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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TL;DR

I add a Next.js server boundary when it protects browser credentials, supports authenticated server rendering, or adapts several backend calls into one UI-specific operation. Server Components can read data for rendering, Server Actions fit UI-coupled mutations, and Route Handlers expose an HTTP endpoint for browser or other callers. Those are transport choices; the ASP.NET Core API still owns shared business rules, durable authorization, validation, and data integrity.

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

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Why this question matters
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 add a Next.js server boundary when it protects browser credentials, supports authenticated server rendering, or adapts several backend calls into one UI-specific operation. Server Components can read data for rendering, Server Actions fit UI-coupled mutations, and Route Handlers expose an HTTP endpoint for browser or other callers. Those are transport choices; the ASP.NET Core API still owns shared business rules, durable authorization, validation, and data integrity.
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The BFF stays thin. It can read secure session cookies, attach backend credentials, shape a page response, and map errors into the UI contract without duplicating a domain use case. A Server Action or Route Handler is still a public-facing entry point, so it validates untrusted input and authenticates and authorizes the action before calling downstream services. User- and tenant-specific responses need explicit cache behavior, and reusable mobile or partner behavior belongs in the API rather than a Next.js-only adapter.

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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.

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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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