diff --git a/docs/guides/overview.md b/docs/guides/overview.md
index f453861..7ca9f5d 100644
--- a/docs/guides/overview.md
+++ b/docs/guides/overview.md
@@ -20,7 +20,8 @@ des verbundenen WordPress-Benutzers.
 Direkt sichtbar: `lit-guide`, `lit-site-status`, `lit-content-types`, `lit-content-query`,
 `lit-content-get`, `lit-content-create`, `lit-content-update`, `lit-post-publish`, `lit-media-search`,
 `lit-media-preview`, `lit-media-download`, `lit-media-upload-from-url`, `lit-page-render-summary`,
-`lit-divi-outline`, `lit-divi-node-get`, `lit-divi-catalog`, `lit-divi-design-system`.
+`lit-divi-outline`, `lit-divi-node-get`, `lit-divi-catalog`, `lit-divi-design-system`, `lit-divi-insert`,
+`lit-divi-update`.
 
 Welche Tools direkt sichtbar sind, lässt sich unter Einstellungen → LIT MCP → Tools ändern (außer bei
 `lit-guide`, das immer direkt bleibt).
diff --git a/src/Abilities/Errors.php b/src/Abilities/Errors.php
index f64576e..a5d1da7 100644
--- a/src/Abilities/Errors.php
+++ b/src/Abilities/Errors.php
@@ -101,6 +101,24 @@ final class Errors
         return self::make('unsupported', $message, $hint, 400);
     }
 
+    /**
+     * Unknown Divi attribute paths (spec §5.4, §8.2). $fields names every problem of the call,
+     * the unknown paths with their closest known ones.
+     *
+     * @param array<string, string> $fields Input path => problem.
+     */
+    public static function invalidAttrPath(array $fields): WP_Error
+    {
+        return self::make(
+            'invalid_attr_path',
+            'Unbekannte Attributpfade; nichts wurde gespeichert.',
+            'Vorschläge unter fields prüfen; alle gültigen Pfade eines Moduls liefert lit/divi-catalog '
+                . '(direkt: lit-divi-catalog) mit module.',
+            400,
+            ['fields' => $fields]
+        );
+    }
+
     /**
      * A write failed and was rolled back completely.
      *
diff --git a/src/Integrations/Divi/Compose/Composer.php b/src/Integrations/Divi/Compose/Composer.php
index 81e9c93..f53107d 100644
--- a/src/Integrations/Divi/Compose/Composer.php
+++ b/src/Integrations/Divi/Compose/Composer.php
@@ -116,6 +116,7 @@ final class Composer
             }
         }
 
+        $this->overlaps($block->shortName(), $change, $field);
         $attrs = $this->presetAndAttrs($block->name(), $attrs, $change, $field);
         if (array_key_exists('label', $change)) {
             $attrs = $change['label'] === null || $change['label'] === ''
@@ -126,6 +127,39 @@ final class Composer
         return ['attrs' => $attrs, 'errors' => $this->errors, 'invalid_paths' => $this->invalidPaths];
     }
 
+    /**
+     * Ruling S38: one change may not reach the same attribute path through "set" and "attrs" –
+     * composite values are replaced whole per breakpoint, so combining them would silently lose
+     * sub-keys. Paths overlap when equal or when one lies below the other.
+     *
+     * @param array<array-key, mixed> $change
+     */
+    private function overlaps(string $type, array $change, string $field): void
+    {
+        $set = $change['set'] ?? null;
+        $full = $change['attrs'] ?? null;
+        if (! is_array($set) || ! is_array($full) || $set === [] || $full === []) {
+            return;
+        }
+
+        $short = $this->shortform->paths($type, array_map('strval', array_keys($set)));
+        foreach (array_keys($full) as $path) {
+            $path = (string) $path;
+            foreach ($short as $name => $shortPath) {
+                if (
+                    $path === $shortPath || str_starts_with($path, $shortPath . '.')
+                    || str_starts_with($shortPath, $path . '.')
+                ) {
+                    $this->errors[$field . '.attrs.' . $path] = sprintf(
+                        'derselbe Attributpfad wird auch über set.%s geändert; bitte als zwei Änderungen senden',
+                        $name
+                    );
+                    break;
+                }
+            }
+        }
+    }
+
     /** Admin label for a new module: "<Abschnitt> – <Modultitel>" or the title alone. */
     public function autoLabel(string $name, string $context): string
     {
diff --git a/src/Integrations/Divi/Compose/ShortformWriter.php b/src/Integrations/Divi/Compose/ShortformWriter.php
index d9134ef..6caf160 100644
--- a/src/Integrations/Divi/Compose/ShortformWriter.php
+++ b/src/Integrations/Divi/Compose/ShortformWriter.php
@@ -72,6 +72,25 @@ final class ShortformWriter
         return ['attrs' => $attrs, 'errors' => $errors];
     }
 
