diff --git a/bin/cfg b/bin/cfg
index 78e84c5..3c7c0ef 100755
--- a/bin/cfg
+++ b/bin/cfg
@@ -22,7 +22,12 @@ foreach ($autoloadFiles as $autoloadFile) {
 	}
 }
 
-$version = '0.3';
+function verboseLog(bool $enabled, string $message): void
+{
+	if ($enabled) fwrite(STDERR, "[verbose] $message".PHP_EOL);
+}
+
+$version = '0.4';
 
 $actions = [ 'show', 'write', 'help' ];
 $settings = ['key' => '', 'value' => ''];
@@ -35,15 +40,55 @@ $collection = (new Input\InputCollection())
 		->description('Display help text')
 	) // }}}
 
-	->add( Input\InputTypeFactory::build('LongOption')->name('in')->short('i') // {{{
-		->flags(AbstractInputType::FLAG_OPTIONAL | Input\AbstractInputType::FLAG_VALUE_REQUIRED)
-		->description('Path to a json data file to to read')
+	->add( Input\InputTypeFactory::build('LongOption')->name('verbose')->short('v') // {{{
+		->flags(AbstractInputType::FLAG_OPTIONAL)
+		->description('Print debug details to STDERR')
 	) // }}}
 
-	->add( Input\InputTypeFactory::build('LongOption')->name('out')->short('o') // {{{
-		->flags(AbstractInputType::FLAG_OPTIONAL | Input\AbstractInputType::FLAG_VALUE_REQUIRED)
-		->description('Path to a json file to to write to')
-	) // }}}
+	->add( Input\InputTypeFactory::build('LongOption')->name('in')->short('i') // {{{
+			->flags(AbstractInputType::FLAG_OPTIONAL | Input\AbstractInputType::FLAG_VALUE_REQUIRED)
+			->description('Path to a JSON data file to read (for write)')
+			->validator(new Input\Validator(
+				function (AbstractInputType $input, AbstractInputHandler $context)
+				{
+					$path = $context->find('in');
+					if ($path === null || $path === '') {
+						return null;
+					}
+
+					if (!is_readable($path))
+					{
+						throw new \Exception("Input file is not readable: $path");
+					}
+
+					$content = file_get_contents($path);
+					if ($content === false)
+					{
+						throw new \Exception("Can not read input file: $path");
+					}
+
+					try
+					{
+						$data = json_decode($content, true, 512, JSON_THROW_ON_ERROR);
+					}
+					catch (\JsonException $ex)
+					{
+						throw new \Exception(sprintf(
+							'Input JSON is invalid (%s): %s',
+							(string)$ex->getCode(),
+							$ex->getMessage()
+						));
+					}
+
+					if (!is_array($data))
+					{
+						throw new \Exception('Input JSON must decode to an object/array');
+					}
+
+					return $data;
+				}
+			))
+		) // }}}
 
 	->add( Input\InputTypeFactory::build('LongOption')->name('mode')->short('m') // {{{
 		->flags(AbstractInputType::FLAG_OPTIONAL | Input\AbstractInputType::FLAG_VALUE_REQUIRED)
@@ -60,6 +105,60 @@ $collection = (new Input\InputCollection())
 		->description('Path where the config/ directory of the package conf files is located, defaults to the working dir')
 	) // }}}
 
