diff --git a/.github/workflows/vortex-release.yml b/.github/workflows/vortex-release.yml
index a799de9cc..0db1a74ce 100644
--- a/.github/workflows/vortex-release.yml
+++ b/.github/workflows/vortex-release.yml
@@ -30,6 +30,13 @@ jobs:
     outputs:
       release-version: ${{ steps.version.outputs.value }}
 
+    # The current major (built from this ref) and the other major (built from
+    # its '{N}.x' branch) are both derived from 'VORTEX_CURRENT_MAJOR' (default
+    # 1), so a single variable bump promotes a new major with no workflow edits.
+    env:
+      CURRENT_MAJOR: ${{ vars.VORTEX_CURRENT_MAJOR || '1' }}
+      OTHER_MAJOR: ${{ (vars.VORTEX_CURRENT_MAJOR || '1') == '1' && '2' || '1' }}
+
     steps:
       - name: Checkout code
         uses: actions/checkout@df4cb1c069e1874edd31b4311f1884172cec0e10 # v6
@@ -85,13 +92,39 @@ jobs:
           ./build/installer.phar --no-interaction --no-cleanup --destination=test || exit 1
         working-directory: .vortex/installer
 
-      - name: Upload artifact
+      - name: Upload v${{ env.CURRENT_MAJOR }} installer artifact
         uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7
         with:
-          name: vortex-installer
+          name: vortex-installer-v${{ env.CURRENT_MAJOR }}
           path: .vortex/installer/build/installer.phar
           if-no-files-found: error
 
+      # Build the other major's installer from its '{N}.x' branch so it is
+      # published alongside the current one. Checked out into a separate path so
+      # the current installer build above is untouched. Mirrors how the docs job
+      # pulls the other major's branch content.
+      - name: Checkout v${{ env.OTHER_MAJOR }} branch
+        uses: actions/checkout@df4cb1c069e1874edd31b4311f1884172cec0e10 # v6
+        with:
+          ref: ${{ env.OTHER_MAJOR }}.x
+          path: vortex-v${{ env.OTHER_MAJOR }}
+          persist-credentials: false
+
+      - name: Build v${{ env.OTHER_MAJOR }} installer
+        run: |
+          composer install
+          sed -i "s/\"vortex-installer-version\": \"development\"/\"vortex-installer-version\": \"${OTHER_MAJOR}.x-dev\"/g" box.json
+          composer build
+          ./build/installer.phar --version
+        working-directory: vortex-v${{ env.OTHER_MAJOR }}/.vortex/installer
+
+      - name: Upload v${{ env.OTHER_MAJOR }} installer artifact
+        uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7
+        with:
+          name: vortex-installer-v${{ env.OTHER_MAJOR }}
+          path: vortex-v${{ env.OTHER_MAJOR }}/.vortex/installer/build/installer.phar
+          if-no-files-found: error
+
   vortex-release-docs:
     needs: vortex-release-installer
 
@@ -109,6 +142,10 @@ jobs:
       run:
         working-directory: docs
 
+    env:
+      CURRENT_MAJOR: ${{ vars.VORTEX_CURRENT_MAJOR || '1' }}
+      OTHER_MAJOR: ${{ (vars.VORTEX_CURRENT_MAJOR || '1') == '1' && '2' || '1' }}
+
     steps:
       - name: Checkout code
         uses: actions/checkout@df4cb1c069e1874edd31b4311f1884172cec0e10 # v6
@@ -134,14 +171,33 @@ jobs:
         with:
           php-version: 8.3
 
-      - name: Download installer
+      - name: Download v1 installer
         uses: actions/download-artifact@3e5f45b2cfb9172054b4087a40e8e0b5a5461e7c # v8
         with:
-          name: vortex-installer
+          name: vortex-installer-v1
+          path: installer-v1
 
-      - name: Copy installer to docs
+      - name: Download v2 installer
+        uses: actions/download-artifact@3e5f45b2cfb9172054b4087a40e8e0b5a5461e7c # v8
+        with:
+          name: vortex-installer-v2
+          path: installer-v2
+
+      # Publish both installers. '/v1/install' and '/v2/install' are the stable
+      # per-major pins, always built from each major's own source. The bare
+      # '/install' is a copy of the current major's pin, selected by the
+      # 'VORTEX_CURRENT_MAJOR' repository variable (default 1) - bumping that one
+      # variable is the only action needed to promote a new major.
+      - name: Copy installers to docs
         run: |
-          cp ../installer.phar ../.vortex/docs/static/install
+          case "${CURRENT_MAJOR}" in
+            1|2) ;;
+            *) echo "Invalid VORTEX_CURRENT_MAJOR='${CURRENT_MAJOR}'. Expected 1 or 2."; exit 1 ;;
+          esac
+          mkdir -p ../.vortex/docs/static/v1 ../.vortex/docs/static/v2
+          cp ../installer-v1/installer.phar ../.vortex/docs/static/v1/install
+          cp ../installer-v2/installer.phar ../.vortex/docs/static/v2/install
+          cp "../.vortex/docs/static/v${CURRENT_MAJOR}/install" ../.vortex/docs/static/install
           php ../.vortex/docs/static/install --version
 
       - name: Check docs up-to-date
@@ -154,22 +210,25 @@ jobs:
         run: yarn install --frozen-lockfile
         working-directory: .vortex/docs
 
