diff --git a/app/Support/PostMediaRules.php b/app/Support/PostMediaRules.php
index 001aa1699..b2200e378 100644
--- a/app/Support/PostMediaRules.php
+++ b/app/Support/PostMediaRules.php
@@ -27,6 +27,16 @@ class PostMediaRules
      */
     public const ALT_TEXT_MAX_LENGTH = 2000;
 
+    /**
+     * Workspace media collections a post may reference by id: uploaded/library
+     * assets, and slides or images the AI pipeline generated for this workspace
+     * (PostImagePipeline, RegeneratePostMediaImage). Other collections such as
+     * `logo` or `avatar` are not post media and stay rejected.
+     *
+     * @var array<int, string>
+     */
+    public const POST_MEDIA_COLLECTIONS = ['assets', 'ai-generated'];
+
     /**
      * @param  bool  $hosted  true (web): items must already be hosted (id + path
      *                        required); false (API): a bare external `url` is
@@ -121,7 +131,7 @@ public static function assertHostedMediaExists(Validator $validator, Workspace $
             $exists = Media::query()
                 ->where('mediable_type', Relation::getMorphAlias(Workspace::class))
                 ->where('mediable_id', $workspace->id)
-                ->where('collection', 'assets')
+                ->whereIn('collection', self::POST_MEDIA_COLLECTIONS)
                 ->whereKey($id)
                 ->exists();
 
diff --git a/tests/Feature/PostMediaExistsValidationWebTest.php b/tests/Feature/PostMediaExistsValidationWebTest.php
index d82ab5ab8..d80fe49bb 100644
--- a/tests/Feature/PostMediaExistsValidationWebTest.php
+++ b/tests/Feature/PostMediaExistsValidationWebTest.php
@@ -108,3 +108,65 @@
     expect($post->fresh()->media)->toHaveCount(1)
         ->and(data_get($post->fresh()->media, '0.id'))->toBe($asset->id);
 });
+
+test('update post accepts media generated by the AI image pipeline (ai-generated collection)', function () {
+    $post = Post::factory()->create([
+        'workspace_id' => $this->workspace->id,
+        'user_id' => $this->user->id,
+    ]);
+
+    $slide = Media::factory()->create([
+        'mediable_type' => (new Workspace)->getMorphClass(),
+        'mediable_id' => $this->workspace->id,
+        'collection' => 'ai-generated',
+    ]);
+
+    $response = $this->actingAs($this->user)->put(route('app.posts.update', $post), [
+        'status' => 'draft',
+        'media' => [['id' => $slide->id, 'path' => $slide->path, 'url' => 'https://example.com/'.$slide->path, 'type' => 'image', 'source' => 'ai']],
+    ]);
+
+    $response->assertSessionDoesntHaveErrors();
+    expect(data_get($post->fresh()->media, '0.id'))->toBe($slide->id);
+});
+
+test('update post rejects another workspace\'s ai-generated media (cross-tenant IDOR)', function () {
+    $post = Post::factory()->create([
+        'workspace_id' => $this->workspace->id,
+        'user_id' => $this->user->id,
+    ]);
+
+    $other = Workspace::factory()->create();
+    $foreignSlide = Media::factory()->create([
+        'mediable_type' => (new Workspace)->getMorphClass(),
+        'mediable_id' => $other->id,
+        'collection' => 'ai-generated',
+    ]);
+
+    $response = $this->actingAs($this->user)->put(route('app.posts.update', $post), [
+        'status' => 'draft',
+        'media' => [['id' => $foreignSlide->id, 'path' => $foreignSlide->path, 'url' => 'https://example.com/'.$foreignSlide->path, 'type' => 'image']],
+    ]);
+
+    $response->assertSessionHasErrors(['media.0.id']);
+    expect($post->fresh()->media)->toBe([]);
+});
+
+test('update post still rejects the workspace logo collection as post media', function () {
+    $post = Post::factory()->create([
+        'workspace_id' => $this->workspace->id,
+        'user_id' => $this->user->id,
+    ]);
+
+    $logo = Media::factory()->logo()->create([
+        'mediable_type' => (new Workspace)->getMorphClass(),
+        'mediable_id' => $this->workspace->id,
+    ]);
+
+    $response = $this->actingAs($this->user)->put(route('app.posts.update', $post), [
+        'status' => 'draft',
+        'media' => [['id' => $logo->id, 'path' => $logo->path, 'url' => 'https://example.com/'.$logo->path, 'type' => 'image']],
+    ]);
+
+    $response->assertSessionHasErrors(['media.0.id']);
+});