+    /**
+     * Attribute paths the given short-form fields of $type write to (unknown fields left out).
+     *
+     * @param list<string> $names Field names.
+     * @return array<string, string> Field => attribute path.
+     */
+    public function paths(string $type, array $names): array
+    {
+        $fields = $this->modules === null ? ShortformMap::fields($type) : ($this->modules[$type] ?? []);
+        $paths = [];
+        foreach ($names as $name) {
+            if (isset($fields[$name]['path'])) {
+                $paths[$name] = (string) $fields[$name]['path'];
+            }
+        }
+
+        return $paths;
+    }
+
     /**
      * The short-form color check (ruling S13; Task 3 reuses it for background_color): a
      * placeholder or reference that is a color reference, or a plain CSS color – checked
diff --git a/src/Integrations/Divi/DiviIntegration.php b/src/Integrations/Divi/DiviIntegration.php
index 52b66fa..749d7f6 100644
--- a/src/Integrations/Divi/DiviIntegration.php
+++ b/src/Integrations/Divi/DiviIntegration.php
@@ -4,11 +4,17 @@ declare(strict_types=1);
 
 namespace LIT\MCP\Integrations\Divi;
 
+use LIT\MCP\ChangeLog\ChangeLog;
 use LIT\MCP\Integrations\Divi\Design\DesignSystemReader;
 use LIT\MCP\Integrations\Divi\Design\DiviDesignSystemAbility;
+use LIT\MCP\Integrations\Divi\Pages\DiviComposition;
+use LIT\MCP\Integrations\Divi\Pages\DiviInsertAbility;
 use LIT\MCP\Integrations\Divi\Pages\DiviNodeGetAbility;
 use LIT\MCP\Integrations\Divi\Pages\DiviOutlineAbility;
 use LIT\MCP\Integrations\Divi\Pages\DiviPageLoader;
+use LIT\MCP\Integrations\Divi\Pages\DiviSaver;
+use LIT\MCP\Integrations\Divi\Pages\DiviUpdateAbility;
+use LIT\MCP\Integrations\Divi\Pages\DiviWriter;
 use LIT\MCP\Integrations\Divi\Schema\DiviCatalogAbility;
 use LIT\MCP\Integrations\Divi\Schema\DiviModules;
 use LIT\MCP\Integrations\Integration;
@@ -16,14 +22,15 @@ use LIT\MCP\Integrations\WordPress\Schema\ContentSchema;
 
 /**
  * Divi 5 pages (spec §5, Etappe 2). Only available when the Divi 5 builder is loaded
- * (spec §2 principle 4); Divi 4 is never edited (spec §3.4).
+ * (spec §2 principle 4); Divi 4 is never edited (spec §3.4). The write abilities share one
+ * DiviWriter; each binds the composition to its entry via DiviWriter::composition().
  */
 final class DiviIntegration implements Integration
 {
     private DiviModules $modules;
     private DesignSystemReader $design;
 
-    public function __construct(private ContentSchema $schema)
+    public function __construct(private ContentSchema $schema, private ChangeLog $log)
     {
         $this->modules = new DiviModules();
         $this->design = new DesignSystemReader();
@@ -42,12 +49,16 @@ final class DiviIntegration implements Integration
     public function abilities(): array
     {
         $loader = new DiviPageLoader($this->schema);
+        $composition = new DiviComposition($this->modules, $this->design);
+        $writer = new DiviWriter($loader, new DiviSaver(), $this->log, $this->design, $composition);
 