-      # Assemble the multi-version site for production: snapshot the released
-      # docs as v1 (served at the bare '/docs') and pull the '2.x'
-      # docs in as v2 (at '/docs/v2'). Mirrors the same step in
-      # 'vortex-test-docs.yml' (development / Netlify).
+      # Assemble the multi-version site for production: snapshot this ref's docs
+      # as the current major (served at the bare '/docs') and pull the other
+      # major's '{N}.x' branch docs in at '/docs/v{other}'. All driven by
+      # 'VORTEX_CURRENT_MAJOR'. Mirrors the same step in 'vortex-test-docs.yml'
+      # (development / Netlify).
       - name: Assemble versioned docs
         run: |
-          yarn docusaurus docs:version 1.x
-          git -C "${{ github.workspace }}" fetch origin 2.x --depth=1 || { echo "Failed to fetch the 2.x branch."; exit 1; }
-          git -C "${{ github.workspace }}" rev-parse --verify origin/2.x || { echo "The 2.x branch does not exist."; exit 1; }
+          yarn docusaurus docs:version "${CURRENT_MAJOR}.x"
+          git -C "${{ github.workspace }}" fetch origin "${OTHER_MAJOR}.x" --depth=1 || { echo "Failed to fetch the ${OTHER_MAJOR}.x branch."; exit 1; }
+          git -C "${{ github.workspace }}" rev-parse --verify "origin/${OTHER_MAJOR}.x" || { echo "The ${OTHER_MAJOR}.x branch does not exist."; exit 1; }
           rm -rf content
-          git -C "${{ github.workspace }}" checkout origin/2.x -- .vortex/docs/content || { echo "Failed to check out content from 2.x."; exit 1; }
+          git -C "${{ github.workspace }}" checkout "origin/${OTHER_MAJOR}.x" -- .vortex/docs/content || { echo "Failed to check out content from ${OTHER_MAJOR}.x."; exit 1; }
         working-directory: .vortex/docs
 
       - name: Build documentation site
         run: yarn run build
         working-directory: .vortex/docs
+        env:
+          VORTEX_CURRENT_MAJOR: ${{ env.CURRENT_MAJOR }}
 
       - name: Generate video for installer (not used in the final artifact)
         run: |
diff --git a/.github/workflows/vortex-test-docs.yml b/.github/workflows/vortex-test-docs.yml
index b98016932..680595502 100644
--- a/.github/workflows/vortex-test-docs.yml
+++ b/.github/workflows/vortex-test-docs.yml
@@ -19,6 +19,10 @@ jobs:
       statuses: write # Post pending/final commit statuses via 'gh api repos/.../statuses/...'.
       pull-requests: write # Post the Netlify preview link comment on the originating PR.
 
+    env:
+      CURRENT_MAJOR: ${{ vars.VORTEX_CURRENT_MAJOR || '1' }}
+      OTHER_MAJOR: ${{ (vars.VORTEX_CURRENT_MAJOR || '1') == '1' && '2' || '1' }}
+
     steps:
       # Post pending status to the PR commit.
       # Workflows triggered by workflow_run don't automatically report status
@@ -88,23 +92,55 @@ jobs:
         working-directory: '${{ github.workspace }}/.vortex/docs'
 
       # On the main (development) deploy, assemble the multi-version site:
-      # snapshot main's docs as v1 (served at the bare '/docs') and pull the
-      # '2.x' docs in as v2 (at '/docs/v2'). Skipped on PR/branch
-      # builds, which stay single-version previews. Mirrors the same step in
-      # 'vortex-release.yml' (production / GitHub Pages).
+      # snapshot main's docs as the current major (served at the bare '/docs')
+      # and pull the other major's '{N}.x' branch docs in at '/docs/v{other}'.
+      # Driven by 'VORTEX_CURRENT_MAJOR'. Skipped on PR/branch builds, which stay
+      # single-version previews. Mirrors the same step in 'vortex-release.yml'
+      # (production / GitHub Pages).
       - name: Assemble versioned docs
         if: github.event.workflow_run.head_branch == 'main'
         run: |
-          yarn docusaurus docs:version 1.x
-          git -C "${{ github.workspace }}" fetch origin 2.x --depth=1 || { echo "Failed to fetch the 2.x branch."; exit 1; }
-          git -C "${{ github.workspace }}" rev-parse --verify origin/2.x || { echo "The 2.x branch does not exist."; exit 1; }
+          yarn docusaurus docs:version "${CURRENT_MAJOR}.x"
+          git -C "${{ github.workspace }}" fetch origin "${OTHER_MAJOR}.x" --depth=1 || { echo "Failed to fetch the ${OTHER_MAJOR}.x branch."; exit 1; }
+          git -C "${{ github.workspace }}" rev-parse --verify "origin/${OTHER_MAJOR}.x" || { echo "The ${OTHER_MAJOR}.x branch does not exist."; exit 1; }
           rm -rf content
-          git -C "${{ github.workspace }}" checkout origin/2.x -- .vortex/docs/content || { echo "Failed to check out content from 2.x."; exit 1; }
+          git -C "${{ github.workspace }}" checkout "origin/${OTHER_MAJOR}.x" -- .vortex/docs/content || { echo "Failed to check out content from ${OTHER_MAJOR}.x."; exit 1; }
         working-directory: '${{ github.workspace }}/.vortex/docs'
 
+      # On the main deploy, publish both installers to match the multi-version
+      # docs: '/v1/install' and '/v2/install' are the stable per-major pins and
+      # the bare '/install' is a copy of the current major's pin (the
+      # 'VORTEX_CURRENT_MAJOR' repository variable, default 1). The downloaded
+      # installer above is the current major (built from 'main'); the other
+      # major is built from its '{N}.x' branch. Mirrors 'vortex-release.yml'.
+      - name: Checkout v${{ env.OTHER_MAJOR }} branch
+        if: github.event.workflow_run.head_branch == 'main'
+        uses: actions/checkout@df4cb1c069e1874edd31b4311f1884172cec0e10 # v6
+        with:
+          ref: ${{ env.OTHER_MAJOR }}.x
+          path: vortex-v${{ env.OTHER_MAJOR }}
+          persist-credentials: false
+
+      - name: Build and publish v${{ env.OTHER_MAJOR }} installer
+        if: github.event.workflow_run.head_branch == 'main'
+        run: |
+          case "${CURRENT_MAJOR}" in
+            1|2) ;;
+            *) echo "Invalid VORTEX_CURRENT_MAJOR='${CURRENT_MAJOR}'. Expected 1 or 2."; exit 1 ;;
+          esac
+          composer --working-dir="vortex-v${OTHER_MAJOR}/.vortex/installer" install
+          sed -i "s/\"vortex-installer-version\": \"development\"/\"vortex-installer-version\": \"${OTHER_MAJOR}.x-dev\"/g" "vortex-v${OTHER_MAJOR}/.vortex/installer/box.json"
+          composer --working-dir="vortex-v${OTHER_MAJOR}/.vortex/installer" build
+          mkdir -p .vortex/docs/static/v1 .vortex/docs/static/v2
+          cp .vortex/docs/static/install ".vortex/docs/static/v${CURRENT_MAJOR}/install"
+          cp "vortex-v${OTHER_MAJOR}/.vortex/installer/build/installer.phar" ".vortex/docs/static/v${OTHER_MAJOR}/install"
+          cp ".vortex/docs/static/v${CURRENT_MAJOR}/install" .vortex/docs/static/install
+
       - name: Build documentation site
         run: yarn run build
         working-directory: '${{ github.workspace }}/.vortex/docs'
+        env:
+          VORTEX_CURRENT_MAJOR: ${{ env.CURRENT_MAJOR }}
 
