diff --git a/src/Netclaw.Actors/Protocol/SessionOutputDtoMapper.cs b/src/Netclaw.Actors/Protocol/SessionOutputDtoMapper.cs
index 0a60f7565..d79ce6856 100644
--- a/src/Netclaw.Actors/Protocol/SessionOutputDtoMapper.cs
+++ b/src/Netclaw.Actors/Protocol/SessionOutputDtoMapper.cs
@@ -349,11 +349,15 @@ public static SessionOutput FromDto(SessionOutputDto dto)
                 IsMessy = dto.InteractionIsMessy ?? false,
                 Options = dto.InteractionOptions ?? []
             },
+            // The client build does not know dto.Type. This is a protocol
+            // read failure, not a session error — flag it so a channel
+            // adapter keeps it off any machine-readable result envelope.
             _ => new ErrorOutput
             {
                 SessionId = sessionId,
                 TimestampMs = dto.TimestampMs,
-                Message = $"Unknown output type from daemon: {dto.Type}"
+                Message = $"Unknown output type from daemon: {dto.Type}",
+                IsProtocolDiagnostic = true
             }
             };
     }
diff --git a/src/Netclaw.Actors/Sessions/SessionProtocol.Outputs.cs b/src/Netclaw.Actors/Sessions/SessionProtocol.Outputs.cs
index 8fa0e4612..30fd69f88 100644
--- a/src/Netclaw.Actors/Sessions/SessionProtocol.Outputs.cs
+++ b/src/Netclaw.Actors/Sessions/SessionProtocol.Outputs.cs
@@ -226,6 +226,16 @@ public sealed record ErrorOutput : SessionOutput
         /// for diagnostic logging by subscribers and adapters.
         /// </summary>
         public Exception? Cause { get; init; }
+
+        /// <summary>
+        /// True when this instance is not a real session error. The client
+        /// mapper sets it when it cannot read a wire message from the daemon
+        /// (for example, an output type the client build does not know).
+        /// A channel adapter must keep a protocol diagnostic out of any
+        /// machine-readable result envelope and must not report it as a turn
+        /// error. Defaults to false for every daemon-originated error.
+        /// </summary>
+        public bool IsProtocolDiagnostic { get; init; }
     }
 
     /// <summary>
diff --git a/src/Netclaw.Cli.Tests/Cli/DaemonClientMappingTests.cs b/src/Netclaw.Cli.Tests/Cli/DaemonClientMappingTests.cs
index 2bf5926ee..f201c046f 100644
--- a/src/Netclaw.Cli.Tests/Cli/DaemonClientMappingTests.cs
+++ b/src/Netclaw.Cli.Tests/Cli/DaemonClientMappingTests.cs
@@ -91,6 +91,43 @@ public void FromDto_unknown_type_becomes_error_output()
         Assert.Equal("signalr/test", error.SessionId.Value);
     }
 
+    [Fact]
+    public void FromDto_unknown_type_flags_a_protocol_diagnostic_not_a_session_error()
+    {
+        // A daemon and CLI on different builds can disagree on the output
+        // type set (for example a newer daemon that streams "tool_activity").
+        // The mapper must mark this a client read failure, not session
+        // output, so a channel adapter never reports it as a turn error.
+        var dto = new SessionOutputDto
+        {
+            Type = "tool_activity",
+            SessionId = "signalr/test",
+            TimestampMs = 123
+        };
+
+        var output = DaemonClient.FromDto(dto);
+
+        var error = Assert.IsType<ErrorOutput>(output);
+        Assert.True(error.IsProtocolDiagnostic);
+    }
+
+    [Fact]
+    public void FromDto_daemon_error_output_is_not_a_protocol_diagnostic()
+    {
+        var dto = new SessionOutputDto
+        {
+            Type = "error",
+            SessionId = "signalr/test",
+            TimestampMs = 123,
+            ErrorMessage = "The provider returned a 500."