         return [
             new DiviOutlineAbility($loader, $this->design),
             new DiviNodeGetAbility($loader, $this->design),
             new DiviCatalogAbility($this->modules),
             new DiviDesignSystemAbility($this->design),
+            new DiviInsertAbility($writer),
+            new DiviUpdateAbility($writer),
         ];
     }
 }
diff --git a/src/Integrations/Divi/Pages/DiviInsertAbility.php b/src/Integrations/Divi/Pages/DiviInsertAbility.php
new file mode 100644
index 0000000..9474f3e
--- /dev/null
+++ b/src/Integrations/Divi/Pages/DiviInsertAbility.php
@@ -0,0 +1,130 @@
+<?php
+
+declare(strict_types=1);
+
+namespace LIT\MCP\Integrations\Divi\Pages;
+
+use LIT\MCP\Abilities\Ability;
+use LIT\MCP\Abilities\Errors;
+use LIT\MCP\Abilities\ToolResult;
+use WP_Error;
+
+/**
+ * lit/divi-insert: inserts nodes in short and/or full form – modules, rows or whole sections –
+ * at parent / before / after (spec §5.7). All or nothing (ruling S39): the whole input is
+ * validated first and every error is returned together; the blocks come only from Composer
+ * (client values pass its choke point) and enter the tree only through TreeEditor::insert(),
+ * which also wraps the first section of an empty page in divi/placeholder (ruling S9).
+ */
+final class DiviInsertAbility implements Ability
+{
+    public function __construct(private DiviWriter $writer)
+    {
+    }
+
+    public function name(): string
+    {
+        return 'lit/divi-insert';
+    }
+
+    public function label(): string
+    {
+        return 'Divi-Knoten einfügen';
+    }
+
+    public function description(): string
+    {
+        return 'Fügt Divi-Knoten ein: Module, Zeilen oder ganze Abschnitte in Kurzform (z. B. {"type": "button", '
+            . '"text": "Termin buchen", "url": "/kontakt", "preset": "CTA primär"}) und/oder Vollform ("attrs"). '
+            . 'Position über parent, before oder after (Pfad-IDs aus lit-divi-outline), sonst ans Seitenende. '
+            . 'Platzhalter wie {{color:primary}} statt fester Werte. tree_rev ist Pflicht; alles oder nichts. '
+            . 'Format: lit-guide topic "divi-shortform".';
+    }
+
+    public function inputSchema(): array
+    {
+        return [
+            'type' => 'object',
+            'properties' => array_merge(DiviWriter::inputProperties(), [
+                'nodes' => [
+                    'type' => 'array',
+                    'minItems' => 1,
+                    'maxItems' => 20,
+                    'items' => ['type' => 'object'],
+                    'description' => 'Einzufügende Knoten in Reihenfolge; Abschnitte mit rows/modules, Zeilen mit '
+                        . 'columns und modules, Module mit Kurzform-Feldern, items (Kind-Module) und attrs.',
+                ],
+            ], DiviWriter::positionProperties()),
+            'required' => ['post_id', 'tree_rev', 'nodes'],
+            'additionalProperties' => false,
+        ];
+    }
+
+    public function outputSchema(): ?array
+    {
+        return ToolResult::schema(['tree_rev' => ['type' => 'string']], ['tree_rev']);
+    }
+
+    public function annotations(): array
+    {
+        return ['readonly' => false, 'destructive' => false, 'idempotent' => false];
+    }
+
+    public function isDirect(): bool
+    {
+        return true;
+    }
+
+    public function permission(array $input): bool|WP_Error
+    {
+        $page = $this->writer->page($input);
+
+        return $page instanceof WP_Error ? $page : true;
+    }
+
+    public function execute(array $input): mixed
+    {
+        $page = $this->writer->page($input);
+        if ($page instanceof WP_Error) {
+            return $page;
+        }
+
+        $composition = $this->writer->composition($page);
+        $editor = $composition->editor();
+        $position = $editor->position($page->index, $input);
+        if (is_string($position)) {
+            // The nodes are checked against their parent – without a valid position there is none.
+            return Errors::invalidInput('Ungültige Position; nichts wurde gespeichert.', ['position' => $position]);
+        }
+
+        $context = DiviWriter::sectionLabel($page->index, $position['parent']);
+        $nodes = is_array($input['nodes'] ?? null) ? array_values($input['nodes']) : [];
+        if ($nodes === []) {
+            return Errors::invalidInput('Keine Knoten angegeben; nichts wurde gespeichert.', [
+                'nodes' => 'mindestens ein Knoten',
+            ]);
+        }
+
+        $composed = $composition->composer()->compose($nodes, $position['parent'], 'nodes', $context);
+        if ($composed['errors'] !== []) {
+            return $composed['invalid_paths'] !== []
+                ? Errors::invalidAttrPath($composed['errors'])
+                : Errors::invalidInput('Knoten sind ungültig; nichts wurde gespeichert.', $composed['errors']);
+        }
+
+        $violation = $editor->insert($position, $composed['blocks']);
+        if ($violation !== null) {
+            return Errors::invalidInput(
+                'Knoten passen nicht an diese Stelle; nichts wurde gespeichert.',
+                ['nodes' => $violation]
+            );
+        }
+
+        $change = new DiviChange(sprintf('%d Divi-Knoten eingefügt', count($composed['blocks'])));
+        foreach ($composed['blocks'] as $block) {
+            $change->changed($block, 'inserted');
+        }
+
+        return $this->writer->save($this->name(), $page, $change, $input);
+    }
+}
diff --git a/src/Integrations/Divi/Pages/DiviUpdateAbility.php b/src/Integrations/Divi/Pages/DiviUpdateAbility.php
new file mode 100644
index 0000000..2792428
--- /dev/null
+++ b/src/Integrations/Divi/Pages/DiviUpdateAbility.php
@@ -0,0 +1,175 @@
+<?php
+
+declare(strict_types=1);
+
+namespace LIT\MCP\Integrations\Divi\Pages;
+
+use LIT\MCP\Abilities\Ability;
+use LIT\MCP\Abilities\Errors;
+use LIT\MCP\Abilities\ToolResult;
+use LIT\MCP\Integrations\Divi\BlockTree\Block;
+use WP_Error;
+
+/**
+ * lit/divi-update: sets or removes values of existing nodes – short-form fields ("set"), full
+ * form ("attrs", null removes a path or breakpoint), preset and admin label (spec §5.7). Several
+ * nodes per call, all or nothing (ruling S39): every change is validated first and all errors
+ * come back together; only then are the attributes Composer::update() computed set. A node may
+ * appear once per call (ruling S14); set and attrs of one change must not reach the same path