       # This workflow runs via 'workflow_run', where 'github.ref' is always the
       # default branch, so the action's branch-based production detection cannot
diff --git a/.vortex/docs/.gitignore b/.vortex/docs/.gitignore
index 88b8f32d2..7fa5d971f 100644
--- a/.vortex/docs/.gitignore
+++ b/.vortex/docs/.gitignore
@@ -28,5 +28,7 @@ npm-debug.log*
 yarn-debug.log*
 yarn-error.log*
 
-# Copied installer file.
+# Copied installer files (the bare active-major path and the per-major pins).
 static/install
+static/v1/install
+static/v2/install
diff --git a/.vortex/docs/content/contributing/maintenance/release.mdx b/.vortex/docs/content/contributing/maintenance/release.mdx
index d0c9f5187..2995e1805 100644
--- a/.vortex/docs/content/contributing/maintenance/release.mdx
+++ b/.vortex/docs/content/contributing/maintenance/release.mdx
@@ -43,13 +43,15 @@ When creating a new release, determine the version based on the changes:
 
 ### Branch strategy
 
-- **`main`** - Current stable release branch (always the latest major version)
-- **`project/X.x`** - Development branch for the next major version (e.g., `project/2.x`)
-- **`X.x`** - Maintenance branches for older major versions (e.g., `1.x` after `2.x` is released)
+- **`main`** - the current major version under active development (`1.x` today)
+- **`2.x`** - a development branch for an upcoming major version, ahead of `main` (future majors follow the same pattern: `3.x`, `4.x`, ...)
+- **`1.x`** - a maintenance branch for a superseded major version, behind `main` (for example `1.x` once `main` holds `2.x`)
 
-When a new major version is ready:
-1. The next major version development in `project/X.x` is merged into `main`
-2. The previous major version code is preserved in its own `X.x` branch for maintenance
+Promoting a new major version is a single action: set the `VORTEX_CURRENT_MAJOR` repository variable to its number. The documentation default and the installer's bare `/install` URL both follow it, while the per-major docs and `/v{N}/install` pins are always published.
+
+When `main` advances to a new major version:
+1. The upcoming major's branch (for example `2.x`) is merged into `main`
+2. The superseded major continues on its own branch (for example `1.x`) for maintenance
 
 :::note
 
diff --git a/.vortex/docs/content/installation.mdx b/.vortex/docs/content/installation.mdx
index 002aadc08..286526e39 100644
--- a/.vortex/docs/content/installation.mdx
+++ b/.vortex/docs/content/installation.mdx
@@ -44,6 +44,19 @@ curl -SsL https://www.vortextemplate.com/install > installer.php && php installe
 
 :::
 
