diff --git a/src/Exception/InvalidConfigException.php b/src/Exception/InvalidConfigException.php
new file mode 100644
index 000000000..49874cc0e
--- /dev/null
+++ b/src/Exception/InvalidConfigException.php
@@ -0,0 +1,8 @@
+<?php
+
+namespace Platformsh\Cli\Exception;
+
+class InvalidConfigException extends \InvalidArgumentException
+{
+    protected $code = 4;
+}
diff --git a/src/Helper/FilesystemHelper.php b/src/Helper/FilesystemHelper.php
index dfb37f863..2a6172703 100644
--- a/src/Helper/FilesystemHelper.php
+++ b/src/Helper/FilesystemHelper.php
@@ -167,6 +167,9 @@ public function copyAll($source, $destination)
      */
     public function symLink($target, $link)
     {
+        if ($target === $link) {
+            throw new \InvalidArgumentException("Cannot symlink $link to itself");
+        }
         if (file_exists($link)) {
             $this->fs->remove($link);
         }
diff --git a/src/Local/LocalBuild.php b/src/Local/LocalBuild.php
index 1718dd42e..7e90461d7 100644
--- a/src/Local/LocalBuild.php
+++ b/src/Local/LocalBuild.php
@@ -1,6 +1,7 @@
 <?php
 namespace Platformsh\Cli\Local;
 
+use Platformsh\Cli\Exception\InvalidConfigException;
 use Platformsh\Cli\Helper\FilesystemHelper;
 use Platformsh\Cli\Helper\GitHelper;
 use Platformsh\Cli\Helper\ShellHelper;
@@ -8,6 +9,7 @@
 use Symfony\Component\Console\Output\NullOutput;
 use Symfony\Component\Console\Output\OutputInterface;
 use Symfony\Component\Finder\Finder;
+use Symfony\Component\Yaml\Exception\ParseException;
 use Symfony\Component\Yaml\Parser;
 
 class LocalBuild
@@ -160,13 +162,53 @@ public function getAppConfig($appRoot)
     {
         $config = array();
         if (file_exists($appRoot . '/.platform.app.yaml')) {
-            $parser = new Parser();
-            $config = (array) $parser->parse(file_get_contents($appRoot . '/.platform.app.yaml'));
+            try {
+                $parser = new Parser();
+                $config = (array) $parser->parse(file_get_contents($appRoot . '/.platform.app.yaml'));
+            }
+            catch (ParseException $e) {
+                throw new InvalidConfigException(
+                  "Parse error in file '$appRoot/.platform.app.yaml'. \n" . $e->getMessage()
+                );
+            }
+        }
+
+        return $this->normalizeConfig($config);
+    }
+
+    /**
+     * Normalize an application's configuration.
+     *
+     * @param array $config
+     *
+     * @return array
+     */
+    public function normalizeConfig(array $config)
+    {
+        if (isset($config['toolstack'])) {
+            $this->deprecationWarning('2015.7');
+            if (!strpos($config['toolstack'], ':')) {
+                throw new InvalidConfigException("Invalid value for 'toolstack'");
+            }
+            list($config['type'], $config['build']['flavor']) = explode(':', $config['toolstack'], 2);
         }
 
         return $config;
     }
 
+    /**
+     * Show a warning about deprecated configuration.
+     *
+     * @param string $version
+     * @param string $file
+     */
+    public function deprecationWarning($version, $file = '.platform.app.yaml')
+    {
+        $this->output->writeln("The format of your <comment>$file</comment> configuration file is deprecated.");
+        $this->output->writeln(sprintf("See how to upgrade at: https://docs.platform.sh/reference/upgrade/#changes-in-%s", $version));
+        $this->output->writeln("");
+    }
+
     /**
      * Get the toolstack for a particular application.
      *
@@ -180,9 +222,22 @@ public function getAppConfig($appRoot)
     public function getToolstack($appRoot, array $appConfig = array())
     {
         $toolstackChoice = false;
-        if (isset($appConfig['toolstack'])) {
-            $toolstackChoice = $appConfig['toolstack'];
+
+        // For now, we reconstruct a toolstack string based on the 'type' and
+        // 'build.flavor' config keys.
+        if (isset($appConfig['type'])) {
+            $toolstackChoice = sprintf(
+              '%s:%s',
+              $appConfig['type'],
+              !empty($appConfig['build']['flavor']) ? $appConfig['build']['flavor'] : 'default'
+            );
+
+            // Alias php:default to php:composer.
+            if ($toolstackChoice === 'php:default') {
+                $toolstackChoice = 'php:composer';
+            }
         }
+
         foreach (self::getToolstacks() as $toolstack) {
             $key = $toolstack->getKey();
             if ((!$toolstackChoice && $toolstack->detect($appRoot))
@@ -319,8 +374,15 @@ protected function buildApp($appRoot, $sourceDir, $destination, array $appConfig
             $this->fsHelper->extractArchive($archive, $buildDir);
         } else {
             $message = "Building application <info>$appIdentifier</info>";
-            if ($key = $toolstack->getKey()) {
-                $message .= " using the toolstack <info>$key</info>";
+            $info = array();
+            if (isset($appConfig['type'])) {
+                $info[] = 'runtime type: ' . $appConfig['type'];
+            }
+            if (!empty($treeId)) {
+                $info[] = "tree: " . substr($treeId, 0, 7);
+            }
+            if (!empty($info)) {
+                $message .= ' (' . implode(', ', $info) . ')';
             }
             $this->output->writeln($message);
 