+ * (ruling S38, checked by Composer::update()).
+ */
+final class DiviUpdateAbility implements Ability
+{
+    public function __construct(private DiviWriter $writer)
+    {
+    }
+
+    public function name(): string
+    {
+        return 'lit/divi-update';
+    }
+
+    public function label(): string
+    {
+        return 'Divi-Knoten ändern';
+    }
+
+    public function description(): string
+    {
+        return 'Ändert vorhandene Divi-Knoten: changes = Liste von {id, set: {Kurzform-Felder}, attrs: {Attributpfad: '
+            . '{Breakpoint: Wert}}, preset, label}. null entfernt einen Wert. Nicht genannte Einstellungen bleiben '
+            . 'unverändert. Jeder Knoten nur einmal pro Aufruf; denselben Pfad nicht zugleich über set und attrs. '
+            . 'tree_rev aus lit-divi-outline ist Pflicht; alles oder nichts.';
+    }
+
+    public function inputSchema(): array
+    {
+        return [
+            'type' => 'object',
+            'properties' => array_merge(DiviWriter::inputProperties(), [
+                'changes' => [
+                    'type' => 'array',
+                    'minItems' => 1,
+                    'maxItems' => 50,
+                    'items' => [
+                        'type' => 'object',
+                        'properties' => [
+                            'id' => ['type' => 'string'],
+                            'set' => ['type' => 'object', 'description' => 'Kurzform-Felder, z. B. {"text": "Neu"}.'],
+                            'attrs' => [
+                                'type' => 'object',
+                                'description' => 'Vollform: Attributpfad → Breakpoint-Werte.',
+                            ],
+                            'preset' => ['type' => ['string', 'null'], 'description' => 'Preset-Name; null entfernt.'],
+                            'label' => [
+                                'type' => ['string', 'null'],
+                                'description' => 'Admin-Label im Divi-Builder; null entfernt.',
+                            ],
+                        ],
+                        'required' => ['id'],
+                        'additionalProperties' => false,
+                    ],
+                ],
+            ]),
+            'required' => ['post_id', 'tree_rev', 'changes'],
+            'additionalProperties' => false,
+        ];
+    }
+
+    public function outputSchema(): ?array
+    {
+        return ToolResult::schema(['tree_rev' => ['type' => 'string']], ['tree_rev']);
+    }
+
+    public function annotations(): array
+    {
+        return ['readonly' => false, 'destructive' => false, 'idempotent' => true];
+    }
+
+    public function isDirect(): bool
+    {
+        return true;
+    }
+
+    public function permission(array $input): bool|WP_Error
+    {
+        $page = $this->writer->page($input);
+
+        return $page instanceof WP_Error ? $page : true;
+    }
+
+    public function execute(array $input): mixed
+    {
+        $page = $this->writer->page($input);
+        if ($page instanceof WP_Error) {
+            return $page;
+        }
+
+        $changes = is_array($input['changes'] ?? null) ? array_values($input['changes']) : [];
+        if ($changes === []) {
+            return Errors::invalidInput('Keine Änderungen angegeben; nichts wurde gespeichert.', [
+                'changes' => 'mindestens eine Änderung',
+            ]);
+        }
+
+        $composer = $this->writer->composition($page)->composer();
+        $errors = [];
+        $invalidPaths = false;
+        /** @var array<string, int> $seen id => position of its first change */
+        $seen = [];
+        /** @var list<array{0: Block, 1: array<string, mixed>}> $updates */
+        $updates = [];
+        foreach ($changes as $position => $change) {
+            $field = sprintf('changes[%d]', $position);
+            if (! is_array($change)) {
+                $errors[$field] = 'Objekt mit id erwartet';
+                continue;
+            }
+
+            $id = is_string($change['id'] ?? null) ? $change['id'] : '';
+            if ($id !== '' && isset($seen[$id])) {
+                // Ruling S14: a second change of the same node would overwrite the first silently.
+                $errors[$field . '.id'] = sprintf(
+                    'Knoten "%s" steht schon in changes[%d]; alle Änderungen eines Knotens in einer Änderung bündeln',
+                    $id,
+                    $seen[$id]
+                );
+                continue;
+            }
+
+            $block = $id === '' ? null : $page->index->find($id);
+            if ($id !== '') {
+                $seen[$id] = $position;
+            }
+
+            if ($block === null) {
+                $errors[$field . '.id'] = sprintf('Knoten "%s" gibt es nicht (lit-divi-outline neu lesen)', $id);
+                continue;
+            }
+
+            $result = $composer->update($block, $change, $field);
+            $errors += $result['errors'];
+            $invalidPaths = $invalidPaths || $result['invalid_paths'] !== [];
+            $updates[] = [$block, $result['attrs']];
+        }
+
+        if ($errors !== []) {
+            return $invalidPaths
+                ? Errors::invalidAttrPath($errors)
+                : Errors::invalidInput('Änderungen sind ungültig; nichts wurde gespeichert.', $errors);
+        }
+
+        $updates = array_values(array_filter(
+            $updates,
+            static fn (array $update): bool => $update[1] !== $update[0]->attrs()
+        ));
+        $change = new DiviChange(sprintf('%d Divi-Knoten geändert', count($updates)));
+        foreach ($updates as [$block, $attrs]) {
+            // Composer::update() output: every client value already passed the choke point.
+            $block->setAttrs($attrs);
+            $change->changed($block, 'updated');
+        }
+
+        return $this->writer->save($this->name(), $page, $change, $input);
+    }
+}
diff --git a/src/Plugin.php b/src/Plugin.php
index 9ed09db..4de2e5d 100644
--- a/src/Plugin.php
+++ b/src/Plugin.php
@@ -68,7 +68,7 @@ final class Plugin
             new CoreIntegration(self::pluginDir() . '/docs/guides'),
             new WordPressIntegration($schema, $changeLog),
             new MediaIntegration($schema, $changeLog, $limits),
-            new DiviIntegration($schema)
+            new DiviIntegration($schema, $changeLog)
         );
 