+        };
+
+        var output = DaemonClient.FromDto(dto);
+
+        var error = Assert.IsType<ErrorOutput>(output);
+        Assert.False(error.IsProtocolDiagnostic);
+    }
+
     [Fact]
     public void FromDto_maps_session_joined_with_recent_messages()
     {
diff --git a/src/Netclaw.Cli.Tests/Cli/HeadlessChannelJsonStdoutHygieneTests.cs b/src/Netclaw.Cli.Tests/Cli/HeadlessChannelJsonStdoutHygieneTests.cs
new file mode 100644
index 000000000..d999c6ee6
--- /dev/null
+++ b/src/Netclaw.Cli.Tests/Cli/HeadlessChannelJsonStdoutHygieneTests.cs
@@ -0,0 +1,152 @@
+// -----------------------------------------------------------------------
+// <copyright file="HeadlessChannelJsonStdoutHygieneTests.cs" company="Petabridge, LLC">
+//      Copyright (C) 2026 - 2026 Petabridge, LLC <https://petabridge.com>
+// </copyright>
+// -----------------------------------------------------------------------
+using System.Text.Json;
+using Microsoft.Extensions.Hosting;
+using Microsoft.Extensions.Logging;
+using Microsoft.Extensions.Time.Testing;
+using Netclaw.Actors.Protocol;
+using Netclaw.Cli.Daemon;
+using Netclaw.Configuration;
+using Netclaw.Tests.Utilities;
+using Xunit;
+
+namespace Netclaw.Cli.Tests.Cli;
+
+/// <summary>
+/// Proves that <c>chat -p --json</c> keeps stdout pure JSON when the daemon
+/// sends an output type this CLI build does not recognize (for example a
+/// newer daemon streaming "tool_activity" to an older client). The client
+/// mapper turns the unrecognized type into a diagnostic <c>ErrorOutput</c>
+/// (<see cref="SessionOutputDtoMapper.FromDto"/>); this suite proves the
+/// headless channel keeps that diagnostic off stdout, still surfaces it
+/// (stderr + logger), and still emits a parseable JSON envelope.
+/// </summary>
+[Collection("Update verification")]
+public sealed class HeadlessChannelJsonStdoutHygieneTests : IDisposable
+{
+    private static readonly TimeSpan[] ImmediateDelays = [TimeSpan.Zero];
+
+    private readonly DisposableTempDir _dir = new();
+    private readonly TextWriter _originalOut = Console.Out;
+    private readonly TextWriter _originalError = Console.Error;
+
+    public void Dispose()
+    {
+        Console.SetOut(_originalOut);
+        Console.SetError(_originalError);
+        _dir.Dispose();
+    }
+
+    [Fact]
+    public async Task Unrecognized_output_type_stays_off_stdout_and_envelope_stays_parseable()
+    {
+        var transport = new FakeDaemonHubTransport();
+
+        // Model a daemon that streams an output type this CLI build does
+        // not know about (e.g. "tool_activity") ahead of the real reply.
+        transport.VoidInvokeHook = (method, args, _) =>
+        {
+            if (method == "SendMessage")
+            {
+                var sessionId = (string)args[0]!;
+                transport.PushOutput(new SessionOutputDto
+                {
+                    Type = "tool_activity",
+                    SessionId = sessionId,
+                    TimestampMs = 1
+                });
+                transport.PushOutput(new SessionOutputDto
+                {
+                    Type = "text",
+                    SessionId = sessionId,
+                    TimestampMs = 2,
+                    Text = "hello"
+                });
+                transport.PushOutput(new SessionOutputDto
+                {
+                    Type = "turn_completed",
+                    SessionId = sessionId,
+                    TimestampMs = 3,
+                    TurnNumber = new TurnNumber(1)
+                });
+            }
+
+            return Task.CompletedTask;
+        };
+
+        await using var daemonClient = new DaemonClient(
+            "http://localhost",
+            transport,
+            reconnectDelays: ImmediateDelays,
+            rpcTimeout: TimeSpan.FromSeconds(5));
+
+        var paths = new NetclawPaths(_dir.Path);
+        var lifetime = new RecordingHostLifetime();
+        var logger = new RecordingLogger<HeadlessChannel>();
+        var options = new HeadlessOptions("hi") { JsonOutput = true };
+
+        var channel = new HeadlessChannel(
+            daemonClient, paths, lifetime, new FakeTimeProvider(), options, logger);
+
+        var stdout = new StringWriter();
+        var stderr = new StringWriter();
+        Console.SetOut(stdout);
+        Console.SetError(stderr);
+
+        await channel.StartAsync(TestContext.Current.CancellationToken);
+        await lifetime.StopRequested.Task.WaitAsync(TimeSpan.FromSeconds(5), TestContext.Current.CancellationToken);
+
+        var stdoutText = stdout.ToString();
+        var stderrText = stderr.ToString();
+
+        // Stdout carries exactly the JSON envelope — nothing else.
