diff --git a/DEVELOPING.md b/DEVELOPING.md
index 16c3ac6..bda2f7a 100644
--- a/DEVELOPING.md
+++ b/DEVELOPING.md
@@ -251,7 +251,9 @@ Editor saves without inputs use `PATCH /api/projects/:id/editor`. Its root
 field updates leave stored `inputConfig` untouched, including a concurrent
 Setup update. Do not copy an earlier Setup snapshot into a replacement write.
 Full GET/PATCH remains the Setup and legacy detail contract; full PATCH keeps
-its replacement semantics. All editor candidates remain available for counts,
+its replacement semantics, except that an omitted `storyboard` leaves stored
+scenes unchanged. Other omitted root fields, including `inputConfig`, are removed.
+All editor candidates remain available for counts,
 selection, generation and composition.
 
 The dedicated editor PATCH path is also a rollback safety boundary: a
diff --git a/orch.py b/orch.py
index 58ee33d..0522100 100644
--- a/orch.py
+++ b/orch.py
@@ -1175,22 +1175,23 @@ def _write_project_doc(
   precondition, scene writes retain their existing set behavior, and the
   stale-scene prune scan is skipped: a brand-new project has no prior scenes
   to prune, and callers cap create at _MAX_CREATE_SCENES so this always fits
-  one atomic batch.
+  one atomic batch. When create is false, an omitted or non-list storyboard
+  (e.g. null) leaves the scenes subcollection untouched; only an explicit list
+  (including []) writes and prunes scenes.
   """
-  scenes = payload.get('storyboard')
-  if not isinstance(scenes, list):
-    scenes = []
   root = dict(payload)
   root['storyboard'] = []
-  scenes_ref = doc_ref.collection(_SCENES_SUBCOLLECTION)
   ops = [('create' if create else 'set', doc_ref, root)]
-  for index, scene in enumerate(scenes):
-    ops.append(('set', scenes_ref.document(_scene_doc_id(index)), scene))
-  if not create:
-    keep_ids = {_scene_doc_id(i) for i in range(len(scenes))}
-    for snapshot in scenes_ref.stream():
-      if snapshot.id not in keep_ids:
-        ops.append(('delete', scenes_ref.document(snapshot.id), None))
+  scenes = payload.get('storyboard')
+  if isinstance(scenes, list):
+    scenes_ref = doc_ref.collection(_SCENES_SUBCOLLECTION)
+    for index, scene in enumerate(scenes):
+      ops.append(('set', scenes_ref.document(_scene_doc_id(index)), scene))
+    if not create:
+      keep_ids = {_scene_doc_id(i) for i in range(len(scenes))}
+      for snapshot in scenes_ref.stream():
+        if snapshot.id not in keep_ids:
+          ops.append(('delete', scenes_ref.document(snapshot.id), None))
   _commit_in_batches(ui_db, ops)
 
 
@@ -1205,29 +1206,31 @@ def _write_editor_project_doc(
   The root update is deliberately field-based: unlike a read-modify-write
   replacement, it cannot copy a stale inputConfig snapshot over a concurrent
   Setup save. Fields omitted by the editor payload retain the legacy full-save
-  replacement behavior through DELETE_FIELD updates.
+  replacement behavior through DELETE_FIELD updates, except storyboard: the
+  root always keeps the [] placeholder, and an omitted or non-list storyboard
+  (e.g. null) leaves the scenes subcollection untouched.
   """
   root_updates = {
       field_path.FieldPath(key).to_api_repr(): firestore.DELETE_FIELD
       for key in stored
-      if key not in payload and key != 'inputConfig'
+      if key not in payload and key not in ('inputConfig', 'storyboard')
   }
   root_updates.update({
       field_path.FieldPath(key).to_api_repr(): value
       for key, value in payload.items()
   })
-  scenes = payload.get('storyboard')
-  if not isinstance(scenes, list):
-    scenes = []
   root_updates[field_path.FieldPath('storyboard').to_api_repr()] = []
-  scenes_ref = doc_ref.collection(_SCENES_SUBCOLLECTION)
-  ops = [('update', doc_ref, root_updates)]
-  for index, scene in enumerate(scenes):
-    ops.append(('set', scenes_ref.document(_scene_doc_id(index)), scene))
-  keep_ids = {_scene_doc_id(i) for i in range(len(scenes))}
-  for snapshot in scenes_ref.stream():
-    if snapshot.id not in keep_ids:
-      ops.append(('delete', scenes_ref.document(snapshot.id), None))
+  ops = []
+  scenes = payload.get('storyboard')
+  if isinstance(scenes, list):
+    scenes_ref = doc_ref.collection(_SCENES_SUBCOLLECTION)
+    for index, scene in enumerate(scenes):
+      ops.append(('set', scenes_ref.document(_scene_doc_id(index)), scene))
+    keep_ids = {_scene_doc_id(i) for i in range(len(scenes))}
+    for snapshot in scenes_ref.stream():
+      if snapshot.id not in keep_ids:
+        ops.append(('delete', scenes_ref.document(snapshot.id), None))
+  ops.insert(0, ('update', doc_ref, root_updates))
   _commit_in_batches(ui_db, ops)
 