+:::tip Choosing a Vortex version
+
+The command above installs the current stable **Vortex** release (the `1.x` line
+today). To target a specific major version explicitly, use its dedicated path:
+
+- `https://www.vortextemplate.com/v1/install` - the current `1.x` line.
+- `https://www.vortextemplate.com/v2/install` - the `2.x` line, currently in development.
+
+Once installed, `ahoy update-vortex` keeps your project on its major version - it
+never jumps across a major release. See [Updating Vortex](./updating-vortex).
+
+:::
+
 ### 2. Commit the initial project structure
 
 ```shell
diff --git a/.vortex/docs/content/updating-vortex.mdx b/.vortex/docs/content/updating-vortex.mdx
index 34847ea18..7faa6fe3f 100644
--- a/.vortex/docs/content/updating-vortex.mdx
+++ b/.vortex/docs/content/updating-vortex.mdx
@@ -21,6 +21,16 @@ After the update, review and commit the changes to your project to maintain vers
 We recommend updating **Vortex** monthly or at least quarterly to keep your project
 secure and up-to-date.
 
+:::note Staying on your major version
+
+`ahoy update-vortex` updates your project **within its current major version** - a
+`1.x` project receives the latest `1.x` release and is never moved to `2.x`
+automatically. Moving to a new major is a deliberate step: run that major's
+installer (for example `https://www.vortextemplate.com/v2/install`) against your
+project.
+
+:::
+
 ## Before updating
 
 Check the release notes for the latest version to understand what has changed
diff --git a/.vortex/docs/docusaurus.config.js b/.vortex/docs/docusaurus.config.js
index d1741b7ec..16d6f338a 100644
--- a/.vortex/docs/docusaurus.config.js
+++ b/.vortex/docs/docusaurus.config.js
@@ -18,6 +18,16 @@ import {themes as prismThemes} from 'prism-react-renderer';
 // CI; it is never committed to a branch.
 const versioned = fs.existsSync('versioned_docs');
 
+// The current major (the 'VORTEX_CURRENT_MAJOR' repository variable, default 1)
+// drives the whole site: its docs are a snapshot under 'versioned_docs/' served
+// as the default at the bare '/docs', and the live 'content/' (pulled from the
+// other major's '{N}.x' branch in CI) is served at '/docs/v{other}'. Bumping
+// that one variable promotes a new major - nothing else changes here.
+const currentMajor = process.env.VORTEX_CURRENT_MAJOR || '1';
+const otherMajor = currentMajor === '1' ? '2' : '1';
+const currentDocsVersion = `${currentMajor}.x`;
+const otherIsNewer = Number(otherMajor) > Number(currentMajor);
+
 /** @type {import('@docusaurus/types').Config} */
 const config = {
   title: 'Vortex - Drupal project template',
@@ -58,22 +68,20 @@ const config = {
           // Please change this to your repo.
           // Remove this to remove the "edit this page" links.
           editUrl: 'https://github.com/drevops/vortex/tree/main/.vortex/docs/',
-          // In versioned (aggregate) builds, v1 is the snapshot in
-          // 'versioned_docs/' served at the bare '/docs' (the default), and
-          // the current 'content/' is v2 at '/docs/v2'. To flip the default
-          // when 2.x is ready: set `lastVersion: 'current'`, give '1.x' a
-          // `path: 'v1'`, drop the v2 `path` so 'current' serves at the bare
-          // '/docs', and swap the '/docs/v1' redirect below for '/docs/v2'.
+          // In versioned (aggregate) builds the current major is the snapshot
+          // in 'versioned_docs/' served at the bare '/docs' (the default), and
+          // the live 'content/' is the other major at '/docs/v{other}'. Both
+          // are derived from 'VORTEX_CURRENT_MAJOR' - no manual edits to flip.
           ...(versioned ? {
-            lastVersion: '1.x',
+            lastVersion: currentDocsVersion,
             versions: {
-              '1.x': {
-                label: 'v1',
+              [currentDocsVersion]: {
+                label: `v${currentMajor}`,
               },
               current: {
-                label: 'v2',
-                path: 'v2',
-                banner: 'unreleased',
+                label: `v${otherMajor}`,
+                path: `v${otherMajor}`,
+                banner: otherIsNewer ? 'unreleased' : 'unmaintained',
               },
             },
           } : {}),
@@ -233,11 +241,10 @@ const config = {
       '@docusaurus/plugin-client-redirects',
       {
         redirects: [
-          // In versioned builds, 'v1' is the default at the bare '/docs', so
-          // its explicit path redirects there. When the default flips to
-          // 'v2', change this to redirect '/docs/v2' instead.
+          // The current major is the default at the bare '/docs', so its
+          // explicit '/docs/v{current}' path redirects there.
           ...(versioned ? [{
-            from: '/docs/v1',
+            from: `/docs/v${currentMajor}`,
             to: '/docs',
           }] : []),
           {
diff --git a/.vortex/installer/src/Command/InstallCommand.php b/.vortex/installer/src/Command/InstallCommand.php
index 4d1f5151b..e8620f894 100644
--- a/.vortex/installer/src/Command/InstallCommand.php
+++ b/.vortex/installer/src/Command/InstallCommand.php
@@ -25,6 +25,7 @@
 use DrevOps\VortexInstaller\Utils\FileManager;
 use DrevOps\VortexInstaller\Utils\OptionsResolver;
 use DrevOps\VortexInstaller\Utils\Tui;
+use DrevOps\VortexInstaller\Utils\Version;
 use Symfony\Component\Console\Command\Command;
 use Symfony\Component\Console\Input\InputInterface;
 use Symfony\Component\Console\Input\InputOption;
@@ -186,6 +187,8 @@ protected function execute(InputInterface $input, OutputInterface $output): int
 
       $this->presenter->header($this->artifact, $this->getApplication()->getVersion());
 
+      $this->assertMajorCompatibility();
+
       // Only validate if using custom repository or custom reference.
       if (!$this->artifact->isDefault()) {
         Task::action(
@@ -218,7 +221,8 @@ protected function execute(InputInterface $input, OutputInterface $output): int
       Task::action(
         label: 'Downloading Vortex',
         action: function (): string {
-          $version = $this->getRepositoryDownloader()->download($this->artifact, $this->config->get(Config::TMP));
+          $release_prefix = Version::releasePrefix($this->getApplication()->getVersion());
