diff --git a/drupal-dev/composer-plugin/src/Plugin.php b/drupal-dev/composer-plugin/src/Plugin.php
index a97d152..6b5e1fd 100644
--- a/drupal-dev/composer-plugin/src/Plugin.php
+++ b/drupal-dev/composer-plugin/src/Plugin.php
@@ -29,18 +29,31 @@ public function activate(Composer $composer, IOInterface $io): void
         $this->composer = $composer;
         $this->io = $io;
 
-        // Seed installer-paths from core's composer.json into the root package
-        // extra so the fallback in getInstallPath() works before the merge
-        // plugin has had a chance to merge core's extra.
-        $rootExtra = $composer->getPackage()->getExtra();
-        if (empty($rootExtra['installer-paths'])) {
-            $coreFile = getcwd() . '/composer.json';
-            if (file_exists($coreFile)) {
-                $coreConfig = json_decode(file_get_contents($coreFile), true);
-                if (!empty($coreConfig['extra']['installer-paths'])) {
-                    $rootExtra['installer-paths'] = $coreConfig['extra']['installer-paths'];
-                    $composer->getPackage()->setExtra($rootExtra);
-                }
+        // Seed installer-paths and config.platform from core's composer.json
+        // into the root package so the fallback in getInstallPath() works
+        // before the merge plugin has had a chance to merge core's extra, and
+        // so PHPStan sees the correct platform PHP version.
+        //
+        // config.platform must also be written to the root composer file on
+        // disk because PHPStan reads it directly from that file (via the
+        // $COMPOSER env var) in a separate process — the in-memory Config
+        // merge is not visible to it.
+        $coreFile = getcwd() . '/composer.json';
+        if (file_exists($coreFile)) {
+            $coreConfig = json_decode(file_get_contents($coreFile), true);
+
+            $rootExtra = $composer->getPackage()->getExtra();
+            if (empty($rootExtra['installer-paths']) && !empty($coreConfig['extra']['installer-paths'])) {
+                $rootExtra['installer-paths'] = $coreConfig['extra']['installer-paths'];
+                $composer->getPackage()->setExtra($rootExtra);
+            }
+
+            if (!empty($coreConfig['config']['platform'])) {
+                $composer->getConfig()->merge(
+                    ['config' => ['platform' => $coreConfig['config']['platform']]],
+                    $coreFile
+                );
+                $this->syncPlatformToRootFile($coreConfig['config']['platform']);
             }
         }
 
@@ -120,6 +133,44 @@ private function registerInstaller(): void
         $this->composer->getInstallationManager()->addInstaller($installer);
     }
 
+    /**
+     * Write config.platform into the root composer file (e.g. composer.local.json)
+     * so that tools like PHPStan, which read that file directly in a separate
+     * process, pick up the correct platform PHP version.
+     *
+     * Only writes when the value differs from what is already on disk to avoid
+     * unnecessary file churn.
+     *
+     * @param array<string, string> $platform
+     */
+    private function syncPlatformToRootFile(array $platform): void
+    {
+        $envComposer = getenv('COMPOSER');
+        $rootFileName = is_string($envComposer) && $envComposer !== '' ? basename($envComposer) : 'composer.json';
+        $rootFile = getcwd() . '/' . $rootFileName;
+
+        // Only act when we're actually running under an overlay root file that
+        // is separate from the core composer.json we just read.
+        if ($rootFile === getcwd() . '/composer.json' || !file_exists($rootFile)) {
+            return;
+        }
+
+        $rootData = json_decode(file_get_contents($rootFile), true);
+        if (!is_array($rootData)) {
+            return;
+        }
+
+        if (($rootData['config']['platform'] ?? null) === $platform) {
+            return;
+        }
+
+        $rootData['config']['platform'] = $platform;
+        file_put_contents(
+            $rootFile,
+            json_encode($rootData, JSON_PRETTY_PRINT | JSON_UNESCAPED_SLASHES) . "\n"
+        );
+    }
+
     public function deactivate(Composer $composer, IOInterface $io): void
     {
     }
diff --git a/tests/test.bats b/tests/test.bats
index 3d2d7e4..e3d4eda 100644
--- a/tests/test.bats
+++ b/tests/test.bats
@@ -359,3 +359,21 @@ PY
     [ "${core_sha}" = "${overlay_sha}" ]
   fi
 }
+
+@test "config-platform" {
+  set -eu -o pipefail
+  addon_setup
+
+  # The plugin must sync config.platform from composer.json into composer.local.json
+  # so that tools like PHPStan, which read the root composer file directly, pick
+  # up the correct platform PHP version instead of falling back to the runtime.
+  core_platform=$(python3 -c "import json; print(json.load(open('${TESTDIR}/composer.json'))['config']['platform']['php'])")
+  overlay_platform=$(python3 -c "import json; print(json.load(open('${TESTDIR}/composer.local.json'))['config']['platform']['php'])")
+  [ "${core_platform}" = "${overlay_platform}" ]
+
+  # PHPStan must report the platform version, not the runtime version.
+  # -vvv emits "PHP version for analysis: X.Y (from config.platform.php in composer.json)"
+  run ddev exec vendor/bin/phpstan -vvv analyze --configuration=core/phpstan.neon.dist -- core/lib/Drupal/Core/Entity/EntityInterface.php
+  assert_success
+  assert_output --partial "from config.platform.php"
+}