diff --git a/framework/core/src/Database/AbstractModel.php b/framework/core/src/Database/AbstractModel.php
index f9ed974506..980bc0e0aa 100644
--- a/framework/core/src/Database/AbstractModel.php
+++ b/framework/core/src/Database/AbstractModel.php
@@ -54,6 +54,18 @@ abstract class AbstractModel extends Eloquent
      */
     public static array $defaults = [];
 
+    /**
+     * Resolved casts, keyed by model class.
+     *
+     * Eloquent asks for the casts on every attribute access, so resolving the
+     * class hierarchy each time is measurable: an index request resolved them
+     * over 130,000 times. The result only changes when an extender registers
+     * new casts, which is what flushCastsCache() is for.
+     *
+     * @var array<class-string, array<string, string>>
+     */
+    protected static array $castsCache = [];
+
     /**
      * An alias for the table name, used in queries.
      *
@@ -102,13 +114,27 @@ public function __construct(array $attributes = [])
 
     public function getCasts(): array
     {
+        if (isset(static::$castsCache[static::class])) {
+            return static::$castsCache[static::class];
+        }
+
         $casts = parent::getCasts();
 
         foreach (array_merge(array_reverse(class_parents($this)), [static::class]) as $class) {
             $casts = array_merge($casts, Arr::get(static::$customCasts, $class, []));
         }
 
-        return $casts;
+        return static::$castsCache[static::class] = $casts;
+    }
+
+    /**
+     * Discard the resolved casts, so that newly registered ones are picked up.
+     *
+     * @internal
+     */
+    public static function flushCastsCache(): void
+    {
+        static::$castsCache = [];
     }
 
     /**
diff --git a/framework/core/src/Extend/Model.php b/framework/core/src/Extend/Model.php
index 087872c85c..60e2564235 100644
--- a/framework/core/src/Extend/Model.php
+++ b/framework/core/src/Extend/Model.php
@@ -181,5 +181,8 @@ public function extend(Container $container, ?Extension $extension = null): void
                 $this->casts
             )
         );
+
+        // Models may already have resolved (and memoised) their casts by now.
+        AbstractModel::flushCastsCache();
     }
 }
diff --git a/framework/core/tests/unit/Database/AbstractModelCastsTest.php b/framework/core/tests/unit/Database/AbstractModelCastsTest.php
new file mode 100644
index 0000000000..f139174db4
--- /dev/null
+++ b/framework/core/tests/unit/Database/AbstractModelCastsTest.php
@@ -0,0 +1,118 @@
+<?php
+
+/*
+ * This file is part of Flarum.
+ *
+ * For detailed copyright and license information, please view the
+ * LICENSE file that was distributed with this source code.
+ */
+
+namespace Flarum\Tests\unit\Database;
+
+use Flarum\Database\AbstractModel;
+use Flarum\Testing\unit\TestCase;
+
+class AbstractModelCastsTest extends TestCase
+{
+    protected function setUp(): void
+    {
+        parent::setUp();
+
+        AbstractModel::$customCasts = [];
+        AbstractModel::flushCastsCache();
+    }
+
+    protected function tearDown(): void
+    {
+        AbstractModel::$customCasts = [];
+        AbstractModel::flushCastsCache();
+
+        parent::tearDown();
+    }
+
+    public function test_it_includes_casts_declared_on_the_model(): void
+    {
+        $model = new AbstractModelCastsTestModel;
+
+        $this->assertSame('bool', $model->getCasts()['is_native'] ?? null);
+    }
+
+    public function test_it_includes_custom_casts_registered_for_the_model(): void
+    {
+        AbstractModel::$customCasts[AbstractModelCastsTestModel::class] = ['extended' => 'datetime'];
+
+        $model = new AbstractModelCastsTestModel;
+
+        $this->assertSame('datetime', $model->getCasts()['extended'] ?? null);
+    }
+
+    public function test_it_inherits_custom_casts_registered_against_a_parent_class(): void
+    {
+        AbstractModel::$customCasts[AbstractModelCastsTestModel::class] = ['from_parent' => 'int'];
+
+        $model = new AbstractModelCastsTestChildModel;
+
+        $this->assertSame('int', $model->getCasts()['from_parent'] ?? null);
+    }
+
+    public function test_casts_registered_on_a_child_override_the_parent(): void
+    {
+        AbstractModel::$customCasts[AbstractModelCastsTestModel::class] = ['overridden' => 'int'];
+        AbstractModel::$customCasts[AbstractModelCastsTestChildModel::class] = ['overridden' => 'bool'];
+
+        $model = new AbstractModelCastsTestChildModel;
+
+        $this->assertSame('bool', $model->getCasts()['overridden'] ?? null);
+    }
+
+    public function test_two_instances_of_the_same_model_resolve_the_same_casts(): void
+    {
+        AbstractModel::$customCasts[AbstractModelCastsTestModel::class] = ['shared' => 'bool'];
+
+        $this->assertEquals(
+            (new AbstractModelCastsTestModel)->getCasts(),
+            (new AbstractModelCastsTestModel)->getCasts()
+        );
+    }
+
+    public function test_sibling_models_do_not_share_each_others_casts(): void
+    {
+        // A per-class cache keyed carelessly (or not keyed at all) would leak
+        // the first model's casts onto the second.
+        AbstractModel::$customCasts[AbstractModelCastsTestModel::class] = ['only_on_parent' => 'bool'];
+        AbstractModel::$customCasts[AbstractModelCastsTestSiblingModel::class] = ['only_on_sibling' => 'int'];
+
+        (new AbstractModelCastsTestModel)->getCasts();
+
+        $sibling = (new AbstractModelCastsTestSiblingModel)->getCasts();
+
+        $this->assertArrayNotHasKey('only_on_parent', $sibling);
+        $this->assertSame('int', $sibling['only_on_sibling'] ?? null);
+    }
+
+    public function test_casts_registered_after_a_first_resolution_are_picked_up(): void
+    {
+        // Extenders mutate $customCasts during boot, which can happen after a
+        // model has already resolved its casts once. A cache that never
+        // invalidates would silently ignore the newly registered cast.
+        $this->assertArrayNotHasKey('late', (new AbstractModelCastsTestModel)->getCasts());
+
+        AbstractModel::$customCasts[AbstractModelCastsTestModel::class] = ['late' => 'bool'];
+        AbstractModel::flushCastsCache();
+
+        $this->assertSame('bool', (new AbstractModelCastsTestModel)->getCasts()['late'] ?? null);
+    }
+}
+
+class AbstractModelCastsTestModel extends AbstractModel
+{
+    protected $casts = ['is_native' => 'bool'];
+}
+
+class AbstractModelCastsTestChildModel extends AbstractModelCastsTestModel
+{
+}
+
+class AbstractModelCastsTestSiblingModel extends AbstractModel
+{
+}