diff --git a/cmd/s3-genindex/main.go b/cmd/s3-genindex/main.go
index a9949e2..4fed48e 100644
--- a/cmd/s3-genindex/main.go
+++ b/cmd/s3-genindex/main.go
@@ -215,8 +215,8 @@ func generateS3HTML(entries []indexgen.FileEntry, opts *indexgen.Options) error
 func main() {
 	var opts indexgen.Options
 	var excludeRegexStr string
-	var s3Mode bool
 	var directory string
+	var s3URL string
 
 	// Set defaults
 	opts.DirAppend = true
@@ -224,33 +224,38 @@ func main() {
 	opts.Recursive = true
 	opts.IncludeHidden = true
 
-	flag.StringVar(&directory, "d", "", "directory or S3 URL to process (required)")
+	flag.StringVar(&directory, "d", "", "local directory to process")
+	flag.StringVar(&s3URL, "s3", "", "S3 URL to process")
 	flag.StringVar(&opts.Filter, "f", "*", "only include files matching glob")
 	flag.StringVar(&excludeRegexStr, "x", "", "exclude files matching regular expression")
 	flag.BoolVar(&opts.Verbose, "v", false, "verbosely list every processed file")
-	flag.BoolVar(&s3Mode, "s3", false, "treat -d argument as S3 URL")
 
 	flag.Usage = func() {
 		fmt.Fprintf(os.Stderr, "Generate directory index files (recursive is ON, hidden files included by default).\n")
 		fmt.Fprintf(os.Stderr, "Output file is 'index.html', directory href appending is enabled.\n")
-		fmt.Fprintf(os.Stderr, "Directory must be specified with -d flag.\n")
-		fmt.Fprintf(os.Stderr, "For S3 mode, use -s3 flag with S3 URL in -d.\n")
+		fmt.Fprintf(os.Stderr, "Specify either -d <directory> OR -s3 <url> (mutually exclusive).\n")
 		fmt.Fprintf(os.Stderr, "S3 URLs: http://host:port/bucket or https://host/bucket\n")
 		fmt.Fprintf(os.Stderr, "For S3, set AWS_ACCESS_KEY_ID and AWS_SECRET_ACCESS_KEY.\n")
 		fmt.Fprintf(os.Stderr, "Optionally filter by file types with -f \"*.py\".\n\n")
 		fmt.Fprintf(os.Stderr, "Usage: %s [OPTIONS]\n\n", os.Args[0])
 		fmt.Fprintf(os.Stderr, "Examples:\n")
 		fmt.Fprintf(os.Stderr, "  %s -d /path/to/dir\n", os.Args[0])
-		fmt.Fprintf(os.Stderr, "  %s -d http://minio.example.com:9000/bucket -s3\n", os.Args[0])
-		fmt.Fprintf(os.Stderr, "  %s -v -f \"*.log\" -d https://s3.amazonaws.com/logs -s3\n\n", os.Args[0])
+		fmt.Fprintf(os.Stderr, "  %s -s3 http://minio.example.com:9000/bucket\n", os.Args[0])
+		fmt.Fprintf(os.Stderr, "  %s -v -f \"*.log\" -s3 https://s3.amazonaws.com/logs\n\n", os.Args[0])
 		flag.PrintDefaults()
 	}
 
 	flag.Parse()
 
-	// Check if directory flag is provided
-	if directory == "" {
-		fmt.Fprintf(os.Stderr, "Error: Directory must be specified with -d flag.\n\n")
+	// Check mutual exclusion and that exactly one option is provided
+	if directory == "" && s3URL == "" {
+		fmt.Fprintf(os.Stderr, "Error: Either -d <directory> or -s3 <url> must be specified.\n\n")
+		flag.Usage()
+		os.Exit(1)
+	}
+
+	if directory != "" && s3URL != "" {
+		fmt.Fprintf(os.Stderr, "Error: -d and -s3 flags are mutually exclusive. Use only one.\n\n")
 		flag.Usage()
 		os.Exit(1)
 	}
@@ -263,9 +268,9 @@ func main() {
 		}
 	}
 
-	if s3Mode {
+	if s3URL != "" {
 		// Parse S3 URL
-		s3Config, err := parseS3URL(directory)
+		s3Config, err := parseS3URL(s3URL)
 		if err != nil {
 			log.Fatal("Failed to parse S3 URL:", err)
 		}
diff --git a/cmd/s3-genindex/main_test.go b/cmd/s3-genindex/main_test.go
index ae02cfc..6b1ffe1 100644
--- a/cmd/s3-genindex/main_test.go
+++ b/cmd/s3-genindex/main_test.go
@@ -117,14 +117,14 @@ func TestS3FlagHandling(t *testing.T) {
 	}{
 		{
 			name: "s3 flag with URL",
-			args: []string{"s3-genindex", "-s3", "-d", "http://minio.example.com:9000/bucket"},
+			args: []string{"s3-genindex", "-s3", "http://minio.example.com:9000/bucket"},
 		},
 		{
 			name: "s3 flag with verbose",
-			args: []string{"s3-genindex", "-v", "-s3", "-d", "https://s3.amazonaws.com/logs"},
+			args: []string{"s3-genindex", "-v", "-s3", "https://s3.amazonaws.com/logs"},
 		},
 		{
-			name: "local directory without s3 flag",
+			name: "local directory flag",
 			args: []string{"s3-genindex", "-d", "/tmp"},
 		},
 	}
@@ -148,25 +148,30 @@ func TestS3FlagHandling(t *testing.T) {
 	}
 }
 
-func TestMandatoryDirectoryArgument(t *testing.T) {
+func TestMutualExclusionAndRequiredFlags(t *testing.T) {
 	tests := []struct {
 		name string
 		args []string
 		shouldExitWithError bool
 	}{
 		{
-			name: "no directory flag",
+			name: "no flags provided",
 			args: []string{"s3-genindex"},
 			shouldExitWithError: true,
 		},
 		{
-			name: "with directory flag",
+			name: "directory flag only",
 			args: []string{"s3-genindex", "-d", "/tmp"},
 			shouldExitWithError: false,
 		},
 		{
-			name: "with s3 flag but no directory",
-			args: []string{"s3-genindex", "-s3"},
+			name: "s3 flag only",
+			args: []string{"s3-genindex", "-s3", "http://example.com/bucket"},
+			shouldExitWithError: false,
+		},
+		{
+			name: "both flags provided (mutual exclusion)",
+			args: []string{"s3-genindex", "-d", "/tmp", "-s3", "http://example.com/bucket"},
 			shouldExitWithError: true,
 		},
 	}