apps/NativeTui.Host/Program.cs
@@ -41,7 +41,7 @@ private static async Task<int> RunSelftestTurnAsync() {
 
         var sw = System.Diagnostics.Stopwatch.StartNew();
         session.Client.Submit("只回复两个字母: ok");
-        Console.WriteLine($"[probe] t={sw.ElapsedMilliseconds}ms 提交回合");
+        Console.WriteLine($"[probe] t={sw.ElapsedMilliseconds}ms 提交 /btw");
 
         string? firstThinking = null;
         string? firstToken = null;
@@ -87,6 +87,22 @@ private static async Task<int> RunSelftestTurnAsync() {
         return 1;
     }
 
+    /// <summary>等待当前回合终态（turn_completed/interrupted/error — 排空事件循环）。</summary>
+    private static async Task WaitTurnCompleteAsync(EngineSession session, TimeSpan timeout) {
+        using var cts = new CancellationTokenSource(timeout);
+        try {
+            while (!cts.Token.IsCancellationRequested) {
+                await Task.Delay(100, cts.Token).ConfigureAwait(false);
+                foreach (var evt in session.Client.DrainEvents()) {
+                    if (evt is BridgeEvent.TurnCompleted or BridgeEvent.TurnInterrupted or BridgeEvent.Error) {
+                        return;
+                    }
+                }
+            }
+        } catch (OperationCanceledException) {
+        }
+    }
+
     /// <summary>交互模式 — TTY 检查 → UTF-8 → raw mode → 引擎组装（Console.Out 重定向 stderr 防引擎污染渲染）→ REPL 主循环。</summary>
     private static async Task<int> RunInteractiveAsync() {
         if (Console.IsInputRedirected || Console.IsOutputRedirected) {
diff --git a/test/unit/host.tests/chat_commands/a_to_f/BtwCommandTests.cs b/test/unit/host.tests/chat_commands/a_to_f/BtwCommandTests.cs
index d9eac97021..e9b4d92bc3 100644
--- a/test/unit/host.tests/chat_commands/a_to_f/BtwCommandTests.cs
+++ b/test/unit/host.tests/chat_commands/a_to_f/BtwCommandTests.cs
@@ -78,20 +78,20 @@ public async Task Execute_WithWhitespaceArgs_Should_Show_Usage() {
     }
 
     [Fact]
-    public async Task Execute_WithQuestion_Should_SendMessage() {
+    public async Task Execute_WithQuestion_Should_InvokeForkQueryEngine() {
+        var engine = new CaptureQueryEngine("侧边回答文本");
+        var services = new ServiceCollection();
+        services.AddSingleton<IQueryEngine>(engine);
         var cmd = new BtwCommand();
-        var chatService = new Mock<IChatService>();
-        chatService.Setup(c => c.SendMessageAsync(It.IsAny<string>(), It.IsAny<CancellationToken>()))
-            .ReturnsAsync("Test answer");
-
         var context = new ChatCommandContext {
             Arguments = "test question",
             CancellationToken = CancellationToken.None,
             Services = new CommandServiceProvider(new CommandServices {
-                ChatService = chatService.Object,
+                ChatService = Mock.Of<IChatService>(),
                 CodeService = Mock.Of<ICodeService>(),
                 PlanService = Mock.Of<IPlanService>(),
                 FileSystem = TestFileSystem.Current,
+                ServiceProvider = services.BuildServiceProvider(),
             }),
         };
 
@@ -99,26 +99,26 @@ public async Task Execute_WithQuestion_Should_SendMessage() {
 
         result.ShouldContinue.Should().BeTrue();
         result.IsHandled.Should().BeTrue();
-        chatService.Verify(c => c.SendMessageAsync(
-            It.Is<string>(s => s.Contains("test question")),
-            It.IsAny<CancellationToken>()), Times.Once);
+        engine.QueryCallCount.Should().Be(1, "fork 路径走 IQueryEngine（对齐基准 runForkedAgent），不再走 ChatService.SendMessageAsync");
+        engine.CapturedPrompt.Should().Contain("test question");
     }
 
+    /// <summary>fork 异常路径 — QueryEngine 抛异常时 HandleError 吞掉，命令仍 Continue 不中断主对话流。</summary>
     [Fact]
-    public async Task Execute_WhenSendMessageThrows_Should_Return_Continue() {
+    public async Task Execute_WhenQueryEngineThrows_Should_Return_Continue() {
         var cmd = new BtwCommand();
-        var chatService = new Mock<IChatService>();
-        chatService.Setup(c => c.SendMessageAsync(It.IsAny<string>(), It.IsAny<CancellationToken>()))
-            .ThrowsAsync(new InvalidOperationException("chat error"));
-
+        var engine = new CaptureQueryEngine(throwOnQuery: true);
+        var services = new ServiceCollection();
+        services.AddSingleton<IQueryEngine>(engine);
         var context = new ChatCommandContext {
             Arguments = "test question",
             CancellationToken = CancellationToken.None,
             Services = new CommandServiceProvider(new CommandServices {
-                ChatService = chatService.Object,
+                ChatService = Mock.Of<IChatService>(),
                 CodeService = Mock.Of<ICodeService>(),
                 PlanService = Mock.Of<IPlanService>(),
                 FileSystem = TestFileSystem.Current,
+                ServiceProvider = services.BuildServiceProvider(),
             }),
         };
 
@@ -127,4 +127,36 @@ public async Task Execute_WhenSendMessageThrows_Should_Return_Continue() {
         result.ShouldContinue.Should().BeTrue();
         result.IsHandled.Should().BeTrue();
     }
+
+    /// <summary>捕获型 QueryEngine 假体 — fork 调用计数与 prompt 捕获（IQueryEngine 最小面）。</summary>
+    private sealed class CaptureQueryEngine(string? fixedResponse = null, bool throwOnQuery = false) : IQueryEngine {
+        public int QueryCallCount { get; private set; }
+        public string CapturedPrompt { get; private set; } = string.Empty;
+
+        public Task<string> ExecuteQueryAsync(string query, CancellationToken cancellationToken = default)
+            => Task.FromResult(fixedResponse ?? string.Empty);
+
+        public async IAsyncEnumerable<QueryStreamChunk> QueryAsync(
+            string userInput, MessageList chatHistory,
+            [System.Runtime.CompilerServices.EnumeratorCancellation] CancellationToken cancellationToken = default) {
+            QueryCallCount++;
+            CapturedPrompt = userInput;
+            if (throwOnQuery) {
+                throw new InvalidOperationException("fork query error");
+            }
+
+            await Task.Yield();
+            yield return new QueryStreamChunk { Type = AgentStreamChunkType.Content, Content = fixedResponse ?? string.Empty };
+        }
+
+        public IAsyncEnumerable<QueryStreamChunk> QueryAsync(
+            string userInput, MessageList chatHistory, QueryOptions? options, CancellationToken cancellationToken = default)
+            => QueryAsync(userInput, chatHistory, cancellationToken);
+
+        public JoinCode.Abstractions.LLM.IQueryService GetChatCompletionService()
+            => throw new NotSupportedException("测试假体不提供查询服务");
+
+        public JoinCode.Abstractions.LLM.IChatClient GetKernel()
+            => throw new NotSupportedException("测试假体不提供 kernel");
+    }
 }
\ No newline at end of file