[BUG] 4.2.0rc3 failure to lanch with flask run: ValueError: The name '_dash_assets' is already registered for a different blueprint.

[ghaarsma]

We have a proper running Dash 4.1 application. Upon testing 4.2.0rc3 and starting the application with flask run we were greeted with: ValueError: The name '_dash_assets' is already registered for a different blueprint. Use 'name=' to provide a unique name.

Full stack:

(venv) PS C:\Users\user\Python\dash-site> flask run
2026-05-14 13:39:02 - INFO - Settings class: dash.config.LocalDevelopmentConfig
2026-05-14 13:39:03 - INFO - Database URI: sqlite:///C:\Users\user\Python\dash-site\instance\dwsc.sqlite3
2026-05-14 13:39:06 - CRITICAL - Uncaught exception
Traceback (most recent call last):
 File "<frozen runpy>", line 198, in _run_module_as_main
 File "<frozen runpy>", line 88, in _run_code
 File "C:\Users\user\Python\dash-site\venv\Scripts\flask.exe\__main__.py", line 5, in <module>
 sys.exit(main())
 ~~~~^^
 File "C:\Users\user\Python\dash-site\venv\Lib\site-packages\flask\cli.py", line 1131, in main
 cli.main()
 ~~~~~~~~^^
 File "C:\Users\user\Python\dash-site\venv\Lib\site-packages\click\core.py", line 1435, in main
 rv = self.invoke(ctx)
 File "C:\Users\user\Python\dash-site\venv\Lib\site-packages\click\core.py", line 1902, in invoke
 return _process_result(sub_ctx.command.invoke(sub_ctx))
 ~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^
 File "C:\Users\user\Python\dash-site\venv\Lib\site-packages\click\core.py", line 1298, in invoke
 return ctx.invoke(self.callback, **ctx.params)
 ~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 File "C:\Users\user\Python\dash-site\venv\Lib\site-packages\click\core.py", line 853, in invoke
 return callback(*args, **kwargs)
 File "C:\Users\user\Python\dash-site\venv\Lib\site-packages\click\decorators.py", line 93, in new_func
 return ctx.invoke(f, obj, *args, **kwargs)
 ~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^
 File "C:\Users\user\Python\dash-site\venv\Lib\site-packages\click\core.py", line 853, in invoke
 return callback(*args, **kwargs)
 File "C:\Users\user\Python\dash-site\venv\Lib\site-packages\flask\cli.py", line 979, in run_command
 raise e from None
 File "C:\Users\user\Python\dash-site\venv\Lib\site-packages\flask\cli.py", line 963, in run_command
 app: WSGIApplication = info.load_app() # pyright: ignore
 ~~~~~~~~~~~~~^^
 File "C:\Users\user\Python\dash-site\venv\Lib\site-packages\flask\cli.py", line 349, in load_app
 app = locate_app(import_name, name)
 File "C:\Users\user\Python\dash-site\venv\Lib\site-packages\flask\cli.py", line 262, in locate_app
 return find_best_app(module)
 File "C:\Users\user\Python\dash-site\venv\Lib\site-packages\flask\cli.py", line 72, in find_best_app
 app = app_factory()
 File "C:\Users\user\Python\dash-site\dash\__init__.py", line 180, in create_app
 dash_app.init_app(app)
 ~~~~~~~~~~~~~~~~~^^^^^
 File "C:\Users\user\Python\dash-site\dash\dash_app.py", line 505, in init_app
 app.init_app(server)
 ~~~~~~~~~~~~^^^^^^^^
 File "C:\Users\user\Python\dash-site\venv\Lib\site-packages\dash\dash.py", line 734, in init_app
 self.backend.register_assets_blueprint(
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^
 assets_blueprint_name,
 ^^^^^^^^^^^^^^^^^^^^^^
 config.routes_pathname_prefix + self.config.assets_url_path.lstrip("/"),
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 self.config.assets_folder,
 ^^^^^^^^^^^^^^^^^^^^^^^^^^
 )
 ^
 File "C:\Users\user\Python\dash-site\venv\Lib\site-packages\dash\backends\_flask.py", line 95, in register_assets_blueprint
 self.server.register_blueprint(bp)
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^
 File "C:\Users\user\Python\dash-site\venv\Lib\site-packages\flask\sansio\scaffold.py", line 47, in wrapper_func
 return f(self, *args, **kwargs)
 File "C:\Users\user\Python\dash-site\venv\Lib\site-packages\flask\sansio\app.py", line 595, in register_blueprint
 blueprint.register(self, options)
 ~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^
 File "C:\Users\user\Python\dash-site\venv\Lib\site-packages\flask\sansio\blueprints.py", line 310, in register
 raise ValueError(
 ...<3 lines>...
 )
ValueError: The name '_dash_assets' is already registered for a different blueprint. Use 'name=' to provide a unique name.
(venv) PS C:\Users\user\Python\dash-site>

