diff --git a/.gitattributes b/.gitattributes
index 3a01b37..8253128 100644
--- a/.gitattributes
+++ b/.gitattributes
@@ -1,2 +1 @@
-/.gitattributes export-ignore
-/.gitignore export-ignore
+/.git* export-ignore
diff --git a/.github/PULL_REQUEST_TEMPLATE.md b/.github/PULL_REQUEST_TEMPLATE.md
new file mode 100644
index 0000000..4689c4d
--- /dev/null
+++ b/.github/PULL_REQUEST_TEMPLATE.md
@@ -0,0 +1,8 @@
+Please do not submit any Pull Requests here. They will be closed.
+---
+
+Please submit your PR here instead:
+https://github.com/symfony/symfony
+
+This repository is what we call a "subtree split": a read-only subset of that main repository.
+We're looking forward to your PR there!
diff --git a/.github/workflows/close-pull-request.yml b/.github/workflows/close-pull-request.yml
new file mode 100644
index 0000000..e55b478
--- /dev/null
+++ b/.github/workflows/close-pull-request.yml
@@ -0,0 +1,20 @@
+name: Close Pull Request
+
+on:
+  pull_request_target:
+    types: [opened]
+
+jobs:
+  run:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: superbrothers/close-pull-request@v3
+      with:
+        comment: |
+          Thanks for your Pull Request! We love contributions.
+
+          However, you should instead open your PR on the main repository:
+          https://github.com/symfony/symfony
+
+          This repository is what we call a "subtree split": a read-only subset of that main repository.
+          We're looking forward to your PR there!
diff --git a/CacheTrait.php b/CacheTrait.php
index 3d41bc8..4c5449b 100644
--- a/CacheTrait.php
+++ b/CacheTrait.php
@@ -38,7 +38,7 @@ public function delete(string $key): bool
     private function doGet(CacheItemPoolInterface $pool, string $key, callable $callback, ?float $beta, ?array &$metadata = null, ?LoggerInterface $logger = null): mixed
     {
         if (0 > $beta ??= 1.0) {
-            throw new class(sprintf('Argument "$beta" provided to "%s::get()" must be a positive number, %f given.', static::class, $beta)) extends \InvalidArgumentException implements InvalidArgumentException { };
+            throw new class(\sprintf('Argument "$beta" provided to "%s::get()" must be a positive number, %f given.', static::class, $beta)) extends \InvalidArgumentException implements InvalidArgumentException {};
         }
 
         $item = $pool->getItem($key);
@@ -54,7 +54,7 @@ private function doGet(CacheItemPoolInterface $pool, string $key, callable $call
                 $item->expiresAt(null);
                 $logger?->info('Item "{key}" elected for early recomputation {delta}s before its expiration', [
                     'key' => $key,
-                    'delta' => sprintf('%.1f', $expiry - $now),
+                    'delta' => \sprintf('%.1f', $expiry - $now),
                 ]);
             }
         }
diff --git a/NamespacedPoolInterface.php b/NamespacedPoolInterface.php
new file mode 100644
index 0000000..cd67bc0
--- /dev/null
+++ b/NamespacedPoolInterface.php
@@ -0,0 +1,31 @@
+<?php
+
+/*
+ * This file is part of the Symfony package.
+ *
+ * (c) Fabien Potencier <fabien@symfony.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Symfony\Contracts\Cache;
+
+use Psr\Cache\InvalidArgumentException;
+
+/**
+ * Enables namespace-based invalidation by prefixing keys with backend-native namespace separators.
+ *
+ * Note that calling `withSubNamespace()` MUST NOT mutate the pool, but return a new instance instead.
+ *
+ * When tags are used, they MUST ignore sub-namespaces.
+ *
+ * @author Nicolas Grekas <p@tchwork.com>
+ */
+interface NamespacedPoolInterface
+{
+    /**
+     * @throws InvalidArgumentException If the namespace contains characters found in ItemInterface's RESERVED_CHARACTERS
+     */
+    public function withSubNamespace(string $namespace): static;
+}
diff --git a/composer.json b/composer.json
index 79a1d2d..b713c29 100644
--- a/composer.json
+++ b/composer.json
@@ -25,7 +25,7 @@
     "minimum-stability": "dev",
     "extra": {
         "branch-alias": {
-            "dev-main": "3.3-dev"
+            "dev-main": "3.6-dev"
         },
         "thanks": {
             "name": "symfony/contracts",