diff --git a/src/Local/Toolstack/Composer.php b/src/Local/Toolstack/Composer.php
index 4540bc64e..8667b25db 100644
--- a/src/Local/Toolstack/Composer.php
+++ b/src/Local/Toolstack/Composer.php
@@ -4,6 +4,10 @@
 
 class Composer extends ToolstackBase
 {
+    public function getKey()
+    {
+        return 'php:composer';
+    }
 
     public function detect($appRoot)
     {
diff --git a/src/Local/Toolstack/ToolstackBase.php b/src/Local/Toolstack/ToolstackBase.php
index 3b55fe5ab..ea7eed0ce 100644
--- a/src/Local/Toolstack/ToolstackBase.php
+++ b/src/Local/Toolstack/ToolstackBase.php
@@ -107,6 +107,9 @@ protected function processSpecialDestinations()
             if (!$matched) {
                 continue;
             }
+            if ($relDestination === '{webroot}' && $this->buildInPlace) {
+                continue;
+            }
 
             // On Platform these replacements would be a bit different.
             $absDestination = str_replace(array('{webroot}', '{approot}'), array($this->getWebRoot(), $this->buildDir), $relDestination);
@@ -117,17 +120,21 @@ protected function processSpecialDestinations()
                 if (in_array($relSource, $this->ignoredFiles)) {
                     continue;
                 }
+                $destination = $absDestination;
+                // Do not overwrite directories with files.
+                if (!is_dir($source) && is_dir($destination)) {
+                    $destination = $destination . '/' . basename($source);
+                }
+                // Ignore if source and destination are the same.
+                if ($destination === $source) {
+                    continue;
+                }
                 if ($this->copy) {
                     $this->output->writeln("Copying $relSource to $relDestination");
                 }
                 else {
                     $this->output->writeln("Symlinking $relSource to $relDestination");
                 }
-                $destination = $absDestination;
-                // Do not overwrite directories with files.
-                if (!is_dir($source) && is_dir($destination)) {
-                    $destination = $destination . '/' . basename($source);
-                }
                 // Delete existing files, emitting a warning.
                 if (file_exists($destination)) {
                     $this->output->writeln(
diff --git a/tests/Local/Toolstack/ComposerTest.php b/tests/Local/Toolstack/ComposerTest.php
index 2e8d4e96f..e740edd8d 100644
--- a/tests/Local/Toolstack/ComposerTest.php
+++ b/tests/Local/Toolstack/ComposerTest.php
@@ -26,4 +26,12 @@ public function testBuildFakeSymfony()
     {
         $this->assertBuildSucceeds('tests/data/apps/fake-symfony');
     }
+
+    /**
+     * Test the deprecated config file format still works.
+     */
+    public function testBuildDeprecatedConfig()
+    {
+        $this->assertBuildSucceeds('tests/data/apps/deprecated-config');
+    }
 }
diff --git a/tests/data/apps/deprecated-config/.platform.app.yaml b/tests/data/apps/deprecated-config/.platform.app.yaml
new file mode 100644
index 000000000..6f0986971
--- /dev/null
+++ b/tests/data/apps/deprecated-config/.platform.app.yaml
@@ -0,0 +1,2 @@
+name: deprecated-config
+toolstack: php:symfony
diff --git a/tests/data/apps/drupal/project-yaml/.platform.app.yaml b/tests/data/apps/drupal/project-yaml/.platform.app.yaml
index 5838d4806..bbf5203ff 100644
--- a/tests/data/apps/drupal/project-yaml/.platform.app.yaml
+++ b/tests/data/apps/drupal/project-yaml/.platform.app.yaml
@@ -1 +1,3 @@
-toolstack: "php:drupal"
+type: php
+build:
+  flavor: drupal
diff --git a/tests/data/apps/drupal/project/.platform.app.yaml b/tests/data/apps/drupal/project/.platform.app.yaml
index e70640625..c2eb729b8 100644
--- a/tests/data/apps/drupal/project/.platform.app.yaml
+++ b/tests/data/apps/drupal/project/.platform.app.yaml
@@ -1,4 +1,6 @@
-toolstack: "php:drupal"
+type: php
+build:
+  flavor: drupal
 
 # Test build hook
 hooks:
diff --git a/tests/data/apps/fake-symfony/.platform.app.yaml b/tests/data/apps/fake-symfony/.platform.app.yaml
index 862c680a4..cfa869d6a 100644
--- a/tests/data/apps/fake-symfony/.platform.app.yaml
+++ b/tests/data/apps/fake-symfony/.platform.app.yaml
@@ -2,4 +2,6 @@
 # composer.json file.
 
 name: fake-symfony
-toolstack: "php:symfony"
+type: php
+build:
+  flavor: symfony
diff --git a/tests/data/repositories/multiple/drupal/.platform.app.yaml b/tests/data/repositories/multiple/drupal/.platform.app.yaml
index d9aab9aaa..07d0f82cf 100644
--- a/tests/data/repositories/multiple/drupal/.platform.app.yaml
+++ b/tests/data/repositories/multiple/drupal/.platform.app.yaml
@@ -1,3 +1,4 @@
 name: drupal
-
-toolstack: "php:drupal"
+type: php
+build:
+  flavor: drupal