Fix MCP CLI logging: apply colors and respect config log-level

src/Cli.Tests/CustomLoggerTests.cs

@@ -4,78 +4,179 @@
 namespace Cli.Tests;
 
 /// <summary>
-/// Tests for CustomLoggerProvider and CustomConsoleLogger, verifying
-/// that log level labels use ASP.NET Core abbreviated format.
+/// Tests for <see cref="CustomLoggerProvider"/> covering both the standard CLI
+/// path (writes to stdout/stderr with abbreviated labels) and the MCP stdio path
+/// (suppressed by default, opt-in via either CLI <c>--LogLevel</c> or the
+/// runtime config's <c>log-level</c>, always routed to stderr to keep the
+/// JSON-RPC channel on stdout uncorrupted).
 /// </summary>
 [TestClass]
 public class CustomLoggerTests
 {
     /// <summary>
-    /// Validates that each enabled log level produces the correct abbreviated label
-    /// matching ASP.NET Core's default console formatter convention.
-    /// Trace and Debug are below the logger's minimum level and produce no output.
+    /// The CustomConsoleLogger reads several static flags from <see cref="Cli.Utils"/>.
+    /// Reset them around every test so cases cannot leak into each other and so the
+    /// rest of the test suite continues to see the default (non-MCP) behavior.
     /// </summary>
-    [DataTestMethod]
-    [DataRow(LogLevel.Information, "info:")]
-    [DataRow(LogLevel.Warning, "warn:")]
-    public void LogOutput_UsesAbbreviatedLogLevelLabels(LogLevel logLevel, string expectedPrefix)
+    [TestInitialize]
+    [TestCleanup]
+    public void ResetMcpStaticState()
     {
-        CustomLoggerProvider provider = new();
-        ILogger logger = provider.CreateLogger("TestCategory");
+        Cli.Utils.IsMcpStdioMode = false;
+        Cli.Utils.IsLogLevelOverriddenByCli = false;
+        Cli.Utils.IsLogLevelOverriddenByConfig = false;
+        Cli.Utils.CliLogLevel = LogLevel.Information;
+        Cli.Utils.ConfigLogLevel = LogLevel.Information;
+    }
 
+    /// <summary>
+    /// Redirects Console.Out and Console.Error around <paramref name="action"/>
+    /// and returns whatever was written to each. Restores the original writers
+    /// on exit.
+    /// </summary>
+    private static (string Stdout, string Stderr) CaptureConsole(Action action)
+    {
         TextWriter originalOut = Console.Out;
+        TextWriter originalError = Console.Error;
+        StringWriter stdout = new();
+        StringWriter stderr = new();
         try
         {
-            StringWriter writer = new();
-            Console.SetOut(writer);
-
-            logger.Log(logLevel, "test message");
-
-            string output = writer.ToString();
-            Assert.IsTrue(
-                output.StartsWith(expectedPrefix),
-                $"Expected output to start with '{expectedPrefix}' but got: '{output}'");
-            Assert.IsTrue(
-                output.Contains("test message"),
-                $"Expected output to contain 'test message' but got: '{output}'");
+            Console.SetOut(stdout);
+            Console.SetError(stderr);
+            action();
         }
         finally
         {
             Console.SetOut(originalOut);
+            Console.SetError(originalError);
         }
+
+        return (stdout.ToString(), stderr.ToString());
     }
 
+    private static ILogger NewLogger() =>
+        new CustomLoggerProvider().CreateLogger("TestCategory");
+
     /// <summary>
-    /// Validates that each log level error and above produces the correct abbreviated
-    /// label matching ASP.NET Core's default console formatter convention.
-    /// Error and Critical logs should go to the stderr stream.
+    /// Standard (non-MCP) path: each log level produces the correct abbreviated
+    /// label matching ASP.NET Core's default console formatter and is routed to
+    /// stdout for Information/Warning and stderr for Error/Critical.
     /// </summary>
     [DataTestMethod]
-    [DataRow(LogLevel.Error, "fail:")]
-    [DataRow(LogLevel.Critical, "crit:")]
-    public void LogError_UsesAbbreviatedLogLevelLabels(LogLevel logLevel, string expectedPrefix)
+    [DataRow(LogLevel.Information, "info:", false)]
+    [DataRow(LogLevel.Warning, "warn:", false)]
+    [DataRow(LogLevel.Error, "fail:", true)]
+    [DataRow(LogLevel.Critical, "crit:", true)]
+    public void LogOutput_UsesAbbreviatedLogLevelLabels(LogLevel logLevel, string expectedPrefix, bool expectStderr)
     {
-        CustomLoggerProvider provider = new();
-        ILogger logger = provider.CreateLogger("TestCategory");
+        const string Message = "test message";
 