I realize this is not a MRE. If this is needed to troubleshoot, I can invest the time to try to create one.

[AnnMarieW]

@ghaarsma
Thanks for reporting - a MRE would be very helpful.

This error is likely caused by initializing Dash twice (or more) against the same Flask instance.

[ghaarsma]

I have attached an MRE, in zip format. When running with Dash 4.1.0 all works fine

(venv) PS C:\Users\gabe\Python\dash_test> flask run
 * Serving Flask app 'dash_test'
 * Debug mode: off

Then swap to Dash 4.2.0rc3 in the pyproject.toml and run pip install .

(venv) PS C:\Users\gabe\Python\dash_test> pip install .
Processing .\.
 Installing build dependencies ... done
 Getting requirements to build wheel ... done
 Preparing metadata (pyproject.toml) ... done
Collecting dash==4.2.0rc3 (from dash_test==2026.0.0)
 Using cached dash-4.2.0rc3-py3-none-any.whl.metadata (11 kB)
Requirement already satisfied: python-dotenv in .\venv\Lib\site-packages (from dash_test==2026.0.0) (1.2.2)
Requirement already satisfied: Flask<3.2,>=1.0.4 in .\venv\Lib\site-packages (from dash==4.2.0rc3->dash_test==2026.0.0) (3.1.3)
Requirement already satisfied: Werkzeug<3.2 in .\venv\Lib\site-packages (from dash==4.2.0rc3->dash_test==2026.0.0) (3.1.8)
Requirement already satisfied: plotly>=5.0.0 in .\venv\Lib\site-packages (from dash==4.2.0rc3->dash_test==2026.0.0) (6.7.0)
Requirement already satisfied: importlib-metadata in .\venv\Lib\site-packages (from dash==4.2.0rc3->dash_test==2026.0.0) (9.0.0)
Requirement already satisfied: typing_extensions>=4.1.1 in .\venv\Lib\site-packages (from dash==4.2.0rc3->dash_test==2026.0.0) (4.15.0)
Requirement already satisfied: requests in .\venv\Lib\site-packages (from dash==4.2.0rc3->dash_test==2026.0.0) (2.34.2)
Requirement already satisfied: retrying in .\venv\Lib\site-packages (from dash==4.2.0rc3->dash_test==2026.0.0) (1.4.2)
Requirement already satisfied: nest-asyncio in .\venv\Lib\site-packages (from dash==4.2.0rc3->dash_test==2026.0.0) (1.6.0)
Requirement already satisfied: setuptools in .\venv\Lib\site-packages (from dash==4.2.0rc3->dash_test==2026.0.0) (82.0.1)
Collecting janus>=1.0.0 (from dash==4.2.0rc3->dash_test==2026.0.0)
 Using cached janus-2.0.0-py3-none-any.whl.metadata (5.3 kB)
Requirement already satisfied: blinker>=1.9.0 in .\venv\Lib\site-packages (from Flask<3.2,>=1.0.4->dash==4.2.0rc3->dash_test==2026.0.0) (1.9.0)
Requirement already satisfied: click>=8.1.3 in .\venv\Lib\site-packages (from Flask<3.2,>=1.0.4->dash==4.2.0rc3->dash_test==2026.0.0) (8.4.0)
Requirement already satisfied: itsdangerous>=2.2.0 in .\venv\Lib\site-packages (from Flask<3.2,>=1.0.4->dash==4.2.0rc3->dash_test==2026.0.0) (2.2.0)
Requirement already satisfied: jinja2>=3.1.2 in .\venv\Lib\site-packages (from Flask<3.2,>=1.0.4->dash==4.2.0rc3->dash_test==2026.0.0) (3.1.6)
Requirement already satisfied: markupsafe>=2.1.1 in .\venv\Lib\site-packages (from Flask<3.2,>=1.0.4->dash==4.2.0rc3->dash_test==2026.0.0) (3.0.3)
Requirement already satisfied: colorama in .\venv\Lib\site-packages (from click>=8.1.3->Flask<3.2,>=1.0.4->dash==4.2.0rc3->dash_test==2026.0.0) (0.4.6)
Requirement already satisfied: narwhals>=1.15.1 in .\venv\Lib\site-packages (from plotly>=5.0.0->dash==4.2.0rc3->dash_test==2026.0.0) (2.21.2)
Requirement already satisfied: packaging in .\venv\Lib\site-packages (from plotly>=5.0.0->dash==4.2.0rc3->dash_test==2026.0.0) (26.2)
Requirement already satisfied: zipp>=3.20 in .\venv\Lib\site-packages (from importlib-metadata->dash==4.2.0rc3->dash_test==2026.0.0) (3.23.1)
Requirement already satisfied: charset_normalizer<4,>=2 in .\venv\Lib\site-packages (from requests->dash==4.2.0rc3->dash_test==2026.0.0) (3.4.7)
Requirement already satisfied: idna<4,>=2.5 in .\venv\Lib\site-packages (from requests->dash==4.2.0rc3->dash_test==2026.0.0) (3.15)
Requirement already satisfied: urllib3<3,>=1.26 in .\venv\Lib\site-packages (from requests->dash==4.2.0rc3->dash_test==2026.0.0) (2.7.0)
Requirement already satisfied: certifi>=2023.5.7 in .\venv\Lib\site-packages (from requests->dash==4.2.0rc3->dash_test==2026.0.0) (2026.4.22)
Using cached dash-4.2.0rc3-py3-none-any.whl (7.4 MB)
Using cached janus-2.0.0-py3-none-any.whl (12 kB)
Building wheels for collected packages: dash_test
 Building wheel for dash_test (pyproject.toml) ... done
 Created wheel for dash_test: filename=dash_test-2026.0.0-py3-none-any.whl size=2048 sha256=640224ad3144b050306c474df9970ebb319b89f224d91c0851431c7d3f91b362
