Set default log level to None for MCP in Stdio mode.

src/Cli/ConfigGenerator.cs

@@ -2606,13 +2606,38 @@ public static bool TryStartEngineWithOptions(StartOptions options, FileSystemRun
             }
             else
             {
-                minimumLogLevel = deserializedRuntimeConfig.GetConfiguredLogLevel();
-                HostMode hostModeType = deserializedRuntimeConfig.IsDevelopmentMode() ? HostMode.Development : HostMode.Production;
+                // When --mcp-stdio is used without explicit --LogLevel:
+                // 1. Check if config has log-level set - use that (Config has priority 2)
+                // 2. Otherwise default to None for clean MCP stdio output
+                if (options.McpStdio)
+                {
+                    // Check if config explicitly sets a log level
+                    if (!deserializedRuntimeConfig.IsLogLevelNull())
+                    {
+                        minimumLogLevel = deserializedRuntimeConfig.GetConfiguredLogLevel();
+                        _logger.LogInformation("MCP stdio mode: Using config log-level: {minimumLogLevel}.", minimumLogLevel);
+                        // Pass --LogLevel to Service with special marker to indicate it's from config, not CLI.
+                        // This allows MCP logging/setLevel to be blocked by config override.
+                        args.Add("--LogLevel");
+                        args.Add(minimumLogLevel.ToString());
+                        args.Add("--LogLevelFromConfig");
+                    }
+                    else
+                    {
+                        _logger.LogInformation("MCP stdio mode: Defaulting to LogLevel.None (no config override).");
+                        // Don't add --LogLevel to args - let Service handle the default.
+                    }
+                }
+                else
+                {
+                    minimumLogLevel = deserializedRuntimeConfig.GetConfiguredLogLevel();
+                    HostMode hostModeType = deserializedRuntimeConfig.IsDevelopmentMode() ? HostMode.Development : HostMode.Production;
 
-                _logger.LogInformation($"Setting default minimum LogLevel: {minimumLogLevel} for {hostModeType} mode.", minimumLogLevel, hostModeType);
+                    _logger.LogInformation($"Setting default minimum LogLevel: {minimumLogLevel} for {hostModeType} mode.", minimumLogLevel, hostModeType);
 
-                // Don't add --LogLevel arg since user didn't explicitly set it.
-                // Service will determine default log level based on config or host mode.
+                    // Don't add --LogLevel arg since user didn't explicitly set it.
+                    // Service will determine default log level based on config or host mode.
+                }
             }
 
             // This will add args to disable automatic redirects to https if specified by user

src/Cli/CustomLoggerProvider.cs

@@ -71,9 +71,16 @@ public class CustomConsoleLogger : ILogger
 
         /// <summary>
         /// Creates Log message by setting console message color based on LogLevel.
+        /// Skips logging when in MCP stdio mode 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, suppress all CLI logging to keep stdout clean for JSON-RPC.
+            if (Cli.Utils.IsMcpStdioMode)
+            {
+                return;
+            }
+
             if (!IsEnabled(logLevel) || logLevel < _minimumLogLevel)
             {
                 return;

src/Cli/Program.cs

@@ -26,6 +26,9 @@ public static int Main(string[] args)
             // Load environment variables from .env file if present.
             DotNetEnv.Env.Load();
 
+            // Check if MCP stdio mode is requested - suppress CLI logging to keep stdout clean for JSON-RPC.
+            Utils.IsMcpStdioMode = args.Any(a => string.Equals(a, "--mcp-stdio", StringComparison.OrdinalIgnoreCase));
+
             // Logger setup and configuration
             ILoggerFactory loggerFactory = Utils.LoggerFactoryForCli;
             ILogger<Program> cliLogger = loggerFactory.CreateLogger<Program>();

src/Cli/Utils.cs

@@ -23,6 +23,11 @@ public class Utils
         public const string WILDCARD = "*";
         public static readonly string SEPARATOR = ":";
 
+        /// <summary>
+        /// When true, CLI logging to stdout is suppressed to keep the MCP stdio channel clean.
+        /// </summary>
+        public static bool IsMcpStdioMode { get; set; }
+
 #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

src/Service/Program.cs

@@ -4,6 +4,7 @@
 using System;
 using System.CommandLine;
 using System.CommandLine.Parsing;
+using System.IO;
 using System.Runtime.InteropServices;
 using System.Text;
 using System.Text.RegularExpressions;
@@ -63,6 +64,28 @@ public static bool StartEngine(string[] args, bool runMcpStdio, string? mcpRole)
         {
             try
             {
+                // Initialize log level EARLY, before building the host.
+                // This ensures logging filters are effective during the entire host build process.
+                LogLevel initialLogLevel = GetLogLevelFromCommandLineArgs(args, runMcpStdio, out bool isCliOverridden, out bool isConfigOverridden);
+                LogLevelProvider.SetInitialLogLevel(initialLogLevel, isCliOverridden, isConfigOverridden);
+
+                // For MCP stdio mode, redirect Console.Out to keep stdout clean for JSON-RPC.
+                // MCP SDK uses Console.OpenStandardOutput() which gets the real stdout, unaffected by this redirect.
+                if (runMcpStdio)
+                {
+                    // When LogLevel.None, redirect to null stream for ZERO output.
+                    // Otherwise redirect to stderr so logs don't pollute JSON-RPC.
+                    if (initialLogLevel == LogLevel.None)
+                    {
+                        Console.SetOut(TextWriter.Null);
+                        Console.SetError(TextWriter.Null);
+                    }
+                    else
+                    {
+                        Console.SetOut(Console.Error);
+                    }
+                }
+
                 IHost host = CreateHostBuilder(args, runMcpStdio, mcpRole).Build();
 