-        TextWriter originalError = Console.Error;
-        try
+        (string stdout, string stderr) = CaptureConsole(() => NewLogger().Log(logLevel, Message));
+
+        string actual = expectStderr ? stderr : stdout;
+        string other = expectStderr ? stdout : stderr;
+
+        Assert.IsTrue(actual.StartsWith(expectedPrefix),
+            $"Expected output to start with '{expectedPrefix}' but got: '{actual}'");
+        StringAssert.Contains(actual, Message);
+        Assert.AreEqual(string.Empty, other,
+            $"Did not expect output on the other stream but got: '{other}'");
+    }
+
+    /// <summary>
+    /// MCP stdio mode with no overrides (neither CLI <c>--LogLevel</c> nor
+    /// config <c>log-level</c>): all output must be suppressed so the JSON-RPC
+    /// channel stays clean.
+    /// </summary>
+    [TestMethod]
+    public void Mcp_NoOverrides_SuppressesAllOutput()
+    {
+        Cli.Utils.IsMcpStdioMode = true;
+
+        (string stdout, string stderr) = CaptureConsole(() =>
         {
-            StringWriter writer = new();
-            Console.SetError(writer);
-            logger.Log(logLevel, "test message");
-
-            string output = writer.ToString();
-            Assert.IsTrue(
-                output.StartsWith(expectedPrefix),
-                $"Expected output to start with '{expectedPrefix}' but got: '{output}'");
-            Assert.IsTrue(
-                output.Contains("test message"),
-                $"Expected output to contain 'test message' but got: '{output}'");
-        }
-        finally
+            ILogger logger = NewLogger();
+            logger.Log(LogLevel.Information, "info should not appear");
+            logger.Log(LogLevel.Error, "error should not appear");
+        });
+
+        Assert.AreEqual(string.Empty, stdout, "MCP mode without overrides must not write to stdout.");
+        Assert.AreEqual(string.Empty, stderr, "MCP mode without overrides must not write to stderr.");
+    }
+
+    /// <summary>
+    /// MCP stdio mode with a CLI-supplied <c>--LogLevel</c>: logs must always
+    /// go to stderr (never stdout) and the level threshold from
+    /// <see cref="Cli.Utils.CliLogLevel"/> must be honored.
+    /// </summary>
+    [TestMethod]
+    public void Mcp_CliOverride_WritesToStderrAndHonorsCliLevel()
+    {
+        Cli.Utils.IsMcpStdioMode = true;
+        Cli.Utils.IsLogLevelOverriddenByCli = true;
+        Cli.Utils.CliLogLevel = LogLevel.Warning;
+
+        (string stdout, string stderr) = CaptureConsole(() =>
         {
-            Console.SetError(originalError);
-        }
+            ILogger logger = NewLogger();
+            logger.Log(LogLevel.Information, "filtered info");   // below threshold
+            logger.Log(LogLevel.Warning, "visible warn");        // at threshold
+            logger.Log(LogLevel.Error, "visible error");         // above threshold
+        });
+
+        Assert.AreEqual(string.Empty, stdout, "MCP mode must never write to stdout.");
+        Assert.IsFalse(stderr.Contains("filtered info"), $"Below-threshold log should be filtered. Got: '{stderr}'");
+        StringAssert.Contains(stderr, "warn: visible warn");
+        StringAssert.Contains(stderr, "fail: visible error");
+    }
+
+    /// <summary>
+    /// Bug fix: MCP stdio mode where only the runtime config (no CLI flag)
+    /// supplied the log level. Previously suppressed; must now emit to stderr
+    /// using <see cref="Cli.Utils.ConfigLogLevel"/>.
+    /// </summary>
+    [TestMethod]
+    public void Mcp_ConfigOverride_WritesToStderrAndHonorsConfigLevel()
+    {
+        Cli.Utils.IsMcpStdioMode = true;
+        Cli.Utils.IsLogLevelOverriddenByConfig = true;
+        Cli.Utils.ConfigLogLevel = LogLevel.Information;
+
+        (string stdout, string stderr) = CaptureConsole(() =>
+        {
+            ILogger logger = NewLogger();
+            logger.Log(LogLevel.Debug, "filtered debug");      // below threshold
+            logger.Log(LogLevel.Information, "visible info");  // at threshold
+        });
+
+        Assert.AreEqual(string.Empty, stdout, "MCP mode must never write to stdout.");
+        Assert.IsFalse(stderr.Contains("filtered debug"), $"Below-threshold log should be filtered. Got: '{stderr}'");
+        StringAssert.Contains(stderr, "info: visible info");
+    }
+
+    /// <summary>
+    /// Precedence: when both CLI and config supply a log level, the CLI value
+    /// wins (CLI &gt; Config &gt; None).
+    /// </summary>
+    [TestMethod]
+    public void Mcp_CliOverridePrecedesConfigOverride()
+    {
+        Cli.Utils.IsMcpStdioMode = true;
+        Cli.Utils.IsLogLevelOverriddenByCli = true;
+        Cli.Utils.CliLogLevel = LogLevel.Warning;
+        Cli.Utils.IsLogLevelOverriddenByConfig = true;
+        Cli.Utils.ConfigLogLevel = LogLevel.Information;
+
+        (_, string stderr) = CaptureConsole(() =>
+        {
+            ILogger logger = NewLogger();
+            logger.Log(LogLevel.Information, "filtered by CLI Warning");
+            logger.Log(LogLevel.Warning, "passes CLI Warning");
+        });
+
+        Assert.IsFalse(stderr.Contains("filtered by CLI Warning"),
+            $"CLI level should override config and filter Information. Got: '{stderr}'");
+        StringAssert.Contains(stderr, "warn: passes CLI Warning");
     }
 }