 Stored in directory: C:\Users\gabe\AppData\Local\Temp\pip-ephem-wheel-cache-3neq6qu9\wheels\64\d4\de\220d469b0ca12820b70175838f0c3b6e35e506993a93095880
Successfully built dash_test
Installing collected packages: janus, dash, dash_test
 Attempting uninstall: dash
 Found existing installation: dash 4.1.0
 Uninstalling dash-4.1.0:
 Successfully uninstalled dash-4.1.0
 Attempting uninstall: dash_test
 Found existing installation: dash_test 2026.0.0
 Uninstalling dash_test-2026.0.0:
 Successfully uninstalled dash_test-2026.0.0
Successfully installed dash-4.2.0rc3 dash_test-2026.0.0 janus-2.0.0

Then run flask run again

(venv) PS C:\Users\gabe\Python\dash_test> flask run
Traceback (most recent call last):
 File "<frozen runpy>", line 203, in _run_module_as_main
 File "<frozen runpy>", line 88, in _run_code
 File "C:\Users\gabe\Python\dash_test\venv\Scripts\flask.exe\__main__.py", line 5, in <module>
 sys.exit(main())
 ~~~~^^
 File "C:\Users\gabe\Python\dash_test\venv\Lib\site-packages\flask\cli.py", line 1131, in main
 cli.main()
 ~~~~~~~~^^
 File "C:\Users\gabe\Python\dash_test\venv\Lib\site-packages\click\core.py", line 1445, in main
 rv = self.invoke(ctx)
 File "C:\Users\gabe\Python\dash_test\venv\Lib\site-packages\click\core.py", line 1912, in invoke
 return _process_result(sub_ctx.command.invoke(sub_ctx))
 ~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^
 File "C:\Users\gabe\Python\dash_test\venv\Lib\site-packages\click\core.py", line 1308, in invoke
 return ctx.invoke(self.callback, **ctx.params)
 ~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 File "C:\Users\gabe\Python\dash_test\venv\Lib\site-packages\click\core.py", line 877, in invoke
 return callback(*args, **kwargs)
 File "C:\Users\gabe\Python\dash_test\venv\Lib\site-packages\click\decorators.py", line 93, in new_func
 return ctx.invoke(f, obj, *args, **kwargs)
 ~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^
 File "C:\Users\gabe\Python\dash_test\venv\Lib\site-packages\click\core.py", line 877, in invoke
 return callback(*args, **kwargs)
 File "C:\Users\gabe\Python\dash_test\venv\Lib\site-packages\flask\cli.py", line 979, in run_command
 raise e from None
 File "C:\Users\gabe\Python\dash_test\venv\Lib\site-packages\flask\cli.py", line 963, in run_command
 app: WSGIApplication = info.load_app() # pyright: ignore
 ~~~~~~~~~~~~~^^
 File "C:\Users\gabe\Python\dash_test\venv\Lib\site-packages\flask\cli.py", line 349, in load_app
 app = locate_app(import_name, name)
 File "C:\Users\gabe\Python\dash_test\venv\Lib\site-packages\flask\cli.py", line 262, in locate_app
 return find_best_app(module)
 File "C:\Users\gabe\Python\dash_test\venv\Lib\site-packages\flask\cli.py", line 72, in find_best_app
 app = app_factory()
 File "C:\Users\gabe\Python\dash_test\dash_test\__init__.py", line 8, in create_app
 dash_app.init_app(app)
 ~~~~~~~~~~~~~~~~~^^^^^
 File "C:\Users\gabe\Python\dash_test\dash_test\dash_app.py", line 32, in init_app
 app.init_app(server)
 ~~~~~~~~~~~~^^^^^^^^
 File "C:\Users\gabe\Python\dash_test\venv\Lib\site-packages\dash\dash.py", line 734, in init_app
 self.backend.register_assets_blueprint(
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^
 assets_blueprint_name,
 ^^^^^^^^^^^^^^^^^^^^^^
 config.routes_pathname_prefix + self.config.assets_url_path.lstrip("/"),
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 self.config.assets_folder,
 ^^^^^^^^^^^^^^^^^^^^^^^^^^
 )
 ^
 File "C:\Users\gabe\Python\dash_test\venv\Lib\site-packages\dash\backends\_flask.py", line 95, in register_assets_blueprint
 self.server.register_blueprint(bp)
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^
 File "C:\Users\gabe\Python\dash_test\venv\Lib\site-packages\flask\sansio\scaffold.py", line 47, in wrapper_func
 return f(self, *args, **kwargs)
 File "C:\Users\gabe\Python\dash_test\venv\Lib\site-packages\flask\sansio\app.py", line 595, in register_blueprint
 blueprint.register(self, options)
 ~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^
 File "C:\Users\gabe\Python\dash_test\venv\Lib\site-packages\flask\sansio\blueprints.py", line 310, in register
 raise ValueError(
 ...<3 lines>...
 )
ValueError: The name '_dash_assets' is already registered for a different blueprint. Use 'name=' to provide a unique name.
(venv) PS C:\Users\gabe\Python\dash_test>

