diff --git a/src/Integrations/Divi/Design/ThemeOptionDiff.php b/src/Integrations/Divi/Design/ThemeOptionDiff.php
new file mode 100644
index 0000000..10164f7
--- /dev/null
+++ b/src/Integrations/Divi/Design/ThemeOptionDiff.php
@@ -0,0 +1,62 @@
+<?php
+
+declare(strict_types=1);
+
+namespace LIT\MCP\Integrations\Divi\Design;
+
+defined('ABSPATH') || exit;
+
+/**
+ * What a theme-options change does (spec §4 step 4, "diff"): per key added (not set before), changed or reset
+ * (removed afterwards – Divi falls back to its default). Values are compared strictly, so false (Divi's default
+ * font) and a type change (1 versus "1") count. before/after carry the raw values, null where the key is not set.
+ * In the order of ThemeOptionSpec::KEYS. Pure PHP.
+ */
+final class ThemeOptionDiff
+{
+    /**
+     * @return list<array{key: string, label: string, action: string, before: mixed, after: mixed}>
+     *     action: added|changed|reset
+     */
+    public static function between(ThemeOptionBook $before, ThemeOptionBook $after): array
+    {
+        $rows = [];
+        foreach (ThemeOptionSpec::KEYS as $key) {
+            $action = match (true) {
+                ! $before->has($key) && $after->has($key) => 'added',
+                $before->has($key) && ! $after->has($key) => 'reset',
+                $before->get($key) !== $after->get($key) => 'changed',
+                default => null,
+            };
+            if ($action !== null) {
+                $rows[] = [
+                    'key' => $key,
+                    'label' => ThemeOptionSpec::definition($key)['label'],
+                    'action' => $action,
+                    'before' => $before->get($key),
+                    'after' => $after->get($key),
+                ];
+            }
+        }
+
+        return $rows;
+    }
+
+    /**
+     * One German sentence for the dry run and the change log (spec §4 step 6).
+     *
+     * @param list<array<string, mixed>> $diff
+     */
+    public static function summary(array $diff): string
+    {
+        $count = array_count_values(array_map(static fn (array $row): string => (string) $row['action'], $diff));
+        $parts = [];
+        foreach (['added' => 'gesetzt', 'changed' => 'geändert', 'reset' => 'zurückgesetzt'] as $action => $word) {
+            if (isset($count[$action])) {
+                $parts[] = $count[$action] . ' ' . $word;
+            }
+        }
+
+        return 'Theme-Optionen: ' . ($parts === [] ? 'keine Änderung' : implode(', ', $parts)) . '.';
+    }
+}
diff --git a/tests/Unit/Integrations/Divi/Design/ThemeOptionDiffTest.php b/tests/Unit/Integrations/Divi/Design/ThemeOptionDiffTest.php
new file mode 100644
index 0000000..f517c0f
--- /dev/null
+++ b/tests/Unit/Integrations/Divi/Design/ThemeOptionDiffTest.php
@@ -0,0 +1,101 @@
+<?php
+
+declare(strict_types=1);
+
+namespace LIT\MCP\Tests\Unit\Integrations\Divi\Design;
+
+use LIT\MCP\Integrations\Divi\Design\ThemeOptionBook;
+use LIT\MCP\Integrations\Divi\Design\ThemeOptionDiff;
+use PHPUnit\Framework\TestCase;
+
+final class ThemeOptionDiffTest extends TestCase
+{
+    public function testNoChangeIsAnEmptyDiffEvenWhenTheOrderOfTheInputDiffers(): void
+    {
+        $book = ThemeOptionBook::fromDivi(ThemeOptionBookTest::stored());
+        $same = ThemeOptionBook::fromDivi(array_reverse(ThemeOptionBookTest::stored(), true));
+
+        self::assertSame([], ThemeOptionDiff::between($book, $same));
+        self::assertSame([], ThemeOptionDiff::between(ThemeOptionBook::fromDivi([]), ThemeOptionBook::fromDivi([])));
+    }
+