+	->add( Input\InputTypeFactory::build('LongOption')->name('siteDir')->short('s') // {{{
+			->flags(AbstractInputType::FLAG_OPTIONAL | Input\AbstractInputType::FLAG_VALUE_REQUIRED)
+			->description('Site/instance directory below config/. Accepts owner_xyz or config/owner_xyz')
+			->validator(new Input\Validator(
+				function (AbstractInputType $input, AbstractInputHandler $context)
+				{
+					$siteInput = $context->find('siteDir');
+					if ($siteInput === null || $siteInput === false) {
+						return null;
+					}
+
+					$siteInput = trim((string)$siteInput);
+					$appPath = $context->find('appPath');
+					if (!$appPath) {
+						$appPath = getcwd().'/';
+					}
+					$appPath = rtrim($appPath, '/').'/';
+
+					// Accept both "owner_xyz" and "config/owner_xyz" and normalize to site key.
+					if (str_starts_with($siteInput, $appPath.'config/'))
+					{
+						$siteInput = substr($siteInput, strlen($appPath.'config/'));
+					}
+					elseif (str_starts_with($siteInput, 'config/'))
+					{
+						$siteInput = substr($siteInput, strlen('config/'));
+					}
+
+					$siteInput = trim($siteInput, '/');
+					if ($siteInput === '')
+					{
+						throw new \Exception('Option --siteDir is empty.');
+					}
+
+					// Block directory traversal and hidden-dot segments.
+					if (str_contains($siteInput, '..') || preg_match('~(^|/)\.(?:/|$)~', $siteInput))
+					{
+						throw new \Exception("Invalid directory in --siteDir: '$siteInput'.");
+					}
+
+					// Allow only predictable path segments for instance directories.
+					foreach (explode('/', $siteInput) as $part)
+					{
+						if ($part === '' || !preg_match('/^[A-Za-z0-9._-]+$/', $part))
+						{
+							throw new \Exception("Invalid directory name segment '$part' in --siteDir.");
+						}
+					}
+
+					return $siteInput;
+				}
+			))
+		) // }}}
+
 	->add( Input\InputTypeFactory::build('Argument')->name('action') // {{{
 		->flags(AbstractInputType::FLAG_REQUIRED)
 		->description(
@@ -102,18 +201,30 @@ $collection = (new Input\InputCollection())
 		->description(
 			'the settings you want to work on. With action "write" you can pass the value to set as JSON after an equal sign.'
 		)
-		->validator(new Input\Validator(
-			function (AbstractInputType $input, AbstractInputHandler $context)
-			{
-				$setting = $context->find('setting');
-				$action = $context->find('action');
-				
+			->validator(new Input\Validator(
+				function (AbstractInputType $input, AbstractInputHandler $context)
+				{
+					$setting = $context->find('setting');
+						$action = $context->find('action');
+						$inputPayload = $context->find('in');
+
+						if ($action === 'write')
+						{
+							if (($setting !== null && $setting !== '') && $inputPayload) {
+								throw new \Exception('Please use either SETTING or --in, not both.');
+							}
+						}
+	
+					if ($setting === null || $setting === '') {
+						return ['key' => '', 'value' => null];
+					}
+
 				$setting = explode('=', $setting);
 				$settings['key'] = $setting[0];
 				if ($action === 'write')
 				{
-					$value = $setting[1];
-					if (! (isset($value) || $context->find('data'))) {
+					$value = $setting[1] ?? null;
+					if (! (isset($value) || $context->find('in'))) {
 						throw new \Exception('You need a value to write');
 					}
 					$specialValues = [ 'true', 'false', 'null' ];
@@ -139,18 +250,50 @@ $collection = (new Input\InputCollection())
 $usage = Cli\manpage( basename(__FILE__), $version,
 	'read write settings',
 	$collection, Colour::FG_GREEN, Colour::FG_WHITE,
-	[
-		'Examples' =>
-			 'cfg show VeruA db:host'.PHP_EOL.
-			 'cfg write VeruA \'db:host="newHost"\''.PHP_EOL
-	]
+		[
+			'Examples' =>
+				'Basic usage:'
+				.PHP_EOL
+				 .'cfg show VeruA db:host'
+				 .PHP_EOL
+				 .'cfg write VeruA \'db:host="newHost"\''
+				 .PHP_EOL
+				 .PHP_EOL
+				.'Advance usage:'
+            	 .PHP_EOL
+				 .'cfg write prefix \'module:enabled=true\''
+				 .PHP_EOL
+				 .PHP_EOL
+				 .'#Site specific write:'
+				 .PHP_EOL
+				 .'cfg write prefix \'module:enabled=true\' --siteDir=owner_xyz'
+				 .PHP_EOL
+				 .PHP_EOL
+				 .'#Batch write from JSON file:'
+				 .PHP_EOL
+				 .'cfg write prefix --siteDir=owner_xyz -i /tmp/extension.json'
+				 .PHP_EOL
+				 .PHP_EOL
+				 .'#Batch write from JSON string:'
+				 .PHP_EOL
+				 .'cfg write prefix \'module={"enabled":true,"timeout":30,"label":"example"}\' --siteDir=owner_xyz'
+				 .PHP_EOL
+				 .PHP_EOL
+				 .'#Read merged value for a site:'
+				 .PHP_EOL
+				 .'cfg show prefix module:enabled --siteDir=owner_xyz'
+				 .PHP_EOL
+		]
 ).PHP_EOL;
 
