chore: update logger out stream to errStream in stdio transport (#1466)

## Description

---
Update out stream in `stdio` transport protocol.


**Before:** When `stdio=true`, log levels will automatically get set to
WARN (even if user explicitly set it as DEBUG or INFO). This is because
the DEBUG and INFO logger send logs to outStream. In `stdio`, server
MUST NOT write anything to `stdout` that is not a valid MCP message.

MCP specifications mentioned that "The server MAY write UTF-8 strings to
its standard error (stderr) for logging purposes. Client MAY capture,
forward, or ignore this logging."

**After:** When `stdio=true`, logger's DEBUG and INFO logger send logs
to outStream. This will still respect the `log-levels` set by user
(default as INFO).

cmd/root.go

@@ -655,20 +655,6 @@ func watchChanges(ctx context.Context, watchDirs map[string]bool, watchedFiles m
 	}
 }
 
-// updateLogLevel checks if Toolbox have to update the existing log level set by users.
-// stdio doesn't support "debug" and "info" logs.
-func updateLogLevel(stdio bool, logLevel string) bool {
-	if stdio {
-		switch strings.ToUpper(logLevel) {
-		case log.Debug, log.Info:
-			return true
-		default:
-			return false
-		}
-	}
-	return false
-}
-
 func resolveWatcherInputs(toolsFile string, toolsFiles []string, toolsFolder string) (map[string]bool, map[string]bool) {
 	var relevantFiles []string
 
@@ -697,10 +683,6 @@ func resolveWatcherInputs(toolsFile string, toolsFiles []string, toolsFolder str
 }
 
 func run(cmd *Command) error {
-	if updateLogLevel(cmd.cfg.Stdio, cmd.cfg.LogLevel.String()) {
-		cmd.cfg.LogLevel = server.StringLevel(log.Warn)
-	}
-
 	ctx, cancel := context.WithCancel(cmd.Context())
 	defer cancel()
 
@@ -724,16 +706,22 @@ func run(cmd *Command) error {
 		cancel()
 	}(ctx)
 
+	// If stdio, set logger's out stream (usually DEBUG and INFO logs) to errStream
+	loggerOut := cmd.outStream
+	if cmd.cfg.Stdio {
+		loggerOut = cmd.errStream
+	}
+
 	// Handle logger separately from config
 	switch strings.ToLower(cmd.cfg.LoggingFormat.String()) {
 	case "json":
-		logger, err := log.NewStructuredLogger(cmd.outStream, cmd.errStream, cmd.cfg.LogLevel.String())
+		logger, err := log.NewStructuredLogger(loggerOut, cmd.errStream, cmd.cfg.LogLevel.String())
 		if err != nil {
 			return fmt.Errorf("unable to initialize logger: %w", err)
 		}
 		cmd.logger = logger
 	case "standard":
-		logger, err := log.NewStdLogger(cmd.outStream, cmd.errStream, cmd.cfg.LogLevel.String())
+		logger, err := log.NewStdLogger(loggerOut, cmd.errStream, cmd.cfg.LogLevel.String())
 		if err != nil {
 			return fmt.Errorf("unable to initialize logger: %w", err)
 		}

cmd/root_test.go

@@ -1597,51 +1597,3 @@ func TestPrebuiltTools(t *testing.T) {
 		})
 	}
 }
-
-func TestUpdateLogLevel(t *testing.T) {
-	tcs := []struct {
-		desc     string
-		stdio    bool
-		logLevel string
-		want     bool
-	}{
-		{
-			desc:     "no stdio",
-			stdio:    false,
-			logLevel: "info",
-			want:     false,
-		},
-		{
-			desc:     "stdio with info log",
-			stdio:    true,
-			logLevel: "info",
-			want:     true,
-		},
-		{
-			desc:     "stdio with debug log",
-			stdio:    true,
-			logLevel: "debug",
-			want:     true,
-		},
-		{
-			desc:     "stdio with warn log",
-			stdio:    true,
-			logLevel: "warn",
-			want:     false,
-		},
-		{
-			desc:     "stdio with error log",
-			stdio:    true,
-			logLevel: "error",
-			want:     false,
-		},
-	}
-	for _, tc := range tcs {
-		t.Run(tc.desc, func(t *testing.T) {
-			got := updateLogLevel(tc.stdio, tc.logLevel)
-			if got != tc.want {
-				t.Fatalf("incorrect indication to update log level: got %t, want %t", got, tc.want)
-			}
-		})
-	}
-}