+          $version = $this->getRepositoryDownloader()->download($this->artifact, $this->config->get(Config::TMP), $release_prefix);
           $this->config->set(Config::VERSION, $version);
           return $version;
         },
@@ -388,6 +392,39 @@ protected function runBuildCommand(OutputInterface $output): bool {
     return $runner->getExitCode() === RunnerInterface::EXIT_SUCCESS;
   }
 
+  /**
+   * Refuse to operate across major versions.
+   *
+   * Each installer build serves a single major line. Updating an existing
+   * project of a different major would cross a breaking boundary, so stop and
+   * point the user at the matching installer instead. Fresh installs and
+   * projects whose major cannot be determined are treated as compatible.
+   *
+   * @throws \RuntimeException
+   *   When the destination project's major differs from this installer's major.
+   */
+  protected function assertMajorCompatibility(): void {
+    if (!$this->config->isVortexProject()) {
+      return;
+    }
+
+    $installer_major = Version::major($this->getApplication()->getVersion());
+    if ($installer_major === NULL) {
+      return;
+    }
+
+    $project_major = Version::detectProjectMajor((string) $this->config->getDst());
+    if ($project_major === NULL || $project_major === $installer_major) {
+      return;
+    }
+
+    throw new \RuntimeException(sprintf(
+      'This installer targets Vortex %1$d.x, but the destination is a Vortex %2$d.x project. Update it with the %2$d.x installer instead: https://www.vortextemplate.com/v%2$d/install',
+      $installer_major,
+      $project_major,
+    ));
+  }
+
   /**
    * Clean up installer artifacts.
    */
diff --git a/.vortex/installer/src/Downloader/RepositoryDownloader.php b/.vortex/installer/src/Downloader/RepositoryDownloader.php
index abb33378a..afcc07dc0 100644
--- a/.vortex/installer/src/Downloader/RepositoryDownloader.php
+++ b/.vortex/installer/src/Downloader/RepositoryDownloader.php
@@ -46,9 +46,9 @@ public function __construct(
   ) {
   }
 
-  public function download(Artifact $artifact, ?string $dst = NULL): string {
+  public function download(Artifact $artifact, ?string $dst = NULL, ?string $release_prefix = NULL): string {
     if ($artifact->isRemote()) {
-      $version = $this->downloadFromRemote($artifact, $dst);
+      $version = $this->downloadFromRemote($artifact, $dst, $release_prefix);
     }
     else {
       $version = $this->downloadFromLocal($artifact, $dst);
@@ -97,7 +97,7 @@ public function validate(Artifact $artifact): void {
     }
   }
 
-  protected function downloadFromRemote(Artifact $artifact, ?string $destination): string {
+  protected function downloadFromRemote(Artifact $artifact, ?string $destination, ?string $release_prefix = NULL): string {
     if ($destination === NULL) {
       throw new \InvalidArgumentException('Destination cannot be null for remote downloads.');
     }
@@ -108,7 +108,15 @@ protected function downloadFromRemote(Artifact $artifact, ?string $destination):
 
     $version = $artifact->getRef();
     if ($artifact->getRef() === RepositoryDownloader::REF_STABLE) {
-      $ref = $this->discoverLatestReleaseRemote($repo_url);
+      $ref = $this->discoverLatestReleaseRemote($repo_url, $release_prefix);
+
+      if ($ref === NULL && $release_prefix !== NULL) {
+        // No tagged release exists yet for this major (for example, an
+        // unreleased newer major). Fall back to the major development branch
+        // (for example "2.x") so the line is still installable.
+        $ref = rtrim($release_prefix, '.') . '.x';
+        $this->validateRemoteRefExists($repo_url, $ref);
+      }
 
       if ($ref === NULL) {
         throw new \RuntimeException(sprintf('Unable to discover the latest release for "%s".', $repo_url));
diff --git a/.vortex/installer/src/Downloader/RepositoryDownloaderInterface.php b/.vortex/installer/src/Downloader/RepositoryDownloaderInterface.php
index 2e4dc3a42..1d35425f1 100644
--- a/.vortex/installer/src/Downloader/RepositoryDownloaderInterface.php
+++ b/.vortex/installer/src/Downloader/RepositoryDownloaderInterface.php
@@ -17,6 +17,10 @@ interface RepositoryDownloaderInterface {
    * @param string|null $dst
    *   The destination directory. If NULL, a temporary directory will be used
    *   for local repositories.
+   * @param string|null $release_prefix
+   *   When the reference is "stable", restrict release discovery to tags
+   *   starting with this prefix (for example "1." for the 1.x line). If no
+   *   matching release exists, the "{major}.x" development branch is used.
    *
    * @return string
    *   The version/reference that was downloaded.
@@ -26,6 +30,6 @@ interface RepositoryDownloaderInterface {
    * @throws \InvalidArgumentException
    *   If the destination is null for remote downloads.
    */
-  public function download(Artifact $artifact, ?string $dst = NULL): string;
+  public function download(Artifact $artifact, ?string $dst = NULL, ?string $release_prefix = NULL): string;
 
 }
diff --git a/.vortex/installer/src/Utils/Version.php b/.vortex/installer/src/Utils/Version.php
new file mode 100644
index 000000000..60be5ca28
--- /dev/null
+++ b/.vortex/installer/src/Utils/Version.php
@@ -0,0 +1,102 @@
+<?php
+
+declare(strict_types=1);
+
+namespace DrevOps\VortexInstaller\Utils;
+
+/**
+ * Helpers for reasoning about Vortex major versions.
+ *
+ * A single installer build serves one major line (for example, the 1.x build
+ * resolves the latest 1.x release, the 2.x build resolves the latest 2.x
+ * release). These helpers derive the major from the installer's own stamped
+ * version and from an installed project, so the installer can both target the
+ * right major and refuse cross-major operations.
+ */
+class Version {
+
+  /**
+   * Extract the major version number from a version string.
+   *
+   * @param string|null $version
+   *   A version string such as "1.40.0", "v2.0.0", "2.x-dev" or
+   *   "1.0.0+2025.11.0". Unstamped or development values (for example
+   *   "develop" or "@vortex-installer-version@") have no derivable major.
+   *
+   * @return int|null
+   *   The major version number, or NULL when it cannot be determined.
+   */
+  public static function major(?string $version): ?int {
+    if ($version === NULL) {
+      return NULL;
+    }
+