+        var envelope = JsonSerializer.Deserialize<JsonElement>(stdoutText.Trim());
+        Assert.Equal("hello", envelope.GetProperty("response").GetString());
+        Assert.DoesNotContain("Unknown output type", stdoutText, StringComparison.Ordinal);
+        Assert.DoesNotContain("[diagnostic]", stdoutText, StringComparison.Ordinal);
+        Assert.DoesNotContain("[error]", stdoutText, StringComparison.Ordinal);
+
+        // The diagnostic is not silently dropped — stderr and the logger both see it.
+        Assert.Contains("Unknown output type from daemon: tool_activity", stderrText, StringComparison.Ordinal);
+        Assert.Contains(logger.Messages, m => m.Contains("tool_activity", StringComparison.Ordinal));
+        Assert.Contains(logger.Levels, l => l == LogLevel.Warning);
+    }
+
+    private sealed class RecordingHostLifetime : IHostApplicationLifetime
+    {
+        public TaskCompletionSource StopRequested { get; } = new(TaskCreationOptions.RunContinuationsAsynchronously);
+
+        public CancellationToken ApplicationStarted => CancellationToken.None;
+
+        public CancellationToken ApplicationStopping { get; } = CancellationToken.None;
+
+        public CancellationToken ApplicationStopped => CancellationToken.None;
+
+        public void StopApplication() => StopRequested.TrySetResult();
+    }
+
+    private sealed class RecordingLogger<T> : ILogger<T>
+    {
+        public List<string> Messages { get; } = [];
+
+        public List<LogLevel> Levels { get; } = [];
+
+        public IDisposable? BeginScope<TState>(TState state) where TState : notnull => null;
+
+        public bool IsEnabled(LogLevel logLevel) => true;
+
+        public void Log<TState>(
+            LogLevel logLevel,
+            EventId eventId,
+            TState state,
+            Exception? exception,
+            Func<TState, Exception?, string> formatter)
+        {
+            Levels.Add(logLevel);
+            Messages.Add(formatter(state, exception));
+        }
+    }
+}
diff --git a/src/Netclaw.Cli/HeadlessChannel.cs b/src/Netclaw.Cli/HeadlessChannel.cs
index 5e80a2885..9f1d373a9 100644
--- a/src/Netclaw.Cli/HeadlessChannel.cs
+++ b/src/Netclaw.Cli/HeadlessChannel.cs
@@ -269,6 +269,17 @@ private void HandleOutput(SessionOutput output, StreamWriter? log)
                 Log(log, $"USAGE: in={msg.InputTokens} out={msg.OutputTokens} total={msg.TotalTokens} cached={msg.CachedInputTokens} reasoning={msg.ReasoningTokens} context_window={msg.ContextWindowTokens} prompt_ms={msg.PromptMs} predicted_tok_s={msg.PredictedPerSecond}");
                 break;
 
+            case ErrorOutput { IsProtocolDiagnostic: true } msg:
+                // The client could not read a daemon wire message (unknown
+                // output type). This is not a turn result, so it must never
+                // reach stdout — json mode stdout carries only the envelope.
+                // It still must not go silent: stderr, the session log, and
+                // the structured logger all see it.
+                _logger.LogWarning("Daemon protocol diagnostic: {Message}", msg.Message);
+                Console.Error.WriteLine($"[diagnostic] {msg.Message}");
+                Log(log, $"DIAGNOSTIC: {msg.Message}");
+                break;
+
             case ErrorOutput msg:
                 Console.Error.WriteLine($"[error] {msg.Message}");
                 Log(log, $"ERROR: {msg.Message}");