diff --git a/src/Integrations/Divi/Design/PresetDiff.php b/src/Integrations/Divi/Design/PresetDiff.php
new file mode 100644
index 0000000..e10ab9f
--- /dev/null
+++ b/src/Integrations/Divi/Design/PresetDiff.php
@@ -0,0 +1,125 @@
+<?php
+
+declare(strict_types=1);
+
+namespace LIT\MCP\Integrations\Divi\Design;
+
+defined('ABSPATH') || exit;
+
+/**
+ * What a preset change does (spec §4 step 4, "diff"): per preset added, changed (with the changed top-level keys),
+ * removed, and per module/group a default that is set or cleared. Keys the plugin derives or Divi maintains
+ * (updated, styleAttrs, renderAttrs, version) are no change of their own. Sorted by kind, scope, id, action.
+ * Pure PHP.
+ */
+final class PresetDiff
+{
+    private const DERIVED = ['updated', 'styleAttrs', 'renderAttrs', 'version'];
+
+    /**
+     * @return list<array{kind: string, scope: string, id: string, name: string, action: string,
+     *     fields?: list<string>}>
+     */
+    public static function between(PresetBook $before, PresetBook $after): array
+    {
+        $old = $before->data();
+        $new = $after->data();
+        $rows = [];
+        foreach ([PresetScope::MODULE, PresetScope::GROUP] as $kind) {
+            $names = array_unique(array_merge(array_keys($old[$kind] ?? []), array_keys($new[$kind] ?? [])));
+            sort($names);
+            foreach ($names as $name) {
+                $was = $old[$kind][$name] ?? ['default' => '', 'items' => []];
+                $is = $new[$kind][$name] ?? ['default' => '', 'items' => []];
+                array_push($rows, ...self::items($kind, (string) $name, $was['items'], $is['items']));
+                array_push($rows, ...self::defaults($kind, (string) $name, $was, $is));
+            }
+        }
+
+        return $rows;
+    }
+
+    /**
+     * @param array<string, array<string, mixed>> $was
+     * @param array<string, array<string, mixed>> $is
+     * @return list<array<string, mixed>>
+     */
+    private static function items(string $kind, string $scope, array $was, array $is): array
+    {
+        $ids = array_unique(array_merge(array_keys($was), array_keys($is)));
+        sort($ids);
+        $rows = [];
+        foreach ($ids as $id) {
+            $key = (string) $id;
+            $before = $was[$key] ?? null;
+            $after = $is[$key] ?? null;
+            if ($before === null || $after === null) {
+                $rows[] = self::row($kind, $scope, $key, $after ?? $before, $before === null ? 'added' : 'removed');
+                continue;
+            }
+            $fields = self::changedFields($before, $after);
+            if ($fields !== []) {
+                $rows[] = self::row($kind, $scope, $key, $after, 'changed') + ['fields' => $fields];
+            }
+        }
+
+        return $rows;
+    }
+
+    /**
+     * @param array{default: string, items: array<string, array<string, mixed>>} $was
+     * @param array{default: string, items: array<string, array<string, mixed>>} $is
+     * @return list<array<string, mixed>>
+     */
+    private static function defaults(string $kind, string $scope, array $was, array $is): array
+    {
+        if ($was['default'] === $is['default']) {
+            return [];
+        }
+        if ($is['default'] === '') {
+            $gone = $was['items'][$was['default']] ?? null;
+
+            return [self::row($kind, $scope, $was['default'], $gone, 'default_cleared')];
+        }
+
+        $item = $is['items'][$is['default']] ?? null;
+
+        return [self::row($kind, $scope, $is['default'], $item, 'default_set')];
+    }
+
+    /**
+     * @param array<string, mixed> $before
+     * @param array<string, mixed> $after
+     * @return list<string>
+     */
+    private static function changedFields(array $before, array $after): array
+    {
+        $keys = array_diff(array_unique(array_merge(array_keys($before), array_keys($after))), self::DERIVED);
+        $fields = [];
+        foreach ($keys as $key) {
+            $was = $before[$key] ?? null;
+            $is = $after[$key] ?? null;
+            if ($was != $is) { // phpcs:ignore Universal.Operators.StrictComparisons
+                $fields[] = (string) $key;
+            }
+        }
+        sort($fields);
+
+        return $fields;
+    }
+
+    /**
+     * @param array<string, mixed>|null $item
+     * @return array{kind: string, scope: string, id: string, name: string, action: string}
+     */
+    private static function row(string $kind, string $scope, string $id, ?array $item, string $action): array
+    {
+        return [
+            'kind' => $kind,
+            'scope' => $scope,
+            'id' => $id,
+            'name' => is_string($item['name'] ?? null) ? $item['name'] : $id,
+            'action' => $action,
+        ];
+    }
+}
diff --git a/tests/Unit/Integrations/Divi/Design/PresetDiffTest.php b/tests/Unit/Integrations/Divi/Design/PresetDiffTest.php
new file mode 100644
index 0000000..c01d4b9
--- /dev/null
+++ b/tests/Unit/Integrations/Divi/Design/PresetDiffTest.php
@@ -0,0 +1,49 @@
+<?php
+
+declare(strict_types=1);
+
+namespace LIT\MCP\Tests\Unit\Integrations\Divi\Design;
+
+use LIT\MCP\Integrations\Divi\Design\PresetBook;
+use LIT\MCP\Integrations\Divi\Design\PresetDiff;
+use LIT\MCP\Integrations\Divi\Design\PresetScope;
+use PHPUnit\Framework\TestCase;
+
+final class PresetDiffTest extends TestCase
+{
+    public function testNoChangeIsAnEmptyDiff(): void
+    {
+        $book = PresetBook::fromDivi(PresetBookTest::stored());
+
+        self::assertSame([], PresetDiff::between($book, PresetBook::fromDivi(PresetBookTest::stored())));
+    }
+
+    public function testAddedChangedRemovedAndDefaults(): void
+    {
+        $before = PresetBook::fromDivi(PresetBookTest::stored());
+        $button = new PresetScope('module', 'divi/button');
+        $text = new PresetScope('module', 'divi/text');
+        $changed = $before->items($button)['aaaaaaaaaa'];
+        $changed['attrs'] = ['button' => ['x' => 2]];
+        $changed['updated'] = 99;
+        $changed['styleAttrs'] = ['ignored' => true];
+        $after = $before
+            ->with($button, $changed)
+            ->with($button, ['id' => 'eeeeeeeeee', 'name' => 'Neu', 'attrs' => []])
+            ->withDefault($button, 'eeeeeeeeee')
+            ->without($button, 'bbbbbbbbbb')
+            ->withDefault($text, '');
+
+        self::assertSame([
+            ['kind' => 'module', 'scope' => 'divi/button', 'id' => 'aaaaaaaaaa', 'name' => 'CTA primär',
+                'action' => 'changed', 'fields' => ['attrs']],
+            ['kind' => 'module', 'scope' => 'divi/button', 'id' => 'bbbbbbbbbb', 'name' => 'Doppelt',
+                'action' => 'removed'],
+            ['kind' => 'module', 'scope' => 'divi/button', 'id' => 'eeeeeeeeee', 'name' => 'Neu', 'action' => 'added'],
+            ['kind' => 'module', 'scope' => 'divi/button', 'id' => 'eeeeeeeeee', 'name' => 'Neu',
+                'action' => 'default_set'],
+            ['kind' => 'module', 'scope' => 'divi/text', 'id' => 'dddddddddd', 'name' => 'Fließtext',
+                'action' => 'default_cleared'],
+        ], PresetDiff::between($before, $after));
+    }
+}