 // Get the supplied input. Passing the collection will make the handler bind values
 // and validate the input according to our collection
-try {
+try
+{
 	$argv = Input\InputHandlerFactory::build('Argv', $collection);
-} catch (\Exception $ex) {
+}
+catch (\Exception $ex)
+{
 	echo $usage;
 
 	if (isset($argv[1])) {
@@ -172,42 +315,41 @@ if ($argv->find( 'help' ) || $argv->find('action') == 'help')
 
 
 $prefix = $argv->find('prefix');
-/* 
-echo $argv->find('action').PHP_EOL;
-echo ($prefix).PHP_EOL;
-echo $argv->find('setting')['key'].PHP_EOL;
-echo $argv->find('setting')['value'].PHP_EOL;
- */
-// var_dump($cfg);
+$verbose = (bool)$argv->find('verbose');
 $appPath = $argv->find('appPath');
 if (!$appPath) $appPath = getcwd().'/';
+$appPath = rtrim($appPath, '/').'/';
 
-/* $it = new RecursiveDirectoryIterator($appPath);
-
-foreach(new RecursiveIteratorIterator($it) as $file)
-{
-	$configDir = $file->getPath();
-	if ($file->isDir() && $file->getFilename() == '.' && basename($configDir) == 'config') {
-		echo "found config dir: $configDir\n";
-	}
-}
- */
 $mode = ($argv->find('mode') == '') ? null : $argv->find('mode');
 $cfg = (new Settings([], $mode))->appPath($appPath)->prefix($prefix);
-if ($pkgPath = $argv->find('pkgPath')) $cfg->pkgPath($pkgPath);
+// pkgPath points to package defaults (e.g. <prefix>.default.conf.php)
+if ($pkgPath = $argv->find('pkgPath')) $cfg->pkgPath(rtrim($pkgPath, '/').'/');
 
-try {
+$site = null;
+$siteFlag = 0x01;
+$site = $argv->find('siteDir');
+if ($site !== null && $site !== false)
+{
+		// Reuse util-settings site resolution: config/<site>/<prefix>.conf.php
+		$cfg->site($site);
+		verboseLog($verbose, "siteDir resolved to '$site'");
+}
+
+try
+{
 	if (is_readable($cfg->buildFileName('default'))) {
 		$cfg->load();
 	}
 	elseif (is_readable($cfgFile = $cfg->buildFileName())) {
 		$cfg->load(require($cfgFile));
 	}
-} catch (Exception $e) {
+}
+catch (\Throwable $e)
+{
 	fwrite(STDERR, "Error: ".$e->getMessage().PHP_EOL);
 	exit(1);
 }
-//var_dump($cfg);
+verboseLog($verbose, 'config bootstrap loaded');
 
 $result = $cfg;
 $settings = $argv->find('setting') ?? $settings;
@@ -232,33 +374,63 @@ if ($result instanceof Settings) $result = $result->toArray();
 switch ($argv->find('action'))
 {
 case 'show':
+	verboseLog($verbose, "show action for prefix '$prefix'");
 	$out = (is_string($result)) ? $result : json_encode($result, JSON_PRETTY_PRINT);
 	echo $out.PHP_EOL;
 	break;
 	
 case 'write':
+	$inputPayload = $argv->find('in');
+	if (!$inputPayload && $settings['key'] === '')
+	{
+		fwrite(STDERR, 'Nothing to write: provide SETTING or --in.'.PHP_EOL);
+		exit(1);
+	}
 	$path = ($settings['key'] !== '') ? explode(':', $settings['key']) : [];
-	var_dump($path);
-	
-	$setting2write = $settings['value'];
+	verboseLog($verbose, 'write source: '.($inputPayload ? '--in' : 'SETTING'));
 	