 
@@ -1394,7 +1397,9 @@ def project_detail_handler(
 
   The default PATCH is the faithful port of the UI's whole-document autosave:
   a full set() with createdBy stripped from the payload (immutable; the stored
-  owner is preserved) and lastEdited refreshed server-side. PATCH
+  owner is preserved) and lastEdited refreshed server-side. An omitted
+  storyboard preserves scenes; other omitted mutable root fields are removed.
+  PATCH
   ?view=editor is the legacy form of the explicit editor exception. The
   dedicated PATCH /api/projects/<id>/editor route is the preferred form; both
   use field updates so the omitted Setup inputConfig cannot be overwritten by
diff --git a/test/test_frontdoor_data.py b/test/test_frontdoor_data.py
index ce74d55..853604e 100644
--- a/test/test_frontdoor_data.py
+++ b/test/test_frontdoor_data.py
@@ -31,6 +31,8 @@
 import threading
 import uuid
 
+import pytest
+
 from google.api_core import exceptions as google_exceptions
 from google.cloud import firestore
 
@@ -1662,6 +1664,235 @@ def test_project_patch_prunes_removed_scenes(monkeypatch, orchestrator_module):
   body = client.get('/api/projects/shrink').get_json()
   assert [s['d'] for s in body['storyboard']] == ['0']
 
