diff --git a/betty/extension/demo/__init__.py b/betty/extension/demo/__init__.py
index cdaa61fd4..b66e0e8f5 100644
--- a/betty/extension/demo/__init__.py
+++ b/betty/extension/demo/__init__.py
@@ -14,7 +14,7 @@
 from betty.model.ancestry import Place, PlaceName, Person, Presence, Subject, PersonName, Link, Source, Citation, Event, \
     Enclosure
 from betty.model.event_type import Marriage, Birth, Death
-from betty.project import LocaleConfiguration, ExtensionConfiguration, EntityReference, ProjectConfiguration
+from betty.project import LocaleConfiguration, ExtensionConfiguration, EntityReference, Project
 from betty.serve import Server, AppServer, NoPublicUrlBecauseServerNotStartedError
 
 
@@ -29,12 +29,12 @@ def _load(self, *entities: Entity) -> None:
         self._app.project.ancestry.add(*entities)
 
     @classmethod
-    def project_configuration(cls) -> ProjectConfiguration:
+    def project(cls) -> Project:
         from betty.extension import CottonCandy, Demo
 
-        configuration = ProjectConfiguration()
-        configuration.extensions.append(ExtensionConfiguration(Demo))
-        configuration.extensions.append(ExtensionConfiguration(CottonCandy, True, CottonCandyConfiguration(
+        project = Project(project_id=cls.name())
+        project.configuration.extensions.append(ExtensionConfiguration(Demo))
+        project.configuration.extensions.append(ExtensionConfiguration(CottonCandy, True, CottonCandyConfiguration(
             featured_entities=[
                 EntityReference(Place, 'betty-demo-amsterdam'),
                 EntityReference(Person, 'betty-demo-liberta-lankester'),
@@ -42,13 +42,13 @@ def project_configuration(cls) -> ProjectConfiguration:
             ],
         )))
         # Include all of the translations Betty ships with.
-        configuration.locales.replace(
+        project.configuration.locales.replace(
             LocaleConfiguration('en-US', 'en'),
             LocaleConfiguration('nl-NL', 'nl'),
             LocaleConfiguration('fr-FR', 'fr'),
             LocaleConfiguration('uk', 'uk'),
         )
-        return configuration
+        return project
 
     async def load(self) -> None:
         netherlands = Place(
@@ -381,9 +381,8 @@ class DemoServer(Server):
     def __init__(self):
         from betty.extension import Demo
 
-        self._app = App()
+        self._app = App(None, Demo.project())
         super().__init__(self._app.localizer)
-        self._app.project.configuration.update(Demo.project_configuration())
         self._server: Server | None = None
         self._exit_stack = AsyncExitStack()
 
diff --git a/betty/extension/npm/__init__.py b/betty/extension/npm/__init__.py
index f69b435fc..2cf62efa5 100644
--- a/betty/extension/npm/__init__.py
+++ b/betty/extension/npm/__init__.py
@@ -1,6 +1,5 @@
 from __future__ import annotations
 
-import hashlib
 import logging
 import os
 import shutil
@@ -198,7 +197,7 @@ async def install(self, extension_type: type[NpmBuilder & Extension], working_di
     def _get_cached_assets_build_directory_path(self, extension_type: type[NpmBuilder & Extension]) -> Path:
         path = self.cache_directory_path / extension_type.name()
         if extension_type.npm_cache_scope() == CacheScope.PROJECT:
-            path /= hashlib.md5(str(self.app.project.configuration.configuration_file_path).encode('utf-8')).hexdigest()
+            path /= self.app.project.id
         return path
 
     async def ensure_assets(self, extension: NpmBuilder & Extension) -> Path:
diff --git a/betty/project.py b/betty/project.py
index aaa1c8ab5..7f7bf3afe 100644
--- a/betty/project.py
+++ b/betty/project.py
@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import hashlib
 from contextlib import suppress
 from pathlib import Path
 from reprlib import recursive_repr
@@ -769,19 +770,28 @@ def dump(self) -> VoidableDictDump[Dump]:
 
 
 class Project(Configurable[ProjectConfiguration]):
-    def __init__(self):
+    def __init__(
+        self,
+        *,
+        project_id: str | None = None,
+    ):
         super().__init__()
+        self._id = project_id
         self._configuration = ProjectConfiguration()
         self._ancestry = Ancestry()
 
-    def __getstate__(self) -> tuple[VoidableDump, Path, Ancestry]:
-        return self._configuration.dump(), self._configuration.configuration_file_path, self._ancestry
+    def __getstate__(self) -> tuple[str | None, VoidableDump, Path, Ancestry]:
+        return self._id, self._configuration.dump(), self._configuration.configuration_file_path, self._ancestry
 
-    def __setstate__(self, state: tuple[Dump, Path, Ancestry]) -> None:
-        dump, configuration_file_path, self._ancestry = state
+    def __setstate__(self, state: tuple[str | None, Dump, Path, Ancestry]) -> None:
+        self._id, dump, configuration_file_path, self._ancestry = state
         self._configuration = ProjectConfiguration.load(dump)
         self._configuration.configuration_file_path = configuration_file_path
 
+    @property
+    def id(self) -> str:
+        return self._id or hashlib.md5(str(self.configuration.configuration_file_path).encode('utf-8')).hexdigest()
+
     @property
     def ancestry(self) -> Ancestry:
         return self._ancestry