Summary Deploying a frontend and backend as separate Databricks Apps is a supported architectural pattern. To resolve the authentication interception, your frontend app must authenticate its server-to-server requests using its dedicated service principal (M2M OAuth) rather than relying on user authentication.

How Inter-App Communication Works Your frontend is receiving the Databricks Sign-In HTML page because the backend app's user authentication layer is intercepting the unauthenticated server-side HTTP request.

To fix this, you need to leverage the dual identity models provided by Databricks Apps:

• Dual Identity Model: Databricks Apps support both user authentication (where users log in with their credentials) and service principal authentication (where the app uses its dedicated service principal).  You can mix and match these authorization models within the same app.
• Service Principal Authentication: Because the frontend is making a server-side request to the backend, it cannot rely on the end-user's session. The frontend app must authenticate using its own dedicated service principal.
• Implementation: You need to grant the frontend app's service principal access to the backend app. The frontend app must then fetch its service principal OAuth token and pass it in the headers of the HTTP request. This will bypass the user login screen and allow the FastAPI backend to successfully return the JSON response.