+    return preg_match('/^\s*v?(\d+)\./', $version, $matches) ? (int) $matches[1] : NULL;
+  }
+
+  /**
+   * Build a release tag prefix for a version's major (for example "1.").
+   *
+   * The trailing dot ensures "1." matches "1.40.0" but not "11.0.0".
+   *
+   * @param string|null $version
+   *   A version string.
+   *
+   * @return string|null
+   *   The dot-terminated major prefix, or NULL when the major is unknown.
+   */
+  public static function releasePrefix(?string $version): ?string {
+    $major = self::major($version);
+
+    return $major === NULL ? NULL : $major . '.';
+  }
+
+  /**
+   * Extract the major version number from a Composer version constraint.
+   *
+   * @param string|null $constraint
+   *   A constraint such as "^1.1.0", "~2.0" or "2.x-dev".
+   *
+   * @return int|null
+   *   The first major version number in the constraint, or NULL.
+   */
+  public static function majorFromConstraint(?string $constraint): ?int {
+    if ($constraint === NULL) {
+      return NULL;
+    }
+
+    return preg_match('/(\d+)/', $constraint, $matches) ? (int) $matches[1] : NULL;
+  }
+
+  /**
+   * Detect the Vortex major of an installed project from its composer.json.
+   *
+   * The project's pinned 'drevops/vortex-tooling' major is the provenance
+   * signal: a 1.x project requires '^1', a 2.x project requires '^2'. When the
+   * package is absent (a fresh directory or a pre-tooling project), the major
+   * is undeterminable and the caller treats it as compatible.
+   *
+   * @param string $dir
+   *   The project directory.
+   *
+   * @return int|null
+   *   The project's major, or NULL when it cannot be determined.
+   */
+  public static function detectProjectMajor(string $dir): ?int {
+    $composer_json = $dir . DIRECTORY_SEPARATOR . 'composer.json';
+
+    if (!is_file($composer_json)) {
+      return NULL;
+    }
+
+    $data = json_decode((string) file_get_contents($composer_json), TRUE);
+    if (!is_array($data)) {
+      return NULL;
+    }
+
+    $constraint = $data['require']['drevops/vortex-tooling'] ?? NULL;
+
+    return is_string($constraint) ? self::majorFromConstraint($constraint) : NULL;
+  }
+
+}
diff --git a/.vortex/installer/tests/Functional/Command/InstallCommandTest.php b/.vortex/installer/tests/Functional/Command/InstallCommandTest.php
index 58433e317..a80cf409e 100644
--- a/.vortex/installer/tests/Functional/Command/InstallCommandTest.php
+++ b/.vortex/installer/tests/Functional/Command/InstallCommandTest.php
@@ -496,4 +496,75 @@ public static function dataProviderInstallCommand(): \Iterator {
     ];
   }
 
+  /**
+   * Test the cross-major compatibility gate.
+   */
+  #[DataProvider('dataProviderInstallCommandMajorGate')]
+  public function testInstallCommandMajorGate(?string $version, string $composer_json, bool $mock_download_fail, string $expected_message): void {
+    $executable_finder = $this->createMock(ExecutableFinder::class);
+    $executable_finder->method('find')->willReturnCallback(fn(string $command): string => '/usr/bin/' . $command);
+
+    $install_command = new InstallCommand();
+    $install_command->setExecutableFinder($executable_finder);
+
+    if ($mock_download_fail) {
+      $mock_downloader = $this->createMock(RepositoryDownloader::class);
+      $mock_downloader->method('download')->willThrowException(new \RuntimeException('Failed to download Vortex.'));
+      $install_command->setRepositoryDownloader($mock_downloader);
+    }
+
+    // Pre-populate the destination so it looks like an existing Vortex project:
+    // the README badge flags it as a Vortex project and the composer.json
+    // 'drevops/vortex-tooling' constraint carries the project's major.
+    $this->assertNotFalse(file_put_contents(self::$sut . '/README.md', '[![Vortex](https://img.shields.io/badge/Vortex-1.40.0-65ACBC.svg)](https://github.com/drevops/vortex)'));
+    $this->assertNotFalse(file_put_contents(self::$sut . '/composer.json', $composer_json));
+
+    static::applicationInitFromCommand($install_command);
+
+    if ($version !== NULL) {
+      $this->applicationGet()->setVersion($version);
+    }
+
+    $this->applicationRun([
+      '--' . InstallCommand::OPTION_NO_INTERACTION => TRUE,
+      '--' . InstallCommand::OPTION_URI => File::dir(static::$root),
+      '--' . InstallCommand::OPTION_DESTINATION => self::$sut,
+    ], [], TRUE);
+
+    $this->assertApplicationAnyOutputContainsOrNot($expected_message);
+  }
+
+  /**
+   * Data provider for testInstallCommandMajorGate().
+   *
+   * @return \Iterator<string, array{(string | null), string, bool, string}>
+   *   Test data.
+   */
+  public static function dataProviderInstallCommandMajorGate(): \Iterator {
+    yield 'v1 installer refuses v2 project' => [
+      '1.40.0',
+      '{"require": {"drevops/vortex-tooling": "^2.0.0"}}',
+      FALSE,
+      'https://www.vortextemplate.com/v2/install',
+    ];
+    yield 'v1 installer accepts v1 project' => [
+      '1.40.0',
+      '{"require": {"drevops/vortex-tooling": "^1.1.0"}}',
+      TRUE,
+      'Failed to download Vortex.',
+    ];
+    yield 'unstamped installer skips gate' => [
+      NULL,
+      '{"require": {"drevops/vortex-tooling": "^2.0.0"}}',
+      TRUE,
+      'Failed to download Vortex.',
+    ];
+    yield 'undetectable project major skips gate' => [
+      '1.40.0',
+      '{"require": {"php": ">=8.3"}}',
+      TRUE,
+      'Failed to download Vortex.',
+    ];
+  }
+
 }
diff --git a/.vortex/installer/tests/Unit/Downloader/RepositoryDownloaderTest.php b/.vortex/installer/tests/Unit/Downloader/RepositoryDownloaderTest.php
index 6a2638fe1..eeef6f88a 100644
--- a/.vortex/installer/tests/Unit/Downloader/RepositoryDownloaderTest.php
+++ b/.vortex/installer/tests/Unit/Downloader/RepositoryDownloaderTest.php
@@ -169,6 +169,60 @@ public function testDownloadFromRemoteWithGitSuffix(): void {
     $this->assertEquals('develop', $version);
   }
 
+  public function testDownloadStableWithReleasePrefixSelectsMajor(): void {
+    $release_json = (string) json_encode([
+      ['tag_name' => '2.0.0', 'draft' => FALSE],
+      ['tag_name' => '1.40.0', 'draft' => FALSE],
+      ['tag_name' => '1.39.0', 'draft' => FALSE],
+    ]);
+
+    $mock_http_client = $this->createMock(ClientInterface::class);