+	if ($inputPayload)
+	{
+		$setting2write = $inputPayload;
+	}
+	else
+	{
+		$setting2write = $settings['value'];
+	}
+	verboseLog($verbose, 'write path: '.json_encode($path));
+
 	while ( ! empty($path))
 	{
 		$setting2write = [array_pop($path) => $setting2write];
 	}
-	if (is_readable($file = $cfg->buildFileName()))
+
+    $writeType = ($site !== null && $site !== false) ? $siteFlag : null;
+	$file = $cfg->buildFileName($writeType);
+	verboseLog($verbose, "write target: $file");
+	if (is_readable($file))
 	{
 		$setting2write = array_replace_recursive(require($file), $setting2write);
 		copy($file, "$file.bak");
+		verboseLog($verbose, "existing config merged from: $file");
 	}
+
+	$targetDir = dirname($file);
+	if (!is_dir($targetDir) && !mkdir($targetDir, 0775, true) && !is_dir($targetDir))
+	{
+		fwrite(STDERR, "Can not create directory: $targetDir".PHP_EOL);
+		exit(1);
+	}
+
 	$writeCfg = $cfg->create($setting2write);
-//	var_dump($writeCfg->toArray());
-	try {
-		(new SettingsWriter($writeCfg))->write();
+	verboseLog($verbose, 'payload prepared for writer');
+	try
+	{
+		(new SettingsWriter($writeCfg, '', $writeType))->write();
 		echo "Written modified settings to: $file".PHP_EOL;
 	}
 	catch (\Exception $e) {
-		echo $e->getMessage().PHP_EOL;
+		fwrite(STDERR, $e->getMessage().PHP_EOL);
+		exit(1);
 	}
 
 	break;
diff --git a/src/Settings.php b/src/Settings.php
index 56e1b0f..ebf7aeb 100644
--- a/src/Settings.php
+++ b/src/Settings.php
@@ -159,7 +159,18 @@ class Settings implements \Iterator, \Countable
 			// if a mode was set in the constructor do not overwrite it
 			if (! isset($this->mode)) {
 				// if a localConf Mode is set use it, or take the default conf mode
-				$this->mode = (isset($localConf['mode'])) ? $localConf['mode'] : $this->settings['mode'];
+                // if local/default config has no mode, fall back to the first known mode (normally "prod")
+				if (isset($localConf['mode']) && is_string($localConf['mode']) && $localConf['mode'] !== '') {
+					$this->mode = $localConf['mode'];
+				}
+				elseif (isset($this->settings['mode']) && is_string($this->settings['mode']) && $this->settings['mode'] !== '') {
+					$this->mode = $this->settings['mode'];
+				}
+				else {
+					// Backward-compatible fallback for configs without explicit mode.
+					$this->mode = (string)array_key_first($this->modes);
+					$this->settings['mode'] = $this->mode;
+				}
 			} else {
 				$this->settings['mode'] = $this->mode;
 			}
@@ -358,4 +369,4 @@ class Settings implements \Iterator, \Countable
 
 /* jEdit buffer local properties {{{
   * :folding=explicit:collapseFolds=1:
-}}}*/
\ No newline at end of file
+}}}*/
diff --git a/src/SettingsWriter.php b/src/SettingsWriter.php
index d6fc41c..6246a9f 100644
--- a/src/SettingsWriter.php
+++ b/src/SettingsWriter.php
@@ -22,6 +22,8 @@
 declare(strict_types=1);
 namespace rabe\Util;
 
