diff --git a/composer.lock b/composer.lock
index be3b54a79f5af6af33536d4464dffa3af9eeeef5..dcb72d9c20371a89f8886bb7c34eb3ed2d369930 100644
--- a/composer.lock
+++ b/composer.lock
@@ -496,7 +496,7 @@
         },
         {
             "name": "drupal/core",
-            "version": "9.1.x-dev",
+            "version": "9.1.0-alpha1",
             "dist": {
                 "type": "path",
                 "url": "core",
@@ -745,11 +745,11 @@
         },
         {
             "name": "drupal/core-project-message",
-            "version": "9.1.x-dev",
+            "version": "9.1.0-alpha1",
             "dist": {
                 "type": "path",
                 "url": "composer/Plugin/ProjectMessage",
-                "reference": "cad5c2853f5b733663dcc1be328f80ee233acc30"
+                "reference": "b4efdbe26634b41a1b89e4f3770a8074769088a6"
             },
             "require": {
                 "composer-plugin-api": "^1.1 || ^2",
@@ -778,11 +778,11 @@
         },
         {
             "name": "drupal/core-vendor-hardening",
-            "version": "9.1.x-dev",
+            "version": "9.1.0-alpha1",
             "dist": {
                 "type": "path",
                 "url": "composer/Plugin/VendorHardening",
-                "reference": "3732b3fa7c1db63f41bc72d847793ea8aebf7735"
+                "reference": "d54f0b3cc8b4237f3a41a0860a808db242f9da9e"
             },
             "require": {
                 "composer-plugin-api": "^1.1 || ^2",
@@ -7319,14 +7319,10 @@
     ],
     "aliases": [],
     "minimum-stability": "dev",
-    "stability-flags": {
-        "drupal/core": 20,
-        "drupal/core-project-message": 20,
-        "drupal/core-vendor-hardening": 20
-    },
+    "stability-flags": [],
     "prefer-stable": true,
     "prefer-lowest": false,
     "platform": [],
     "platform-dev": [],
-    "plugin-api-version": "1.1.0"
+    "plugin-api-version": "2.0.0"
 }
diff --git a/composer/Metapackage/CoreRecommended/composer.json b/composer/Metapackage/CoreRecommended/composer.json
index 43298d84557d66af59775d8c1baccf5f7bc9202f..3f2e7d448a74b12eed4a8b2ab010b9e452ca8c15 100644
--- a/composer/Metapackage/CoreRecommended/composer.json
+++ b/composer/Metapackage/CoreRecommended/composer.json
@@ -7,7 +7,7 @@
         "webflo/drupal-core-strict": "*"
     },
     "require": {
-        "drupal/core": "9.1.x-dev",
+        "drupal/core": "9.1.0-alpha1",
         "asm89/stack-cors": "1.3.0",
         "composer/semver": "1.7.1",
         "doctrine/annotations": "1.10.4",
diff --git a/composer/Metapackage/PinnedDevDependencies/composer.json b/composer/Metapackage/PinnedDevDependencies/composer.json
index a91a293d8f66dcf37abd574f8d463539f740c798..662422ce062360624774d2fcdb0f82550bd04fd5 100644
--- a/composer/Metapackage/PinnedDevDependencies/composer.json
+++ b/composer/Metapackage/PinnedDevDependencies/composer.json
@@ -7,7 +7,7 @@
         "webflo/drupal-core-require-dev": "*"
     },
     "require": {
-        "drupal/core": "9.1.x-dev",
+        "drupal/core": "9.1.0-alpha1",
         "behat/mink": "v1.8.1",
         "behat/mink-browserkit-driver": "v1.3.4",
         "behat/mink-goutte-driver": "v1.2.1",
diff --git a/composer/Template/LegacyProject/composer.json b/composer/Template/LegacyProject/composer.json
index 0d293d9a9e4b53a7569c672b78c2740af264a6c7..609268970f75dc0d77c88f6ef87c08a0fc90aa3e 100644
--- a/composer/Template/LegacyProject/composer.json
+++ b/composer/Template/LegacyProject/composer.json
@@ -27,7 +27,7 @@
     "conflict": {
         "drupal/drupal": "*"
     },
-    "minimum-stability": "dev",
+    "minimum-stability": "alpha",
     "prefer-stable": true,
     "config": {
         "sort-packages": true
diff --git a/composer/Template/RecommendedProject/composer.json b/composer/Template/RecommendedProject/composer.json
index 86f1ad8bbba293403e9f6ee0c81f9074654089c6..517c60e6875250b084b939d4695d54786bb646be 100644
--- a/composer/Template/RecommendedProject/composer.json
+++ b/composer/Template/RecommendedProject/composer.json
@@ -26,7 +26,7 @@
     "conflict": {
         "drupal/drupal": "*"
     },
-    "minimum-stability": "dev",
+    "minimum-stability": "alpha",
     "prefer-stable": true,
     "config": {
         "sort-packages": true
diff --git a/core/lib/Drupal.php b/core/lib/Drupal.php
index c2fd03d1ee5bb0126ed6ebc4e1d69d2afaa4ed76..f92bf5c9320cfd7b68cce90efcb7c4cfcbd5cda4 100644
--- a/core/lib/Drupal.php
+++ b/core/lib/Drupal.php
@@ -80,7 +80,7 @@ class Drupal {
   /**
    * The current system version.
    */
-  const VERSION = '9.1.0-dev';
+  const VERSION = '9.1.0-alpha1';
 
   /**
    * Core API compatibility.