Note that flask run is not the only way it fails. flask --app dash_test run will also fail.

dash_test.zip

[T4rk1n]

For this pattern to work properly, you need to init dash with server=False. eg:

app = Dash(__name__, server=False, title="Dash Test")

This is the relevant docstring explaining the behavior:

dash/dash/dash.py

Lines 240 to 244 in d2b77b0

	:param name: The name Flask should use for your app. Even if you provide
	your own ``server``, ``name`` will be used to help find assets.
	Typically ``__name__`` (the magic global var, not a string) is the
	best value to use. Default ``'__main__'``, env: ``DASH_APP_NAME``
	:type name: string

• True (default): Dash creates a server immediately
• False: Server will be added later via init_app(server)

The user's app is using the default (server=True), which already creates and initializes a Flask server, but then they're calling init_app() with a different Flask app - that's the wrong pattern.

The fix is in the user's app - they need server=False:

app = Dash(name, title="Dash Test", server=False)

This worked before probably because the old code path didn't fully initialize everything in init when server=True, so calling init_app again happened to work. Now the backend refactor correctly separates the two patterns, exposing the
incorrect usage.

That being said, someone could put a safeguard to init_app to prevent such issues.

[ghaarsma]

Hi @T4rk1n , Thank you for the help. I have implemented your suggested fix server=False in the Dash call in both Dash 4.1 and Dash 4.2.0rc3

Using the MRE with Dash 4.1 and server=False everything works as it should.

With Dash 4.2.0.rc3 and 'server=False`, the reported ValueError: The name '_dash_assets' is already registered for a different blueprint. goes away, but now the Flask/Dash server will not serve up the page. Everything is returning a 404 Not Found HTTP error.