+use ErrorException;
+
 /**
  * Write Settings into a File
  * @author Norbert.e.Wagner dev@norb.me
@@ -45,13 +47,16 @@ class SettingsWriter
 	 *
 	 * @param Settings $settings an object of type settings
 	 * @param String $name The name midfix for the settings File
-	 */
-	public function __construct( Settings $settings, $name='' )
+     * @param String|int|null $type Optional type for Settings::buildFileName(), e.g. site flag
+     */
+    public function __construct( Settings $settings, $name='', $type=null )
 	{
 		$this->settings = $settings;
-		
-		$file = $settings->buildFileName( $name );
-		
+
+        $file = ( $type === null )
+            ? $settings->buildFileName( $name )
+            : $settings->buildFileName( $type );
+
 		if ( ! $this->handle = fopen( $file, 'w' ) )
 		{
 			throw new ErrorException( "Can not open File: $file" );
diff --git a/tests/CfgTest.php b/tests/CfgTest.php
new file mode 100644
index 0000000..646bcb2
--- /dev/null
+++ b/tests/CfgTest.php
@@ -0,0 +1,457 @@
+<?php
+declare(strict_types=1);
+namespace rabe\Util\tests;
+
+use PHPUnit\Framework\TestCase;
+
+class CfgTest extends TestCase
+{
+	private string $tmpDir;
+
+	/* Lifecycle {{{ */
+	protected function setUp(): void
+	{
+		$this->tmpDir = rtrim(sys_get_temp_dir(), '/').'/util-settings-cfg-'.bin2hex(random_bytes(8));
+		mkdir($this->tmpDir, 0775, true);
+		mkdir($this->tmpDir.'/config', 0775, true);
+
+		file_put_contents(
+			$this->tmpDir.'/config/Extension.default.conf.php',
+			"<?php return [\n\t'mode' => 'prod',\n\t'module' => [\n\t\t'code' => '',\n\t\t'label' => '',\n\t\t'flags' => [\n\t\t\t'enabled' => false,\n\t\t],\n\t],\n\t'feature' => [\n\t\t'endpoint' => '',\n\t],\n];\n"
+		);
+	}
+
+	protected function tearDown(): void
+	{
+		$this->removeDir($this->tmpDir);
+	}
+	/* }}} */
+
+	/* Write Flow {{{ */
+	public function testWriteWithoutDirectoryNameUsesLocalConfigFile(): void
+	{
+		$result = $this->runWrite([
+			'module:code="X100"',
+		]);
+
+		$this->assertCfgSuccess($result);
+
+		$localFile = $this->tmpDir.'/config/Extension.conf.php';
+		$this->assertFileExists($localFile);
+
+		$cfg = require $localFile;
+		$this->assertSame('X100', $cfg['module']['code']);
+	}
+
+	public function testWriteWithoutModeFallsBackToDefaultMode(): void
+	{
+		file_put_contents(
+			$this->tmpDir.'/config/Extension.default.conf.php',
+			"<?php return [\n\t'module' => [\n\t\t'code' => '',\n\t],\n];\n"
+		);
+
+		$result = $this->runWrite([
+			'module:code="X100"',
+		]);
+
+		$this->assertCfgSuccess($result);
+		$this->assertFileExists($this->tmpDir.'/config/Extension.conf.php');
+	}
+
+	public function testWriteWithInputFileWritesMultipleSettings(): void
+	{
+		$inFile = $this->createJsonInputFile([
+			'module' => [
+				'code' => 'X100',
+				'label' => 'demo-module',
+			],
+			'feature' => [
+				'endpoint' => 'https://example.invalid/v1/resource',
+			],
+		]);
+
+		$result = $this->runWrite([
+			'--siteDir=owner_xyz',
+			'-i',
+			$inFile,
+		]);
+
+		$this->assertCfgSuccess($result);
+		$cfg = $this->loadWrittenConfig('owner_xyz');
+		$this->assertSame('X100', $cfg['module']['code']);
+		$this->assertSame('demo-module', $cfg['module']['label']);
+		$this->assertSame('https://example.invalid/v1/resource', $cfg['feature']['endpoint']);
+	}
+
+	public function testWriteAcceptsJsonStringValueInSetting(): void
+	{
+		$result = $this->runWrite([
+			'module={"code":"X100","label":"demo-module"}',
+			'--siteDir=owner_xyz',
+		]);
+
+		$this->assertCfgSuccess($result);
+		$cfg = $this->loadWrittenConfig('owner_xyz');
+		$this->assertSame('X100', $cfg['module']['code']);
+		$this->assertSame('demo-module', $cfg['module']['label']);
+	}
+
+	public function testWriteSupportsNestedPathWithColonNotation(): void
+	{
+		$result = $this->runWrite([
+			'module:flags:enabled=true',
+			'--siteDir=owner_xyz',
+		]);
+
+		$this->assertCfgSuccess($result);
+		$cfg = $this->loadWrittenConfig('owner_xyz');
+		$this->assertTrue($cfg['module']['flags']['enabled']);
+	}
+
+	public function testWriteCoercesSimpleStringValueToJsonString(): void
+	{
+		$result = $this->runWrite([
+			'module:code=X100',
+			'--siteDir=owner_xyz',
+		]);
+
+		$this->assertCfgSuccess($result);
+		$cfg = $this->loadWrittenConfig('owner_xyz');
+		$this->assertSame('X100', $cfg['module']['code']);
+	}
+
+	public function testWriteRejectsInvalidJsonStringValue(): void
+	{
+		$result = $this->runWrite([
+			'module={"code":',
+			'--siteDir=owner_xyz',
+		]);
+
