diff --git a/internal/cmd/drive.go b/internal/cmd/drive.go
index 2937522..d93784a 100644
--- a/internal/cmd/drive.go
+++ b/internal/cmd/drive.go
@@ -1052,6 +1052,14 @@ func guessMimeType(path string) string {
 
 func downloadDriveFile(ctx context.Context, svc *drive.Service, meta *drive.File, destPath string, format string) (string, int64, error) {
 	isGoogleDoc := strings.HasPrefix(meta.MimeType, "application/vnd.google-apps.")
+	normalizedFormat := strings.ToLower(strings.TrimSpace(format))
+	if normalizedFormat == "auto" {
+		normalizedFormat = ""
+	}
+
+	if !isGoogleDoc && normalizedFormat != "" {
+		return "", 0, fmt.Errorf("--format %q not supported for non-Google Workspace files (mimeType=%q); file can only be downloaded as-is", format, meta.MimeType)
+	}
 
 	var (
 		resp    *http.Response
@@ -1061,11 +1069,11 @@ func downloadDriveFile(ctx context.Context, svc *drive.Service, meta *drive.File
 
 	if isGoogleDoc {
 		var exportMimeType string
-		if strings.TrimSpace(format) == "" {
+		if normalizedFormat == "" {
 			exportMimeType = driveExportMimeType(meta.MimeType)
 		} else {
 			var mimeErr error
-			exportMimeType, mimeErr = driveExportMimeTypeForFormat(meta.MimeType, format)
+			exportMimeType, mimeErr = driveExportMimeTypeForFormat(meta.MimeType, normalizedFormat)
 			if mimeErr != nil {
 				return "", 0, mimeErr
 			}
@@ -1129,7 +1137,7 @@ func driveExportMimeType(googleMimeType string) string {
 
 func driveExportMimeTypeForFormat(googleMimeType string, format string) (string, error) {
 	format = strings.ToLower(strings.TrimSpace(format))
-	if format == "" {
+	if format == "" || format == "auto" {
 		return driveExportMimeType(googleMimeType), nil
 	}
 
diff --git a/internal/cmd/drive_download_test.go b/internal/cmd/drive_download_test.go
index e53e0ae..2e06ecd 100644
--- a/internal/cmd/drive_download_test.go
+++ b/internal/cmd/drive_download_test.go
@@ -57,6 +57,36 @@ func TestDownloadDriveFile_NonGoogleDoc(t *testing.T) {
 	}
 }
 
+func TestDownloadDriveFile_NonGoogleDocFormatRejected(t *testing.T) {
+	origDownload := driveDownload
+	t.Cleanup(func() { driveDownload = origDownload })
+
+	called := false
+	driveDownload = func(context.Context, *drive.Service, string) (*http.Response, error) {
+		called = true
+		return &http.Response{
+			Status:     "200 OK",
+			StatusCode: http.StatusOK,
+			Body:       io.NopCloser(strings.NewReader("ok")),
+		}, nil
+	}
+
+	dest := filepath.Join(t.TempDir(), "file.html")
+	_, _, err := downloadDriveFile(context.Background(), &drive.Service{}, &drive.File{Id: "id1", MimeType: "application/pdf"}, dest, "html")
+	if err == nil {
+		t.Fatalf("expected error")
+	}
+	if !strings.Contains(err.Error(), "non-Google Workspace") {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if called {
+		t.Fatalf("download should not be called on format error")
+	}
+	if _, statErr := os.Stat(dest); !os.IsNotExist(statErr) {
+		t.Fatalf("expected no file written, stat=%v", statErr)
+	}
+}
+
 func TestDownloadDriveFile_GoogleDocExport(t *testing.T) {
 	body := "exported"
 	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
diff --git a/internal/cmd/drive_export_format_test.go b/internal/cmd/drive_export_format_test.go
index 6957dc1..280d251 100644
--- a/internal/cmd/drive_export_format_test.go
+++ b/internal/cmd/drive_export_format_test.go
@@ -28,6 +28,12 @@ func TestDriveExportMimeTypeForFormat(t *testing.T) {
 			format:     "",
 			wantMime:   "application/pdf",
 		},
+		{
+			name:       "doc_auto",
+			googleMime: "application/vnd.google-apps.document",
+			format:     "auto",
+			wantMime:   "application/pdf",
+		},
 		{
 			name:       "doc_pdf",
 			googleMime: "application/vnd.google-apps.document",
@@ -59,6 +65,12 @@ func TestDriveExportMimeTypeForFormat(t *testing.T) {
 			format:     "",
 			wantMime:   "text/csv",
 		},
+		{
+			name:       "sheet_auto",
+			googleMime: "application/vnd.google-apps.spreadsheet",
+			format:     "auto",
+			wantMime:   "text/csv",
+		},
 		{
 			name:       "sheet_pdf",
 			googleMime: "application/vnd.google-apps.spreadsheet",