src/Cli/ConfigGenerator.cs

@@ -2752,6 +2752,12 @@ public static bool TryStartEngineWithOptions(StartOptions options, FileSystemRun
             /// - MCP stdio mode: Service defaults to None for clean stdout output
             /// - Non-MCP mode: Service defaults to Debug (Development) or Error (Production) based on config
             LogLevel minimumLogLevel;
+
+            // Reset the config-based override flag so stale state from a prior call
+            // (these are static) cannot leak into the current run.
+            Utils.IsLogLevelOverriddenByConfig = false;
+            Utils.ConfigLogLevel = LogLevel.Information;
+
             if (options.LogLevel is not null)
             {
                 if (options.LogLevel is < LogLevel.Trace or > LogLevel.None)
@@ -2770,6 +2776,15 @@ public static bool TryStartEngineWithOptions(StartOptions options, FileSystemRun
             else
             {
                 minimumLogLevel = deserializedRuntimeConfig.GetConfiguredLogLevel();
+
+                // Track whether config explicitly set a log level. In MCP stdio mode this
+                // allows CLI logs to be emitted to stderr (instead of being suppressed)
+                // when the user expressed intent via the config file rather than --LogLevel.
+                if (deserializedRuntimeConfig.HasExplicitLogLevel())
+                {
+                    Utils.IsLogLevelOverriddenByConfig = true;
+                    Utils.ConfigLogLevel = minimumLogLevel;
+                }
             }
 
             options.CliBuffer.BufferLog(LogLevel.Information, $"Setting minimum LogLevel: {minimumLogLevel}.");

src/Cli/CustomLoggerProvider.cs

@@ -28,14 +28,18 @@ public class CustomConsoleLogger : ILogger
         private readonly LogLevel _minimumLogLevel;
 