         $policy = ExposurePolicy::fromOption();
diff --git a/tests/Unit/Integrations/Divi/Compose/ComposerTest.php b/tests/Unit/Integrations/Divi/Compose/ComposerTest.php
index f3f22bd..fe47b7e 100644
--- a/tests/Unit/Integrations/Divi/Compose/ComposerTest.php
+++ b/tests/Unit/Integrations/Divi/Compose/ComposerTest.php
@@ -183,6 +183,26 @@ final class ComposerTest extends TestCase
         self::assertStringContainsString('nur lesbar', $readOnly['errors']['changes[2].id']);
     }
 
+    /** Ruling S38: one change may not reach the same attribute path through set and attrs. */
+    public function testUpdateRefusesSetAndAttrsOnTheSamePath(): void
+    {
+        $block = Block::create('divi/button', ['button' => ['innerContent' => ['desktop' => ['value' => ['text' => 'Alt']]]]]);
+
+        $result = ComposeFixture::composer()->update($block, [
+            'set' => ['text' => 'Neu'],
+            'attrs' => ['button.innerContent' => ['desktop' => ['linkUrl' => '/kontakt']]],
+        ], 'changes[0]');
+
+        self::assertStringContainsString('set.text', $result['errors']['changes[0].attrs.button.innerContent']);
+        self::assertSame([], $result['invalid_paths']);
+
+        $separate = ComposeFixture::composer()->update($block, [
+            'set' => ['text' => 'Neu'],
+            'attrs' => ['button.decoration.background' => ['desktop' => ['color' => '#ffffff']]],
+        ], 'changes[0]');
+        self::assertSame([], $separate['errors'], 'different paths may be combined');
+    }
+
     /** Ruling S13: labels go through the choke point; a label that fails is an error, never stored raw. */
     public function testLabelsAreCleanedOrReported(): void
     {
diff --git a/tests/Unit/Integrations/Divi/DiviIntegrationTest.php b/tests/Unit/Integrations/Divi/DiviIntegrationTest.php
index d007b41..6572107 100644
--- a/tests/Unit/Integrations/Divi/DiviIntegrationTest.php
+++ b/tests/Unit/Integrations/Divi/DiviIntegrationTest.php
@@ -5,6 +5,7 @@ declare(strict_types=1);
 namespace LIT\MCP\Tests\Unit\Integrations\Divi;
 
 use LIT\MCP\Abilities\Ability;
+use LIT\MCP\ChangeLog\ChangeLog;
 use LIT\MCP\Integrations\Divi\DiviIntegration;
 use LIT\MCP\Integrations\WordPress\Schema\ContentSchema;
 use LIT\MCP\Tests\Unit\TestCase;
@@ -18,11 +19,13 @@ final class DiviIntegrationTest extends TestCase
         'lit/divi-node-get',
         'lit/divi-catalog',
         'lit/divi-design-system',
+        'lit/divi-insert',
+        'lit/divi-update',
     ];
 
     public function testProvidesTheDiviAbilitiesOnlyWithDivi5(): void
     {
-        $integration = new DiviIntegration(Mockery::mock(ContentSchema::class));
+        $integration = new DiviIntegration(Mockery::mock(ContentSchema::class), Mockery::mock(ChangeLog::class));
 