+		$this->assertCfgFailure($result, 'The value does not appear to be a valid JSON string.');
+	}
+
+	public function testWriteWithSiteDirCreatesAndWritesSiteConfig(): void
+	{
+		$result = $this->runWrite([
+			'module:code="X100"',
+			'--siteDir=owner_xyz',
+		]);
+
+		$this->assertCfgSuccess($result);
+
+		$siteFile = $this->tmpDir.'/config/owner_xyz/Extension.conf.php';
+		$this->assertFileExists($siteFile);
+		$this->assertFileDoesNotExist($this->tmpDir.'/config/Extension.conf.php');
+
+		$cfg = require $siteFile;
+		$this->assertSame('X100', $cfg['module']['code']);
+	}
+
+	public function testWriteWithSiteDirMergesIntoExistingSiteConfig(): void
+	{
+		$firstWrite = $this->runWrite([
+			'module:code="X100"',
+			'--siteDir=owner_xyz',
+		]);
+		$this->assertCfgSuccess($firstWrite);
+
+		$secondWrite = $this->runWrite([
+			'module:label="demo-module"',
+			'--siteDir=owner_xyz',
+		]);
+		$this->assertCfgSuccess($secondWrite);
+
+		$cfg = $this->loadWrittenConfig('owner_xyz');
+
+		$this->assertSame('X100', $cfg['module']['code']);
+		$this->assertSame('demo-module', $cfg['module']['label']);
+	}
+
+	public function testWriteWithSiteDirUsingConfigPrefixIsNormalized(): void
+	{
+		$result = $this->runWrite([
+			'module:code="X100"',
+			'--siteDir=config/owner_xyz',
+		]);
+
+		$this->assertCfgSuccess($result);
+		$this->assertFileExists($this->tmpDir.'/config/owner_xyz/Extension.conf.php');
+		$this->assertFileDoesNotExist($this->tmpDir.'/config/config/owner_xyz/Extension.conf.php');
+	}
+
+	public function testWriteWithSiteDirUsingAbsoluteConfigPrefixIsNormalized(): void
+	{
+		$result = $this->runWrite([
+			'module:code="X100"',
+			'--siteDir='.$this->tmpDir.'/config/owner_xyz',
+		]);
+
+		$this->assertCfgSuccess($result);
+		$this->assertFileExists($this->tmpDir.'/config/owner_xyz/Extension.conf.php');
+	}
+
+	public function testWriteWithNestedSiteDirWritesToNestedSiteConfig(): void
+	{
+		$result = $this->runWrite([
+			'module:code="X100"',
+			'--siteDir=owner_xyz/sub_a',
+		]);
+
+		$this->assertCfgSuccess($result);
+		$this->assertFileExists($this->tmpDir.'/config/owner_xyz/sub_a/Extension.conf.php');
+	}
+
+	public function testWriteWithSiteDirWritesToSiteConfig(): void
+	{
+		$result = $this->runWrite([
+			'module:code="X100"',
+			'--siteDir=owner_xyz',
+		]);
+
+		$this->assertCfgSuccess($result);
+		$this->assertFileExists($this->tmpDir.'/config/owner_xyz/Extension.conf.php');
+	}
+
+	public function testWriteToExistingSiteConfigCreatesBackup(): void
+	{
+		$firstWrite = $this->runWrite([
+			'module:code="first"',
+			'--siteDir=owner_xyz',
+		]);
+		$this->assertCfgSuccess($firstWrite);
+
+		$secondWrite = $this->runWrite([
+			'module:code="second"',
+			'--siteDir=owner_xyz',
+		]);
+		$this->assertCfgSuccess($secondWrite);
+
+		$siteFile = $this->tmpDir.'/config/owner_xyz/Extension.conf.php';
+		$backupFile = $siteFile.'.bak';
+		$this->assertFileExists($backupFile);
+
+		$current = require $siteFile;
+		$backup = require $backupFile;
+		$this->assertSame('second', $current['module']['code']);
+		$this->assertSame('first', $backup['module']['code']);
+	}
+	/* }}} */
+
+	/* Show Flow {{{ */
+	public function testShowWithSiteReturnsSiteSpecificValue(): void
+	{
+		$this->runWrite([
+			'module:code="X100"',
+			'--siteDir=owner_xyz',
+		]);
+
+		$show = $this->runShow([
+			'module:code',
+			'--siteDir=owner_xyz',
+		]);
+
+		$this->assertCfgSuccess($show);
+		$this->assertStringContainsString('X100', $show['output']);
+	}
+	/* }}} */
+
+	/* Validation {{{ */
+	public function testInvalidActionReturnsError(): void
+	{
+		$result = $this->runCfg([
+			'-a',
+			$this->tmpDir,
+			'delete',
+			'Extension',
+		]);
+
+		$this->assertCfgFailure($result, "'delete' not found");
+	}
+
+	public function testSiteDirRejectsEmptyValue(): void
+	{
+		$result = $this->runWrite([
+			'module:code="X100"',
+			'--siteDir=',
+		]);
+
+		$this->assertCfgFailure($result, 'a value is required for --siteDir');
+	}
+
+	/**
+	 * @dataProvider siteDirValidationDataProvider
+	 */
+	public function testSiteDirValidation(string $siteDir, string $expectedMessage): void
+	{
+		$result = $this->runWrite([
+			'module:code="X100"',
+			'--siteDir='.$siteDir,
+		]);
+
+		$this->assertCfgFailure($result, $expectedMessage);
+	}
+
+	/**
+	 * @dataProvider inputFileValidationDataProvider
+	 */
+	public function testInputFileValidation(?string $fileContents, string $pathSuffix, string $expectedMessage): void
+	{