         // Minimum LogLevel for CLI output.
-        // For MCP mode: use CLI's --LogLevel if specified, otherwise suppress all.
-        // For non-MCP mode: always use Information.
+        // For MCP mode: prefer CLI's --LogLevel, fall back to config's log-level, otherwise suppress all.
+        // For non-MCP mode: always use the level passed to the constructor.
         // Note: --LogLevel is meant for the ENGINE's log level, not CLI's output.
         public CustomConsoleLogger(LogLevel minimumLogLevel = LogLevel.Information)
         {
             _minimumLogLevel = Cli.Utils.IsMcpStdioMode
-            ? (Cli.Utils.IsLogLevelOverriddenByCli ? Cli.Utils.CliLogLevel : LogLevel.None)
-            : minimumLogLevel;
+                ? (Cli.Utils.IsLogLevelOverriddenByCli
+                    ? Cli.Utils.CliLogLevel
+                    : Cli.Utils.IsLogLevelOverriddenByConfig
+                        ? Cli.Utils.ConfigLogLevel
+                        : LogLevel.None)
+                : minimumLogLevel;
         }
 
         //  Color values based on LogLevel
@@ -89,16 +93,17 @@ public CustomConsoleLogger(LogLevel minimumLogLevel = LogLevel.Information)
         /// <summary>
         /// Creates Log message by setting console message color based on LogLevel.
         /// In MCP stdio mode:
-        /// - If user explicitly set --LogLevel: write to stderr (colored output)
+        /// - If user explicitly set --LogLevel (CLI) or log-level (config): write to stderr (colored output)
         /// - Otherwise: suppress entirely to keep stdout clean for JSON-RPC protocol.
         /// </summary>
         public void Log<TState>(LogLevel logLevel, EventId eventId, TState state, Exception? exception, Func<TState, Exception?, string> formatter)
         {
-            // In MCP stdio mode, only output logs if user explicitly requested a log level.
+            // In MCP stdio mode, only output logs if user explicitly requested a log level
+            // via either the CLI --LogLevel flag or the runtime config file's log-level.
             // In that case, write to stderr to keep stdout clean for JSON-RPC.
             if (Cli.Utils.IsMcpStdioMode)
             {
-                if (!Cli.Utils.IsLogLevelOverriddenByCli)
+                if (!Cli.Utils.IsLogLevelOverriddenByCli && !Cli.Utils.IsLogLevelOverriddenByConfig)
                 {
                     return; // Suppress entirely when no explicit log level
                 }
@@ -116,7 +121,15 @@ public void Log<TState>(LogLevel logLevel, EventId eventId, TState state, Except
 
                 // In MCP stdio mode, stdout is reserved for JSON-RPC protocol messages.
                 // Logs must go to stderr to avoid corrupting the MCP communication channel.
-                Console.Error.WriteLine($"{mcpAbbreviation}: {formatter(state, exception)}");
+                // Apply colors so the abbreviation matches the visual style of engine logs.
+                ConsoleColor mcpOriginalForeGroundColor = Console.ForegroundColor;
+                ConsoleColor mcpOriginalBackGroundColor = Console.BackgroundColor;
+                Console.ForegroundColor = _logLevelToForeGroundConsoleColorMap.GetValueOrDefault(logLevel, ConsoleColor.White);
+                Console.BackgroundColor = _logLevelToBackGroundConsoleColorMap.GetValueOrDefault(logLevel, ConsoleColor.Black);
+                Console.Error.Write($"{mcpAbbreviation}:");
+                Console.ForegroundColor = mcpOriginalForeGroundColor;
+                Console.BackgroundColor = mcpOriginalBackGroundColor;
+                Console.Error.WriteLine($" {formatter(state, exception)}");
                 return;
             }
 

src/Cli/Utils.cs

@@ -40,6 +40,18 @@ public class Utils
         /// </summary>
         public static LogLevel CliLogLevel { get; set; } = LogLevel.Information;
 
+        /// <summary>
+        /// When true, the runtime config file explicitly set a log-level value.
+        /// This allows CLI logs to be written to stderr in MCP mode even when no --LogLevel flag was provided.
+        /// </summary>
+        public static bool IsLogLevelOverriddenByConfig { get; set; }
+
+        /// <summary>
+        /// The log level specified via runtime config file's log-level setting.
+        /// Only valid when IsLogLevelOverriddenByConfig is true.
+        /// </summary>
+        public static LogLevel ConfigLogLevel { get; set; } = LogLevel.Information;
+
 #pragma warning disable CS8618 // Non-nullable field must contain a non-null value when exiting constructor. Consider declaring as nullable.
         private static ILogger<Utils> _logger;
 #pragma warning restore CS8618