diff --git a/wrolpi/files/api.py b/wrolpi/files/api.py
index 0031805c..c6742710 100644
--- a/wrolpi/files/api.py
+++ b/wrolpi/files/api.py
@@ -120,17 +120,17 @@ async def delete_file(_: Request, body: schema.DeleteRequest):
 async def delete_file_groups(request: Request, body: schema.DeleteFileGroupsRequest):
     if not body.file_group_ids:
         raise InvalidFile('file_group_ids cannot be empty')
-    session = request.ctx.session
+    # Read-then-write, so the transaction must begin as a writer (see `RequestDB.write`).
+    with request.ctx.db.write() as session:
+        if not body.force:
+            tagged = lib.get_tagged_file_groups_by_ids(session, body.file_group_ids)
+            if tagged:
+                return json_response(
+                    {'code': 'FILE_GROUP_IS_TAGGED', 'file_groups': tagged},
+                    HTTPStatus.CONFLICT,
+                )
 
-    if not body.force:
-        tagged = lib.get_tagged_file_groups_by_ids(session, body.file_group_ids)
-        if tagged:
-            return json_response(
-                {'code': 'FILE_GROUP_IS_TAGGED', 'file_groups': tagged},
-                HTTPStatus.CONFLICT,
-            )
-
-    affected_models = lib.delete_file_groups(session, body.file_group_ids, force=body.force)
+        affected_models = lib.delete_file_groups(session, body.file_group_ids, force=body.force)
 
     if 'video' in affected_models:
         # A Channel's tracked downloads may have changed; rewrite channels.yaml.
@@ -391,16 +391,18 @@ async def post_tag_file_group(request: Request, body: schema.TagFileGroupPost):
             HTTPStatus.BAD_REQUEST,
         )
 
-    await lib.add_file_group_tag(request.ctx.session, body.file_group_id, body.file_group_primary_path, body.tag_name,
-                                 body.tag_id)
+    with request.ctx.db.write() as session:
+        await lib.add_file_group_tag(session, body.file_group_id, body.file_group_primary_path, body.tag_name,
+                                     body.tag_id)
     return response.empty(HTTPStatus.CREATED)
 
 
 @files_bp.post('/untag')
 @validate(schema.TagFileGroupPost)
 async def post_untag_file_group(request: Request, body: schema.TagFileGroupPost):
-    await lib.remove_file_group_tag(request.ctx.session, body.file_group_id, body.file_group_primary_path,
-                                    body.tag_name, body.tag_id)
+    with request.ctx.db.write() as session:
+        await lib.remove_file_group_tag(session, body.file_group_id, body.file_group_primary_path,
+                                        body.tag_name, body.tag_id)
     return response.empty(HTTPStatus.NO_CONTENT)
 
 
diff --git a/wrolpi/files/test/test_api.py b/wrolpi/files/test/test_api.py
index 4ead049e..6bd16bf2 100644
--- a/wrolpi/files/test/test_api.py
+++ b/wrolpi/files/test/test_api.py
@@ -9,6 +9,7 @@ import pytest
 
 from modules.videos import Video
 from wrolpi.common import get_wrolpi_config
+from wrolpi.conftest import production_like_sessions, write_lock_held_briefly
 from wrolpi.files.lib import get_mimetype
 from wrolpi.files.models import FileGroup, Directory
 from wrolpi.tags import TagFile
@@ -1636,3 +1637,35 @@ async def test_get_file_ignored_directory_not_tracked(test_session, async_client
     # Verify the ignored file is not in the database (because it was never refreshed)
     ignored_fgs = test_session.query(FileGroup).filter(FileGroup.primary_path.like('%config%')).all()
     assert len(ignored_fgs) == 0, "Files in ignored directories should not be in the database"
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize('step', ['tag', 'untag', 'delete_groups'])
+async def test_file_group_writes_wait_for_a_concurrent_writer(test_session, async_client, test_directory,
+                                                              example_singlefile, tag_factory, step):
+    """Tagging, untagging and deleting FileGroups read before they write, so each must begin as a writer:
+    SQLite fails a deferred transaction's lock upgrade instantly ("database is locked") if another connection
+    wrote meanwhile."""
+    file_group = FileGroup.from_paths(test_session, example_singlefile)
+    tag = await tag_factory()
+    if step == 'untag':
+        file_group.add_tag(test_session, tag.id)
+    test_session.commit()
+    db_file = test_session.get_bind().url.database
+
+    with production_like_sessions(test_session) as maker:
+        # Held past the slow work these requests do before they write (e.g. a config save).
+        with write_lock_held_briefly(db_file, seconds=3):
+            if step == 'delete_groups':
+                body = dict(file_group_ids=[file_group.id])
+                _, response = await async_client.post('/api/files/delete_groups', content=json.dumps(body))
+            else:
+                body = dict(file_group_id=file_group.id, tag_id=tag.id)
+                _, response = await async_client.post(f'/api/files/{step}', content=json.dumps(body))
+        assert response.status_code in (HTTPStatus.CREATED, HTTPStatus.NO_CONTENT), response.body
+
+        with maker() as session:
+            if step == 'delete_groups':
+                assert session.query(FileGroup).filter_by(id=file_group.id).one_or_none() is None
+            else:
+                assert session.query(TagFile).count() == (1 if step == 'tag' else 0)