When would you put a Next.js route handler or server action between the React UI and an ASP.NET Core API?

Use a Next.js backend-for-frontend with an ASP.NET API

Explains when a Next.js route layer helps with cookies, tokens, aggregation, and UI-specific API adaptation.

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I would use a BFF layer when the browser should not handle backend tokens directly, when server-rendered pages need authenticated data, or when the UI needs a small aggregation tailored to a page. The Next.js layer can read secure cookies, call the ASP.NET API with a bearer token or server credential, normalize errors, and return a frontend-friendly response. The ASP.NET API still owns business rules and authorization.

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BFF boundaries, auth cookies, token handling, API aggregation, caching, server/client rendering, and operational ownership.

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Modern full-stack apps often have both a frontend server and a backend API. The design needs a clear boundary so the BFF layer helps the UI without becoming a second unowned backend.
I would use a BFF layer when the browser should not handle backend tokens directly, when server-rendered pages need authenticated data, or when the UI needs a small aggregation tailored to a page. The Next.js layer can read secure cookies, call the ASP.NET API with a bearer token or server credential, normalize errors, and return a frontend-friendly response. The ASP.NET API still owns business rules and authorization.
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I would keep the BFF thin. It can adapt transport concerns, session cookies, caching headers, and UI-specific composition, but it should not duplicate domain decisions. If multiple clients need the same behavior, that logic belongs in the API. Caching needs care because user-specific data, permissions, and tenant context can make shared caching dangerous. Server components can fetch data directly on the server, while client components call route handlers for mutations or browser-triggered refresh.

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Putting business authorization only in the Next.js layer.
Using the BFF as an unstructured duplicate backend.
Caching user-specific responses as if they were public.
Losing correlation between frontend and API logs.

When would you put a Next.js route handler or server action between the React UI and an ASP.NET Core API?

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