         self::assertSame('divi', $integration->id());
         self::assertFalse($integration->isAvailable(), 'no Divi classes in unit tests');
diff --git a/tests/Unit/Integrations/Divi/Pages/DiviWriteAbilitiesTest.php b/tests/Unit/Integrations/Divi/Pages/DiviWriteAbilitiesTest.php
new file mode 100644
index 0000000..0672cfb
--- /dev/null
+++ b/tests/Unit/Integrations/Divi/Pages/DiviWriteAbilitiesTest.php
@@ -0,0 +1,295 @@
+<?php
+
+declare(strict_types=1);
+
+namespace LIT\MCP\Tests\Unit\Integrations\Divi\Pages;
+
+use Brain\Monkey\Functions;
+use LIT\MCP\Abilities\Rev;
+use LIT\MCP\ChangeLog\ChangeLog;
+use LIT\MCP\Integrations\Divi\BlockTree\NodeIndex;
+use LIT\MCP\Integrations\Divi\BlockTree\Parser;
+use LIT\MCP\Integrations\Divi\Design\DesignSystemReader;
+use LIT\MCP\Integrations\Divi\Pages\DiviComposition;
+use LIT\MCP\Integrations\Divi\Pages\DiviInsertAbility;
+use LIT\MCP\Integrations\Divi\Pages\DiviPageLoader;
+use LIT\MCP\Integrations\Divi\Pages\DiviSaver;
+use LIT\MCP\Integrations\Divi\Pages\DiviUpdateAbility;
+use LIT\MCP\Integrations\Divi\Pages\DiviWriter;
+use LIT\MCP\Integrations\Divi\Schema\DiviModules;
+use LIT\MCP\Tests\Unit\Integrations\Divi\Compose\ComposeFixture;
+use LIT\MCP\Tests\Unit\Integrations\Divi\Design\DesignSystemTest;
+use LIT\MCP\Tests\Unit\Integrations\Divi\Schema\ModuleCatalogTest;
+use LIT\MCP\Tests\Unit\TestCase;
+use Mockery;
+use WP_Error;
+use WP_Post;
+
+final class DiviWriteAbilitiesTest extends TestCase
+{
+    private WP_Post $post;
+    private string $original = '';
+
+    /** @var list<string> Contents handed to DiviSaver::save(). */
+    private array $saved = [];
+
+    private DiviWriter $writer;
+
+    protected function setUp(): void
+    {
+        parent::setUp();
+        $this->original = DiviPageLoaderTest::fixture('synthetic-basic.html');
+        $this->post = DiviPageLoaderTest::givenPost($this->original);
+        Functions\when('get_preview_post_link')->justReturn('https://kunde.test/?page_id=7&preview=true');
+
+        $modules = Mockery::mock(DiviModules::class);
+        $modules->allows('catalog')->andReturn(ModuleCatalogTest::sampleCatalog());
+        $modules->allows('builderVersion')->andReturn('5.2.1');
+        $design = Mockery::mock(DesignSystemReader::class);
+        $design->allows('read')->andReturn(DesignSystemTest::sample());
+        $composition = new DiviComposition($modules, $design, ComposeFixture::sanitizer(), ComposeFixture::media());
+
+        $saver = Mockery::mock(DiviSaver::class);
+        $saver->allows('save')->andReturnUsing(function (WP_Post $post, string $content): array {
+            $this->saved[] = $content;
+            $post->post_content = $content;
+
+            return ['revision_id' => 811, 'warnings' => []];
+        });
+        $log = Mockery::mock(ChangeLog::class);
+        $log->allows('record')->andReturn(4711);
+
+        $this->writer = new DiviWriter(
+            new DiviPageLoader(DiviPageLoaderTest::schema()),
+            $saver,
+            $log,
+            $design,
+            $composition
+        );
+    }
+
+    private function treeRev(): string
+    {
+        return Rev::of($this->original);
+    }
+
+    private function insert(): DiviInsertAbility
+    {
+        return new DiviInsertAbility($this->writer);
+    }
+
+    private function update(): DiviUpdateAbility
+    {
+        return new DiviUpdateAbility($this->writer);
+    }
+
+    /** @return array<string, mixed> The structured error the client sees (message JSON, spec §8.2). */
+    private static function clientError(WP_Error $error): array
+    {
+        return (array) json_decode($error->get_error_message(), true);
+    }
+
+    public function testInsertComposesAndSavesWithTreeRevAndPostRev(): void
+    {
+        $result = $this->insert()->execute([
+            'post_id' => 7,
+            'tree_rev' => $this->treeRev(),
+            'after' => 's0.r0.c1.m1',
+            'nodes' => [['type' => 'text', 'content' => '<p>Neu</p>']],
+        ]);
+
+        self::assertTrue($result['ok'], (string) json_encode($result));
+        self::assertCount(1, $this->saved);
+        self::assertSame([['id' => 's0.r0.c1.m2', 'type' => 'divi/text', 'action' => 'inserted', 'label' => 'Hero – Text']], $result['changed']);
+        self::assertSame(Rev::of($this->saved[0]), $result['tree_rev']);
+        self::assertSame(Rev::ofPost($this->post), $result['rev']);
+        self::assertSame(811, $result['revision_id']);
+        self::assertSame(4711, $result['log_id']);
+        self::assertStringStartsWith('s0 section "Hero"', (string) $result['outline']);
+        self::assertSame('https://kunde.test/?page_id=7&et_fb=1', $result['edit_url']);
+
+        // Byte identity: only the new block was added, every other byte is unchanged.