                 if (runMcpStdio)
@@ -115,13 +138,20 @@ public static IHostBuilder CreateHostBuilder(string[] args, bool runMcpStdio, st
                 })
                 .ConfigureLogging(logging =>
                 {
-                    logging.AddFilter("Microsoft", logLevel => LogLevelProvider.ShouldLog(logLevel));
-                    logging.AddFilter("Microsoft.Hosting.Lifetime", logLevel => LogLevelProvider.ShouldLog(logLevel));
+                    // Set minimum level at the framework level - this affects all loggers.
+                    // For MCP stdio mode, Console.Out is redirected to stderr in Main(),
+                    // so any logging output goes to stderr and doesn't pollute the JSON-RPC channel.
+                    logging.SetMinimumLevel(LogLevelProvider.CurrentLogLevel);
+
+                    // Add filter for dynamic log level changes (e.g., via MCP logging/setLevel)
+                    logging.AddFilter(logLevel => LogLevelProvider.ShouldLog(logLevel));
                 })
                 .ConfigureWebHostDefaults(webBuilder =>
                 {
-                    Startup.MinimumLogLevel = GetLogLevelFromCommandLineArgs(args, out Startup.IsLogLevelOverriddenByCli);
-                    LogLevelProvider.SetInitialLogLevel(Startup.MinimumLogLevel, Startup.IsLogLevelOverriddenByCli);
+                    // LogLevelProvider was already initialized in StartEngine before CreateHostBuilder.
+                    // Use the already-set values to avoid re-parsing args.
+                    Startup.MinimumLogLevel = LogLevelProvider.CurrentLogLevel;
+                    Startup.IsLogLevelOverriddenByCli = LogLevelProvider.IsCliOverridden;
                     ILoggerFactory loggerFactory = GetLoggerFactoryForLogLevel(Startup.MinimumLogLevel, stdio: runMcpStdio);
                     ILogger<Startup> startupLogger = loggerFactory.CreateLogger<Startup>();
                     DisableHttpsRedirectionIfNeeded(args);
@@ -133,19 +163,59 @@ public static IHostBuilder CreateHostBuilder(string[] args, bool runMcpStdio, st
         /// Using System.CommandLine Parser to parse args and return
         /// the correct log level. We save if there is a log level in args through
         /// the out param. For log level out of range we throw an exception.
+        /// When in MCP stdio mode without explicit --LogLevel, defaults to None without CLI override.
         /// </summary>
         /// <param name="args">array that may contain log level information.</param>
-        /// <param name="isLogLevelOverridenByCli">sets if log level is found in the args.</param>
+        /// <param name="runMcpStdio">whether running in MCP stdio mode.</param>
+        /// <param name="isLogLevelOverridenByCli">sets if log level is found in the args from CLI.</param>
+        /// <param name="isLogLevelOverridenByConfig">sets if log level came from config file.</param>
         /// <returns>Appropriate log level.</returns>
-        private static LogLevel GetLogLevelFromCommandLineArgs(string[] args, out bool isLogLevelOverridenByCli)
+        private static LogLevel GetLogLevelFromCommandLineArgs(string[] args, bool runMcpStdio, out bool isLogLevelOverridenByCli, out bool isLogLevelOverridenByConfig)
         {
             Command cmd = new(name: "start");
             Option<LogLevel> logLevelOption = new(name: "--LogLevel");
+            Option<bool> logLevelFromConfigOption = new(name: "--LogLevelFromConfig");
             cmd.AddOption(logLevelOption);
+            cmd.AddOption(logLevelFromConfigOption);
             ParseResult result = GetParseResult(cmd, args);
             bool matchedToken = result.Tokens.Count - result.UnmatchedTokens.Count - result.UnparsedTokens.Count > 1;
-            LogLevel logLevel = matchedToken ? result.GetValueForOption(logLevelOption) : LogLevel.Error;
-            isLogLevelOverridenByCli = matchedToken;
+
+            // Check if --LogLevelFromConfig flag is present (indicates config override, not CLI)
+            bool isFromConfig = result.GetValueForOption(logLevelFromConfigOption);
+
+            LogLevel logLevel;
+            if (matchedToken)
+            {
+                logLevel = result.GetValueForOption(logLevelOption);
+
+                if (isFromConfig)
+                {
+                    // Log level came from config file (passed by CLI with --LogLevelFromConfig marker)
+                    isLogLevelOverridenByCli = false;
+                    isLogLevelOverridenByConfig = true;
+                }
+                else
+                {
+                    // User explicitly set --LogLevel via CLI (highest priority)
+                    isLogLevelOverridenByCli = true;
+                    isLogLevelOverridenByConfig = false;
+                }
+            }
+            else if (runMcpStdio)
+            {
+                // MCP stdio mode without explicit --LogLevel: default to None to keep stdout clean.
+                // This is NOT a CLI or config override, so MCP logging/setLevel can still change it.
+                logLevel = LogLevel.None;
+                isLogLevelOverridenByCli = false;
+                isLogLevelOverridenByConfig = false;
+            }
+            else
+            {
+                // Normal mode without explicit --LogLevel
+                logLevel = LogLevel.Error;
+                isLogLevelOverridenByCli = false;
+                isLogLevelOverridenByConfig = false;
+            }
 
             if (logLevel is > LogLevel.None or < LogLevel.Trace)
             {