+    $mock_http_client->method('request')->willReturnCallback(function (string $method, string $url) use ($release_json): ResponseInterface {
+      $response = $this->createMock(ResponseInterface::class);
+      $response->method('getStatusCode')->willReturn(200);
+      $body = $this->createMock(StreamInterface::class);
+      $body->method('getContents')->willReturn(str_contains($url, '/releases') ? $release_json : 'ok');
+      $response->method('getBody')->willReturn($body);
+      return $response;
+    });
+
+    $mock_archiver = $this->createMock(ArchiverInterface::class);
+    $mock_file_downloader = $this->createMockFileDownloader();
+    $destination = self::$tmp . '/destination_' . uniqid();
+    File::mkdir($destination);
+    File::dump($destination . '/composer.json', '{}');
+
+    $downloader = new RepositoryDownloader($mock_http_client, $mock_archiver, NULL, $mock_file_downloader);
+    $version = $downloader->download(Artifact::create('https://github.com/user/repo', 'stable'), $destination, '1.');
+    $this->assertEquals('1.40.0', $version);
+  }
+
+  public function testDownloadStableWithReleasePrefixFallsBackToBranch(): void {
+    $release_json = (string) json_encode([
+      ['tag_name' => '1.40.0', 'draft' => FALSE],
+    ]);
+
+    $mock_http_client = $this->createMock(ClientInterface::class);
+    $mock_http_client->method('request')->willReturnCallback(function (string $method, string $url) use ($release_json): ResponseInterface {
+      $response = $this->createMock(ResponseInterface::class);
+      $response->method('getStatusCode')->willReturn(200);
+      $body = $this->createMock(StreamInterface::class);
+      $body->method('getContents')->willReturn(str_contains($url, '/releases') ? $release_json : 'ok');
+      $response->method('getBody')->willReturn($body);
+      return $response;
+    });
+
+    $mock_archiver = $this->createMock(ArchiverInterface::class);
+    $mock_file_downloader = $this->createMockFileDownloader();
+    $destination = self::$tmp . '/destination_' . uniqid();
+    File::mkdir($destination);
+    File::dump($destination . '/composer.json', '{}');
+
+    $downloader = new RepositoryDownloader($mock_http_client, $mock_archiver, NULL, $mock_file_downloader);
+    $version = $downloader->download(Artifact::create('https://github.com/user/repo', 'stable'), $destination, '2.');
+    $this->assertEquals('2.x', $version);
+  }
+
   #[DataProvider('dataProviderDownloadWithNullDestination')]
   public function testDownloadWithNullDestination(string $repo, string $expectedMessage): void {
     $downloader = new RepositoryDownloader();
diff --git a/.vortex/installer/tests/Unit/VersionTest.php b/.vortex/installer/tests/Unit/VersionTest.php
new file mode 100644
index 000000000..25fee2290
--- /dev/null
+++ b/.vortex/installer/tests/Unit/VersionTest.php
@@ -0,0 +1,110 @@
+<?php
+
+declare(strict_types=1);
+
+namespace DrevOps\VortexInstaller\Tests\Unit;
+
+use AlexSkrypnyk\File\File;
+use DrevOps\VortexInstaller\Utils\Version;
+use PHPUnit\Framework\Attributes\CoversClass;
+use PHPUnit\Framework\Attributes\DataProvider;
+
+#[CoversClass(Version::class)]
+class VersionTest extends UnitTestCase {
+
+  #[DataProvider('dataProviderMajor')]
+  public function testMajor(?string $version, ?int $expected): void {
+    $this->assertSame($expected, Version::major($version));
+  }
+
+  /**
+   * Data provider for testMajor().
+   *
+   * @return \Iterator<string, array{(string | null), (int | null)}>
+   *   Test data.
+   */
+  public static function dataProviderMajor(): \Iterator {
+    yield 'null' => [NULL, NULL];
+    yield 'empty' => ['', NULL];
+    yield 'develop' => ['develop', NULL];
+    yield 'unstamped token' => ['@vortex-installer-version@', NULL];
+    yield 'semver 1.x' => ['1.40.0', 1];
+    yield 'semver with v prefix' => ['v1.2.3', 1];
+    yield 'semver 2.x' => ['2.0.0', 2];
+    yield 'dev branch' => ['2.x-dev', 2];
+    yield 'semver+calver' => ['1.0.0+2025.11.0', 1];
+    yield 'legacy calver' => ['25.10.0', 25];
+    yield 'leading whitespace' => ['  1.0.0', 1];
+  }
+
+  #[DataProvider('dataProviderReleasePrefix')]
+  public function testReleasePrefix(?string $version, ?string $expected): void {
+    $this->assertSame($expected, Version::releasePrefix($version));
+  }
+
+  /**
+   * Data provider for testReleasePrefix().
+   *
+   * @return \Iterator<string, array{(string | null), (string | null)}>
+   *   Test data.
+   */
+  public static function dataProviderReleasePrefix(): \Iterator {
+    yield 'null' => [NULL, NULL];
+    yield 'develop' => ['develop', NULL];
+    yield 'major 1' => ['1.40.0', '1.'];
+    yield 'major 2 dev' => ['2.x-dev', '2.'];
+    yield 'legacy calver' => ['25.10.0', '25.'];
+  }
+
+  #[DataProvider('dataProviderMajorFromConstraint')]
+  public function testMajorFromConstraint(?string $constraint, ?int $expected): void {
+    $this->assertSame($expected, Version::majorFromConstraint($constraint));
+  }
+
+  /**
+   * Data provider for testMajorFromConstraint().
+   *
+   * @return \Iterator<string, array{(string | null), (int | null)}>
+   *   Test data.
+   */
+  public static function dataProviderMajorFromConstraint(): \Iterator {
+    yield 'null' => [NULL, NULL];
+    yield 'empty' => ['', NULL];
+    yield 'no digits' => ['dev-main', NULL];
+    yield 'caret 1' => ['^1.1.0', 1];
+    yield 'caret 2' => ['^2.0.0', 2];
+    yield 'tilde 2' => ['~2.0', 2];
+    yield 'dev branch' => ['2.x-dev', 2];
+    yield 'range' => ['>=1.2 <3.0', 1];
+  }
+
+  #[DataProvider('dataProviderDetectProjectMajor')]
+  public function testDetectProjectMajor(?string $composer_json, ?int $expected): void {
+    $dir = self::$tmp . '/project_' . uniqid();
+    File::mkdir($dir);
+
+    if ($composer_json !== NULL) {
+      File::dump($dir . '/composer.json', $composer_json);
+    }
+
+    $this->assertSame($expected, Version::detectProjectMajor($dir));
+  }
+
+  /**
+   * Data provider for testDetectProjectMajor().
+   *
+   * @return \Iterator<string, array{(string | null), (int | null)}>
+   *   Test data.
+   */
+  public static function dataProviderDetectProjectMajor(): \Iterator {
+    yield 'no composer.json' => [NULL, NULL];
+    yield 'invalid json' => ['not json', NULL];
+    yield 'empty object' => ['{}', NULL];
+    yield 'no require' => ['{"name": "test/test"}', NULL];