+@pytest.mark.parametrize(
+    'path_template',
+    [
+        '/api/projects/{id}',
+        '/api/projects/{id}/editor',
+        '/api/projects/{id}?view=editor',
+    ],
+)
+def test_project_patch_omitting_storyboard_leaves_scenes_intact(
+    monkeypatch, orchestrator_module, path_template
+):
+  del orchestrator_module
+  orch, fake_db, _ = _load_app(monkeypatch)
+  client = orch.app.test_client()
+  project_id = f'keep-scenes-{uuid.uuid4().hex[:8]}'
+
+  assert (
+      client.post(
+          '/api/projects',
+          json={
+              'id': project_id,
+              'name': 'Original',
+              'inputConfig': {'productDescription': 'Keep for editor saves'},
+              'storyboard': [{'d': '0'}, {'d': '1'}, {'d': '2'}],
+          },
+      ).status_code
+      == 200
+  )
+  assert len(_scenes_docs(fake_db, project_id)) == 3
+
+  # PATCH {"name": "Renamed"} must leave all 3 scenes intact (SM-6 fix).
+  url = path_template.format(id=project_id)
+  response = client.patch(url, json={'name': 'Renamed'})
+  assert response.status_code == 200
+
+  # The GET below rebuilds storyboard from the subcollection, so check the
+  # stored root placeholder directly.
+  assert fake_db.collection('projects').docs[project_id]['storyboard'] == []
+  scenes = _scenes_docs(fake_db, project_id)
+  assert len(scenes) == 3
+  assert [scenes[f'{i:06d}']['d'] for i in range(3)] == ['0', '1', '2']
+
+  body = client.get(f'/api/projects/{project_id}').get_json()
+  assert body['name'] == 'Renamed'
+  assert [s['d'] for s in body['storyboard']] == ['0', '1', '2']
+
+  if path_template == '/api/projects/{id}':
+    # Full PATCH replaces root fields; only an omitted storyboard is preserved.
+    assert 'inputConfig' not in body
+  else:
+    assert body['inputConfig'] == {
+        'productDescription': 'Keep for editor saves'
+    }
+
+
+@pytest.mark.parametrize(
+    'path_template',
+    [
+        '/api/projects/{id}',
+        '/api/projects/{id}/editor',
+        '/api/projects/{id}?view=editor',
+    ],
+)
+def test_project_patch_empty_payload_leaves_scenes_intact(
+    monkeypatch, orchestrator_module, path_template
+):
+  del orchestrator_module
+  orch, fake_db, _ = _load_app(monkeypatch)
+  client = orch.app.test_client()
+  project_id = f'empty-patch-{uuid.uuid4().hex[:8]}'
+
+  assert (
+      client.post(
+          '/api/projects',
+          json={
+              'id': project_id,
+              'name': 'Original',
+              'storyboard': [{'d': '0'}, {'d': '1'}, {'d': '2'}],
+          },
+      ).status_code
+      == 200
+  )
+  assert len(_scenes_docs(fake_db, project_id)) == 3
+
+  url = path_template.format(id=project_id)
+  response = client.patch(url, json={})
+  assert response.status_code == 200
+
+  scenes = _scenes_docs(fake_db, project_id)
+  assert len(scenes) == 3
+  body = client.get(f'/api/projects/{project_id}').get_json()
+  assert [s['d'] for s in body['storyboard']] == ['0', '1', '2']
+
+
+@pytest.mark.parametrize('storyboard', [None, 'not-a-list', {'d': '0'}])
+@pytest.mark.parametrize(
+    'path_template',
+    [
+        '/api/projects/{id}',
+        '/api/projects/{id}/editor',
+        '/api/projects/{id}?view=editor',
+    ],
+)
+def test_project_patch_non_list_storyboard_leaves_scenes_intact(
+    monkeypatch, orchestrator_module, path_template, storyboard
+):
+  del orchestrator_module
+  orch, fake_db, _ = _load_app(monkeypatch)
+  client = orch.app.test_client()
+  project_id = f'null-storyboard-{uuid.uuid4().hex[:8]}'
+
+  assert (
+      client.post(
+          '/api/projects',
+          json={
+              'id': project_id,
+              'name': 'Original',
+              'storyboard': [{'d': '0'}, {'d': '1'}, {'d': '2'}],
+          },
+      ).status_code
+      == 200
+  )
+
+  # Clients that serialize unset optional fields as null must not wipe scenes.
+  url = path_template.format(id=project_id)
+  response = client.patch(url, json={'name': 'n', 'storyboard': storyboard})
+  assert response.status_code == 200
+
+  assert fake_db.collection('projects').docs[project_id]['storyboard'] == []
+  scenes = _scenes_docs(fake_db, project_id)
+  assert [scenes[f'{i:06d}']['d'] for i in range(3)] == ['0', '1', '2']
+
+
+@pytest.mark.parametrize(
+    'path_template',
+    [
+        '/api/projects/{id}',
+        '/api/projects/{id}/editor',
+        '/api/projects/{id}?view=editor',
+    ],
+)
+def test_project_patch_explicit_empty_storyboard_deletes_all_scenes(
+    monkeypatch, orchestrator_module, path_template
+):
+  del orchestrator_module
+  orch, fake_db, _ = _load_app(monkeypatch)
+  client = orch.app.test_client()
+  project_id = f'clear-scenes-{uuid.uuid4().hex[:8]}'
+
+  assert (
+      client.post(
+          '/api/projects',
+          json={
+              'id': project_id,
+              'name': 'Original',
+              'storyboard': [{'d': '0'}, {'d': '1'}, {'d': '2'}],
+          },
+      ).status_code
+      == 200
+  )
+  assert len(_scenes_docs(fake_db, project_id)) == 3
+
+  url = path_template.format(id=project_id)
+  response = client.patch(url, json={'storyboard': []})
+  assert response.status_code == 200
+
+  assert len(_scenes_docs(fake_db, project_id)) == 0
+  body = client.get(f'/api/projects/{project_id}').get_json()
+  assert body['storyboard'] == []
+
+
+@pytest.mark.parametrize(
+    'path_template',
+    [
+        '/api/projects/{id}',
+        '/api/projects/{id}/editor',
+        '/api/projects/{id}?view=editor',
+    ],
+)
+def test_project_patch_shrinking_storyboard_deletes_trailing_scenes(
+    monkeypatch, orchestrator_module, path_template
+):
+  del orchestrator_module
+  orch, fake_db, _ = _load_app(monkeypatch)
+  client = orch.app.test_client()
+  project_id = f'shrink-5-to-3-{uuid.uuid4().hex[:8]}'
+
+  assert (
+      client.post(
+          '/api/projects',
+          json={
+              'id': project_id,
+              'name': 'Original',
+              'storyboard': [{'d': str(i)} for i in range(5)],
+          },
+      ).status_code
+      == 200
+  )
+  assert len(_scenes_docs(fake_db, project_id)) == 5
+
+  url = path_template.format(id=project_id)
+  response = client.patch(
+      url,
+      json={
+          'storyboard': [{'d': f'{i}-updated'} for i in range(3)],
+      },
+  )
+  assert response.status_code == 200
+
+  # Scenes live only in the subcollection, never inline on the root doc.
+  assert fake_db.collection('projects').docs[project_id]['storyboard'] == []
+  scenes = _scenes_docs(fake_db, project_id)
+  assert len(scenes) == 3
+  assert set(scenes.keys()) == {'000000', '000001', '000002'}
+  assert [scenes[f'{i:06d}']['d'] for i in range(3)] == [
+      '0-updated',
+      '1-updated',
+      '2-updated',
+  ]
+  assert '000003' not in scenes
+  assert '000004' not in scenes
+
+  body = client.get(f'/api/projects/{project_id}').get_json()
+  assert [s['d'] for s in body['storyboard']] == [
+      '0-updated',
+      '1-updated',
+      '2-updated',
+  ]
+
 
 def test_project_delete_clears_scenes_subcollection(
     monkeypatch, orchestrator_module