+		$inFile = $this->tmpDir.'/'.$pathSuffix;
+		if ($fileContents !== null) {
+			file_put_contents($inFile, $fileContents);
+		}
+
+		$result = $this->runWrite([
+			'-i',
+			$inFile,
+		]);
+
+		$this->assertCfgFailure($result, $expectedMessage);
+	}
+
+	/**
+	 * @dataProvider writePayloadSourceValidationDataProvider
+	 */
+	public function testWritePayloadSourceValidation(array $extraArgs, string $expectedMessage): void
+	{
+		$inFile = $this->createJsonInputFile(['module' => ['code' => 'X100']]);
+
+		$args = array_merge(
+			[
+				'-a',
+				$this->tmpDir,
+				'write',
+				'Extension',
+			],
+			array_map(
+				fn (string $arg): string => $arg === '__INPUT_FILE__' ? $inFile : $arg,
+				$extraArgs
+			)
+		);
+
+		$result = $this->runCfg($args);
+
+		$this->assertCfgFailure($result, $expectedMessage);
+	}
+
+	public function siteDirValidationDataProvider(): array
+	{
+		return [
+			'traversal' => ['../owner_xyz', 'Invalid directory in --siteDir'],
+			'invalid segment' => ['owner xyz', "Invalid directory name segment 'owner xyz' in --siteDir."],
+			'hidden dot segment' => ['owner_xyz/./secret', "Invalid directory in --siteDir: 'owner_xyz/./secret'."],
+			'config root only' => ['config/', 'Option --siteDir is empty.'],
+		];
+	}
+
+	public function inputFileValidationDataProvider(): array
+	{
+		return [
+			'missing file' => [null, 'missing.json', 'Input file is not readable'],
+			'invalid json' => ['{"module":', 'invalid.json', 'Input JSON is invalid'],
+			'scalar json' => ['true', 'scalar.json', 'Input JSON must decode to an object/array'],
+		];
+	}
+
+	public function writePayloadSourceValidationDataProvider(): array
+	{
+		return [
+			'missing payload source' => [[], 'Nothing to write: provide SETTING or --in.'],
+			'both payload sources' => [['module:code="x"', '-i', '__INPUT_FILE__'], 'Please use either SETTING or --in, not both.'],
+		];
+	}
+	/* }}} */
+
+	/* Helpers {{{ */
+	private function runWrite(array $args): array
+	{
+		return $this->runCfg(array_merge([
+			'-a',
+			$this->tmpDir,
+			'write',
+			'Extension',
+		], $args));
+	}
+
+	private function runShow(array $args): array
+	{
+		return $this->runCfg(array_merge([
+			'-a',
+			$this->tmpDir,
+			'show',
+			'Extension',
+		], $args));
+	}
+
+	private function createJsonInputFile(array $contents, string $name = 'extension-in.json'): string
+	{
+		$path = $this->tmpDir.'/'.$name;
+		file_put_contents($path, json_encode($contents, JSON_PRETTY_PRINT));
+
+		return $path;
+	}
+
+	private function loadWrittenConfig(?string $siteDir = null): array
+	{
+		$path = $siteDir === null
+			? $this->tmpDir.'/config/Extension.conf.php'
+			: $this->tmpDir.'/config/'.$siteDir.'/Extension.conf.php';
+
+		return require $path;
+	}
+
+	private function assertCfgSuccess(array $result): void
+	{
+		$this->assertSame(0, $result['code'], $result['output']);
+	}
+
+	private function assertCfgFailure(array $result, string $message): void
+	{
+		$this->assertSame(1, $result['code'], $result['output']);
+		$this->assertStringContainsString($message, $result['output']);
+	}
+
+	private function runCfg(array $args): array
+	{
+		$script = realpath(__DIR__.'/../bin/cfg');
+		$this->assertNotFalse($script);
+
+		$command = escapeshellarg(PHP_BINARY).' '.escapeshellarg($script);
+		foreach ($args as $arg) {
+			$command .= ' '.escapeshellarg($arg);
+		}
+		$command .= ' 2>&1';
+
+		$outputLines = [];
+		$exitCode = 0;
+		exec($command, $outputLines, $exitCode);
+
+		return [
+			'code' => $exitCode,
+			'output' => implode(PHP_EOL, $outputLines),
+		];
+	}
+
+	private function removeDir(string $dir): void
+	{
+		if (!is_dir($dir)) {
+			return;
+		}
+
+		foreach (scandir($dir) ?: [] as $entry) {
+			if ($entry === '.' || $entry === '..') {
+				continue;
+			}
+
+			$path = $dir.'/'.$entry;
+			if (is_dir($path)) {
+				$this->removeDir($path);
+			} else {
+				unlink($path);
+			}
+		}
+
+		rmdir($dir);
+	}
+	/* }}} */
+}
diff --git a/tests/SettingsTest.php b/tests/SettingsTest.php
index 64ed411..09f311c 100644
--- a/tests/SettingsTest.php
+++ b/tests/SettingsTest.php
@@ -192,6 +192,15 @@ class SettingsTest extends TestCase
 		$cfg = $this->appPath($cfg, 'localOverride')->load();
 		$this->assertEquals(42, $cfg->answer);
 	}
+
+	public function testLoadWithoutModeFallsBackToFirstKnownMode(): void
+	{
+		$cfg = new Settings();
+		$cfg = $this->appPath($cfg, 'noMode')->load();
+
+		$this->assertEquals('prod', $cfg->mode);
+		$this->assertEquals('default', $cfg->testFiles);
+	}
 	