+        $inserted = '<!-- wp:divi/text {"builderVersion":"5.2.1","content":{"innerContent":{"desktop":{"value":'
+            . '"\u003cp\u003eNeu\u003c/p\u003e"}}},"module":{"meta":{"adminLabel":{"desktop":{"value":"Hero – Text"}}}}} '
+            . '--><!-- /wp:divi/text -->';
+        self::assertSame(str_replace('<!-- /wp:divi/text --><!-- wp:divi/button', '<!-- /wp:divi/text -->' . $inserted . '<!-- wp:divi/button', $this->original), $this->saved[0]);
+    }
+
+    public function testStaleTreeRevAndInvalidNodesWriteNothing(): void
+    {
+        $stale = $this->insert()->execute(['post_id' => 7, 'tree_rev' => '000000000000', 'nodes' => [['type' => 'section']]]);
+        self::assertSame('lit_rev_mismatch', $stale->get_error_code());
+        self::assertSame('rev_mismatch', self::clientError($stale)['code'], 'the client sees the code without lit_');
+        self::assertStringContainsString('lit-divi-outline', $stale->get_error_data()['hint']);
+        self::assertSame($stale->get_error_code(), $this->insert()->permission(['post_id' => 7, 'tree_rev' => 'x'])->get_error_code());
+
+        $invalid = $this->insert()->execute([
+            'post_id' => 7,
+            'tree_rev' => $this->treeRev(),
+            'parent' => 's0.r0.c0',
+            'nodes' => [
+                ['type' => 'heading', 'attrs' => ['title.fnt' => ['desktop' => 'x']]],
+                ['type' => 'button', 'url' => 'javascript:alert(1)', 'text' => 'x', 'nope' => 1],
+            ],
+        ]);
+        self::assertSame('lit_invalid_attr_path', $invalid->get_error_code());
+        $client = self::clientError($invalid);
+        self::assertSame('invalid_attr_path', $client['code']);
+        self::assertArrayHasKey('nodes[0].attrs.title.fnt', $client['fields']);
+        self::assertGreaterThanOrEqual(2, count($client['fields']), 'every error of the call in one response (S39)');
+        self::assertStringContainsString('lit-divi-catalog', $client['hint']);
+
+        $wrongPlace = $this->insert()->execute(['post_id' => 7, 'tree_rev' => $this->treeRev(), 'nodes' => [['type' => 'button']]]);
+        self::assertSame('lit_validation_failed', $wrongPlace->get_error_code());
+
+        $badPosition = $this->insert()->execute([
+            'post_id' => 7,
+            'tree_rev' => $this->treeRev(),
+            'parent' => 's0.r0.c0',
+            'after' => 's0.r0.c1.m1',
+            'nodes' => [['type' => 'text']],
+        ]);
+        self::assertSame('lit_validation_failed', $badPosition->get_error_code());
+        self::assertArrayHasKey('position', self::clientError($badPosition)['fields']);
+
+        $unknown = $this->insert()->execute([
+            'post_id' => 7,
+            'tree_rev' => $this->treeRev(),
+            'parent' => 's9',
+            'nodes' => [['type' => 'section']],
+        ]);
+        self::assertSame('lit_validation_failed', $unknown->get_error_code());
+        self::assertSame([], $this->saved);
+        self::assertSame($this->original, $this->post->post_content);
+    }
+
+    /** Ruling S9: the first section of an empty page goes into a divi/placeholder. */
+    public function testInsertIntoAnEmptyPageWrapsInAPlaceholder(): void
+    {
+        $this->post->post_content = '';
+
+        $result = $this->insert()->execute([
+            'post_id' => 7,
+            'tree_rev' => Rev::of(''),
+            'nodes' => [['type' => 'section', 'label' => 'Start', 'modules' => [['type' => 'heading', 'text' => 'Hallo']]]],
+        ]);
+
+        self::assertTrue($result['ok'], (string) json_encode($result));
+        self::assertCount(1, $this->saved);
+        self::assertStringStartsWith('<!-- wp:divi/placeholder --><!-- wp:divi/section ', $this->saved[0]);
+        self::assertStringEndsWith('<!-- /wp:divi/section --><!-- /wp:divi/placeholder -->', $this->saved[0]);
+        self::assertSame('s0', $result['changed'][0]['id']);
+        self::assertSame('Start', $result['changed'][0]['label']);
+    }
+
+    public function testUpdateChangesSeveralNodesAllOrNothing(): void
+    {
+        $failed = $this->update()->execute(['post_id' => 7, 'tree_rev' => $this->treeRev(), 'changes' => [
+            ['id' => 's0.r0.c1.m2', 'set' => ['text' => 'Jetzt buchen']],
+            ['id' => 's0.r0.c1.m0', 'set' => ['level' => 'h9']],
+            ['id' => 's0.r0.c1.m9', 'label' => 'x'],
+        ]]);
+        self::assertSame('lit_validation_failed', $failed->get_error_code());
+        $fields = self::clientError($failed)['fields'];
+        self::assertArrayHasKey('changes[1].set.level', $fields);
+        self::assertArrayHasKey('changes[2].id', $fields, 'every error of the call in one response (S39)');
+        self::assertSame([], $this->saved);
+
+        $result = $this->update()->execute(['post_id' => 7, 'tree_rev' => $this->treeRev(), 'changes' => [