+    yield 'no tooling' => ['{"require": {"php": ">=8.3"}}', NULL];
+    yield 'tooling v1' => ['{"require": {"drevops/vortex-tooling": "^1.1.0"}}', 1];
+    yield 'tooling v2' => ['{"require": {"drevops/vortex-tooling": "^2.0.0"}}', 2];
+    yield 'tooling non-string' => ['{"require": {"drevops/vortex-tooling": 1}}', NULL];
+  }
+
+}
diff --git a/.vortex/tooling/src/update-vortex b/.vortex/tooling/src/update-vortex
index d4cdc0f4d..e16e25434 100755
--- a/.vortex/tooling/src/update-vortex
+++ b/.vortex/tooling/src/update-vortex
@@ -29,7 +29,11 @@ set -eu
 VORTEX_INSTALLER_TEMPLATE_REPO="${VORTEX_INSTALLER_TEMPLATE_REPO:-https://github.com/drevops/vortex.git#stable}"
 
 # The URL of the installer script.
-VORTEX_INSTALLER_URL="${VORTEX_INSTALLER_URL:-https://www.vortextemplate.com/install}"
+#
+# Pinned to the major-specific path so an existing project always updates
+# within its own major line. The bare '/install' tracks the active major and
+# would jump a project across a major boundary once the next major is released.
+VORTEX_INSTALLER_URL="${VORTEX_INSTALLER_URL:-https://www.vortextemplate.com/v1/install}"
 
 # Cache busting parameter for the installer URL.
 VORTEX_INSTALLER_URL_CACHE_BUST="${VORTEX_INSTALLER_URL_CACHE_BUST:-"$(date +%s)"}"
diff --git a/.vortex/tooling/tests/unit/update-vortex.bats b/.vortex/tooling/tests/unit/update-vortex.bats
index 42e4fcb6f..4c094877e 100644
--- a/.vortex/tooling/tests/unit/update-vortex.bats
+++ b/.vortex/tooling/tests/unit/update-vortex.bats
@@ -18,9 +18,9 @@ load ../_helper.bash
 
   # Test default values when no environment variables are set.
   declare -a STEPS=(
-    "@curl -fsSL https://www.vortextemplate.com/install?1234567890 -o installer.php # 0"
+    "@curl -fsSL https://www.vortextemplate.com/v1/install?1234567890 -o installer.php # 0"
     "@php installer.php --no-interaction --uri=https://github.com/drevops/vortex.git\#stable # 0"
-    "Using installer script from URL: https://www.vortextemplate.com/install"
+    "Using installer script from URL: https://www.vortextemplate.com/v1/install"
     "Downloading installer to installer.php"
   )
 
@@ -42,9 +42,9 @@ load ../_helper.bash
   export VORTEX_INSTALLER_URL_CACHE_BUST="1234567890"
 
   declare -a STEPS=(
-    "@curl -fsSL https://www.vortextemplate.com/install?1234567890 -o installer.php # 0"
+    "@curl -fsSL https://www.vortextemplate.com/v1/install?1234567890 -o installer.php # 0"
     "@php installer.php --no-interaction --uri=https://github.com/custom/repo.git\#main # 0"
-    "Using installer script from URL: https://www.vortextemplate.com/install"
+    "Using installer script from URL: https://www.vortextemplate.com/v1/install"
     "Downloading installer to installer.php"
   )
 
@@ -117,9 +117,9 @@ load ../_helper.bash
   export VORTEX_INSTALLER_URL_CACHE_BUST="1234567890"
 
   declare -a STEPS=(
-    "@curl -fsSL https://www.vortextemplate.com/install?1234567890 -o installer.php # 0"
+    "@curl -fsSL https://www.vortextemplate.com/v1/install?1234567890 -o installer.php # 0"
     "@php installer.php --no-interaction --uri=file:///local/path/to/vortex # 0"
-    "Using installer script from URL: https://www.vortextemplate.com/install"
+    "Using installer script from URL: https://www.vortextemplate.com/v1/install"
     "Downloading installer to installer.php"
   )
 
@@ -141,9 +141,9 @@ load ../_helper.bash
   export VORTEX_INSTALLER_URL_CACHE_BUST="1234567890"
 
   declare -a STEPS=(
-    "@curl -fsSL https://www.vortextemplate.com/install?1234567890 -o installer.php # 0"
+    "@curl -fsSL https://www.vortextemplate.com/v1/install?1234567890 -o installer.php # 0"
     "@php installer.php --no-interaction --uri=/local/path/to/vortex\#stable # 0"
-    "Using installer script from URL: https://www.vortextemplate.com/install"
+    "Using installer script from URL: https://www.vortextemplate.com/v1/install"
     "Downloading installer to installer.php"
   )
 
@@ -165,9 +165,9 @@ load ../_helper.bash
   export VORTEX_INSTALLER_URL_CACHE_BUST="1234567890"
 
   declare -a STEPS=(
-    "@curl -fsSL https://www.vortextemplate.com/install?1234567890 -o installer.php # 0"
+    "@curl -fsSL https://www.vortextemplate.com/v1/install?1234567890 -o installer.php # 0"
     "@php installer.php --no-interaction --uri=git@github.com:drevops/vortex.git\#v1.2.3 # 0"
-    "Using installer script from URL: https://www.vortextemplate.com/install"
+    "Using installer script from URL: https://www.vortextemplate.com/v1/install"
     "Downloading installer to installer.php"
   )
 
@@ -189,9 +189,9 @@ load ../_helper.bash
   export VORTEX_INSTALLER_URL_CACHE_BUST="1234567890"
 
   declare -a STEPS=(
-    "@curl -fsSL https://www.vortextemplate.com/install?1234567890 -o installer.php # 0"
+    "@curl -fsSL https://www.vortextemplate.com/v1/install?1234567890 -o installer.php # 0"
     "@php installer.php --no-interaction --uri=https://github.com/drevops/vortex.git\#stable # 1"
-    "Using installer script from URL: https://www.vortextemplate.com/install"
+    "Using installer script from URL: https://www.vortextemplate.com/v1/install"
     "Downloading installer to installer.php"
   )
 
@@ -213,9 +213,9 @@ load ../_helper.bash
   export VORTEX_INSTALLER_URL_CACHE_BUST="1234567890"
 
   declare -a STEPS=(
-    "@curl -fsSL https://www.vortextemplate.com/install?1234567890 -o installer.php # 0"
+    "@curl -fsSL https://www.vortextemplate.com/v1/install?1234567890 -o installer.php # 0"
     "@php installer.php --uri=https://github.com/drevops/vortex.git\#stable # 0"
-    "Using installer script from URL: https://www.vortextemplate.com/install"
+    "Using installer script from URL: https://www.vortextemplate.com/v1/install"
     "Downloading installer to installer.php"
   )
 
@@ -237,9 +237,9 @@ load ../_helper.bash
   export VORTEX_INSTALLER_URL_CACHE_BUST="1234567890"
 
   declare -a STEPS=(
-    "@curl -fsSL https://www.vortextemplate.com/install?1234567890 -o installer.php # 0"
+    "@curl -fsSL https://www.vortextemplate.com/v1/install?1234567890 -o installer.php # 0"
     "@php installer.php --uri=https://github.com/custom/repo.git\#main # 0"
-    "Using installer script from URL: https://www.vortextemplate.com/install"
+    "Using installer script from URL: https://www.vortextemplate.com/v1/install"
     "Downloading installer to installer.php"
   )