From d23130ad121a9b1201b440011160c79283e4511a Mon Sep 17 00:00:00 2001
From: Joshua Blum <hallo@joshuablum.de>
Date: Tue, 2 Jun 2026 18:11:16 +0200
Subject: [PATCH 1/2] Scope shared static cache errors to site

---
 src/StaticCaching/ServiceProvider.php | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/src/StaticCaching/ServiceProvider.php b/src/StaticCaching/ServiceProvider.php
index 3cb2726cc17..960a14ffec8 100644
--- a/src/StaticCaching/ServiceProvider.php
+++ b/src/StaticCaching/ServiceProvider.php
@@ -8,6 +8,7 @@
 use Illuminate\Support\ServiceProvider as LaravelServiceProvider;
 use Illuminate\Support\Str;
 use Statamic\Facades\Cascade;
+use Statamic\Facades\Site;
 use Statamic\StaticCaching\NoCache\DatabaseSession;
 use Statamic\StaticCaching\NoCache\Session;
 
@@ -97,7 +98,10 @@ public function boot()
         });
 
         Request::macro('fakeStaticCacheStatus', function (int $status) {
-            $url = '/__shared-errors/'.$status;
+            // Namespace the shared error by the current site so that multisite
+            // installs serve a correctly localized error page per site, rather
+            // than whichever site happened to render the error first.
+            $url = '/__shared-errors/'.Site::current()->handle().'/'.$status;
             $this->pathInfo = $url;
             $this->requestUri = $url;
             app(Session::class)->setUrl($url);

From b67ebbdd95d3c1b8a98b543f5179512df4c34f09 Mon Sep 17 00:00:00 2001
From: Joshua Blum <hallo@joshuablum.de>
Date: Tue, 2 Jun 2026 18:16:07 +0200
Subject: [PATCH 2/2] Add test

---
 .../SharedErrorsStaticCachingTest.php         | 78 +++++++++++++++++++
 1 file changed, 78 insertions(+)
 create mode 100644 tests/StaticCaching/SharedErrorsStaticCachingTest.php

diff --git a/tests/StaticCaching/SharedErrorsStaticCachingTest.php b/tests/StaticCaching/SharedErrorsStaticCachingTest.php
new file mode 100644
index 00000000000..71a5daaae27
--- /dev/null
+++ b/tests/StaticCaching/SharedErrorsStaticCachingTest.php
@@ -0,0 +1,78 @@
+<?php
+
+namespace Tests\StaticCaching;
+
+use Illuminate\Http\Request;
+use Illuminate\Routing\Events\ResponsePrepared;
+use PHPUnit\Framework\Attributes\Test;
+use Statamic\Facades\Site;
+use Statamic\StaticCaching\Cachers\ApplicationCacher;
+use Tests\TestCase;
+
+class SharedErrorsStaticCachingTest extends TestCase
+{
+    private ApplicationCacher $cacher;
+
+    protected function setUp(): void
+    {
+        parent::setUp();
+
+        $this->cacher = new ApplicationCacher($this->app['cache']->store('array'), []);
+    }
+
+    #[Test]
+    public function the_shared_error_is_scoped_to_the_current_site()
+    {
+        $this->setSites([
+            'english' => ['url' => 'http://localhost/', 'locale' => 'en'],
+            'french' => ['url' => 'http://localhost/fr/', 'locale' => 'fr'],
+        ]);
+
+        // Render and share the english 404 first.
+        Site::setCurrent('english');
+        $this->shareError(404, 'English not found');
+
+        // The french site must not be considered to already have a shared error.
+        // Before scoping by site, the english error would leak here and break the
+        // localization (e.g. the language picker) for every other site.
+        Site::setCurrent('french');
+        $this->assertFalse($this->cacher->hasCachedPage($this->sharedErrorRequest(404)));
+
+        $this->shareError(404, 'French not found');
+
+        // Each site now serves its own localized shared error.
+        Site::setCurrent('english');
+        $this->assertTrue($this->cacher->hasCachedPage($this->sharedErrorRequest(404)));
+        $this->assertEquals('English not found', $this->cacher->getCachedPage($this->sharedErrorRequest(404))->content);
+
+        Site::setCurrent('french');
+        $this->assertTrue($this->cacher->hasCachedPage($this->sharedErrorRequest(404)));
+        $this->assertEquals('French not found', $this->cacher->getCachedPage($this->sharedErrorRequest(404))->content);
+    }
+
+    /**
+     * Replicates the request that's built when a shared error is written
+     * (the Cache middleware's copyError) and read (RendersHttpExceptions's
+     * getCachedError) for the current site.
+     */
+    private function sharedErrorRequest(int $status): Request
+    {
+        return Request::createFrom(Request::create('http://localhost/'))->fakeStaticCacheStatus($status);
+    }
+
+    /**
+     * Cache a shared error for the current site, mirroring how the cacher
+     * persists pages once the response has been prepared.
+     */
+    private function shareError(int $status, string $content): void
+    {
+        $request = $this->sharedErrorRequest($status);
+        $response = response($content, $status);
+
+        $this->cacher->cachePage($request, $response);
+
+        // The cacher writes to the store on the ResponsePrepared event, which
+        // the framework fires during the real response lifecycle.
+        event(new ResponsePrepared($request, $response));
+    }
+}