+            ['id' => 's0.r0.c1.m2', 'set' => ['text' => 'Jetzt buchen']],
+            ['id' => 's0.r0.c1.m0', 'attrs' => ['title.decoration.font.font' => ['phone' => null]]],
+        ]]);
+
+        self::assertTrue($result['ok']);
+        self::assertSame(['s0.r0.c1.m2', 's0.r0.c1.m0'], array_column($result['changed'], 'id'));
+        self::assertSame(['updated', 'updated'], array_column($result['changed'], 'action'));
+        self::assertSame(Rev::of($this->saved[0]), $result['tree_rev']);
+        $index = NodeIndex::build((new Parser())->parse($this->saved[0]));
+        self::assertSame('Jetzt buchen', $index->find('s0.r0.c1.m2')?->attrs()['button']['innerContent']['desktop']['value']['text']);
+        self::assertArrayNotHasKey('phone', $index->find('s0.r0.c1.m0')?->attrs()['title']['decoration']['font']['font']);
+        // Untouched section s1 keeps its exact bytes.
+        self::assertStringContainsString(substr($this->original, (int) strpos($this->original, '<!-- wp:divi/section {"module":{"meta":{"adminLabel":{"desktop":{"value":"Leistungen"'), -1), $this->saved[0]);
+    }
+
+    public function testUpdateWithoutRealChangeSavesNothing(): void
+    {
+        $result = $this->update()->execute([
+            'post_id' => 7,
+            'tree_rev' => $this->treeRev(),
+            'changes' => [['id' => 's0.r0.c1.m2', 'set' => ['text' => 'Termin buchen']]],
+        ]);
+
+        self::assertStringStartsWith('Keine Änderung', $result['summary']);
+        self::assertSame($this->treeRev(), $result['tree_rev']);
+        self::assertSame([], $this->saved);
+    }
+
+    public function testUpdateRefusesAStaleTreeRev(): void
+    {
+        $result = $this->update()->execute([
+            'post_id' => 7,
+            'tree_rev' => '000000000000',
+            'changes' => [['id' => 's0.r0.c1.m2', 'label' => 'x']],
+        ]);
+
+        self::assertSame('lit_rev_mismatch', $result->get_error_code());
+        self::assertSame([], $this->saved);
+    }
+
+    public function testReadOnlyNodesCannotBeUpdated(): void
+    {
+        $this->post->post_content = DiviPageLoaderTest::fixture('synthetic-mixed.html');
+
+        $result = $this->update()->execute([
+            'post_id' => 7,
+            'tree_rev' => Rev::of($this->post->post_content),
+            'changes' => [['id' => 's1.g1', 'label' => 'x']],
+        ]);
+
+        self::assertSame('lit_validation_failed', $result->get_error_code());
+        self::assertStringContainsString('nur lesbar', $result->get_error_message());
+        self::assertSame([], $this->saved);
+    }
+
+    /** Ruling S14: the same node twice in one call would silently lose the first change. */
+    public function testUpdateRefusesADuplicateId(): void
+    {
+        $result = $this->update()->execute(['post_id' => 7, 'tree_rev' => $this->treeRev(), 'changes' => [
+            ['id' => 's0.r0.c1.m2', 'set' => ['text' => 'Eins']],
+            ['id' => 's0.r0.c1.m2', 'set' => ['url' => '/zwei']],
+        ]]);
+
+        self::assertSame('lit_validation_failed', $result->get_error_code());
+        self::assertStringContainsString('changes[0]', self::clientError($result)['fields']['changes[1].id']);
+        self::assertSame([], $this->saved);
+    }
+
+    /** Ruling S38: set and attrs of one change must not reach the same attribute path. */
+    public function testUpdateRefusesSetAndAttrsOnTheSamePath(): void
+    {
+        $result = $this->update()->execute(['post_id' => 7, 'tree_rev' => $this->treeRev(), 'changes' => [[
+            'id' => 's0.r0.c1.m2',
+            'set' => ['text' => 'Neu'],
+            'attrs' => ['button.innerContent' => ['desktop' => ['linkUrl' => '/neu']]],
+        ]]]);
+
+        self::assertSame('lit_validation_failed', $result->get_error_code());
+        self::assertArrayHasKey('changes[0].attrs.button.innerContent', self::clientError($result)['fields']);
+        self::assertSame([], $this->saved);
+    }
+
+    /** Ruling S7: builder-only paths are refused for users without unfiltered_html – visible to the client. */
+    public function testBuilderOnlyPathsAreRefusedForEditors(): void
+    {
+        $result = $this->update()->execute(['post_id' => 7, 'tree_rev' => $this->treeRev(), 'changes' => [[
+            'id' => 's0.r0.c1.m1',
+            'attrs' => ['module.decoration.attributes' => ['desktop' => ['attributes' => [['name' => 'onclick', 'value' => 'x']]]]],
+        ]]]);
+
+        self::assertSame('lit_validation_failed', $result->get_error_code());
+        $client = self::clientError($result);
+        self::assertSame('validation_failed', $client['code']);
+        self::assertStringContainsString('nur im Divi-Builder', $client['fields']['changes[0].attrs.module.decoration.attributes']);
+        self::assertSame([], $this->saved);
+    }
+}