+    public function testAddedChangedAndResetInTheOrderOfTheKeys(): void
+    {
+        $before = ThemeOptionBook::fromDivi(ThemeOptionBookTest::stored());
+        $after = $before
+            ->with('content_width', 1200)
+            ->with('body_font_size', 18)
+            ->with('body_font', 'Roboto')
+            ->without('body_header_style');
+
+        self::assertSame([
+            ['key' => 'body_font', 'label' => 'Fließtext-Schrift', 'action' => 'changed',
+                'before' => 'Lato', 'after' => 'Roboto'],
+            ['key' => 'body_font_size', 'label' => 'Fließtext-Schriftgröße', 'action' => 'changed',
+                'before' => 16, 'after' => 18],
+            ['key' => 'body_header_style', 'label' => 'Überschrift-Schriftstil', 'action' => 'reset',
+                'before' => 'uppercase', 'after' => null],
+            ['key' => 'content_width', 'label' => 'Inhaltsbreite', 'action' => 'added',
+                'before' => null, 'after' => 1200],
+        ], ThemeOptionDiff::between($before, $after));
+    }
+
+    public function testFalseIsAValueOfItsOwn(): void
+    {
+        $unset = ThemeOptionBook::fromDivi([]);
+        $default = ThemeOptionBook::fromDivi(['heading_font' => false]);
+        $font = ThemeOptionBook::fromDivi(['heading_font' => 'Lato']);
+
+        self::assertSame(
+            [['key' => 'heading_font', 'label' => 'Überschrift-Schrift', 'action' => 'added',
+                'before' => null, 'after' => false]],
+            ThemeOptionDiff::between($unset, $default)
+        );
+        self::assertSame(
+            [['key' => 'heading_font', 'label' => 'Überschrift-Schrift', 'action' => 'changed',
+                'before' => false, 'after' => 'Lato']],
+            ThemeOptionDiff::between($default, $font)
+        );
+        self::assertSame(
+            [['key' => 'heading_font', 'label' => 'Überschrift-Schrift', 'action' => 'reset',
+                'before' => false, 'after' => null]],
+            ThemeOptionDiff::between($default, $unset)
+        );
+    }
+
+    public function testATypeChangeCountsAsAChange(): void
+    {
+        $int = ThemeOptionBook::fromDivi(['gutter_width' => 1, 'body_header_height' => 1]);
+        $text = ThemeOptionBook::fromDivi(['gutter_width' => '1', 'body_header_height' => 1.0]);
+
+        self::assertSame(
+            [
+                ['key' => 'body_header_height', 'label' => 'Überschrift-Zeilenhöhe', 'action' => 'changed',
+                    'before' => 1, 'after' => 1.0],
+                ['key' => 'gutter_width', 'label' => 'Spaltenabstand', 'action' => 'changed',
+                    'before' => 1, 'after' => '1'],
+            ],
+            ThemeOptionDiff::between($int, $text)
+        );
+    }
+
+    public function testSummaryCountsPerAction(): void
+    {
+        $before = ThemeOptionBook::fromDivi(ThemeOptionBookTest::stored());
+        $after = $before->with('content_width', 1200)->with('body_font_size', 18)->with('body_font', 'Roboto')
+            ->without('body_header_style');
+
+        self::assertSame(
+            'Theme-Optionen: 1 gesetzt, 2 geändert, 1 zurückgesetzt.',
+            ThemeOptionDiff::summary(ThemeOptionDiff::between($before, $after))
+        );
+        self::assertSame(
+            'Theme-Optionen: 1 gesetzt.',
+            ThemeOptionDiff::summary(ThemeOptionDiff::between(
+                ThemeOptionBook::fromDivi([]),
+                ThemeOptionBook::fromDivi(['content_width' => 1200])
+            ))
+        );
+        self::assertSame('Theme-Optionen: keine Änderung.', ThemeOptionDiff::summary([]));
+    }
+}