 	/**
 	 * @dataProvider fileNameData
@@ -211,6 +220,22 @@ class SettingsTest extends TestCase
 							  
 		$this->assertEquals($path.$expected, $cfg->buildFileName($type));
 	}
+
+	public function testBuildFileNameWithPrefixAndSite(): void
+	{
+		$cfg = new Settings();
+		$cfg->appPath('./')->prefix('Extension')->site('owner_xyz');
+
+		$this->assertEquals('./config/owner_xyz/Extension.conf.php', $cfg->buildFileName(0x01));
+	}
+
+	public function testBuildFileNameWithPrefixAndNestedSite(): void
+	{
+		$cfg = new Settings();
+		$cfg->appPath('./')->prefix('Extension')->site('owner_xyz/sub_a');
+
+		$this->assertEquals('./config/owner_xyz/sub_a/Extension.conf.php', $cfg->buildFileName(0x01));
+	}
 	
 	public function fileNameData()
 	{
diff --git a/tests/cfg/noMode/config/default.conf.php b/tests/cfg/noMode/config/default.conf.php
new file mode 100644
index 0000000..c0a0d9b
--- /dev/null
+++ b/tests/cfg/noMode/config/default.conf.php
@@ -0,0 +1,3 @@
+<?php return [
+	'testFiles' => 'default',
+];