When would you use a Next.js route handler instead of calling the backend API directly from the browser?

Use Next.js route handlers as an API boundary

Explains route handlers as a frontend server boundary for cookies, API adaptation, auth, and backend calls.

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I would use a route handler when the frontend server needs to protect tokens, read secure cookies, adapt backend errors, aggregate a page-specific response, or hide backend topology from the browser. The browser calls the route handler, and the route handler calls the ASP.NET API with the right credentials. The backend API still owns domain rules, authorization, validation, and data integrity.

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Route handlers, BFF pattern, cookies, token handling, backend API calls, error adaptation, and boundary ownership.

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Route handlers are useful, but they can become a confusing second backend if the boundary is unclear. This drill tests practical ownership.
I would use a route handler when the frontend server needs to protect tokens, read secure cookies, adapt backend errors, aggregate a page-specific response, or hide backend topology from the browser. The browser calls the route handler, and the route handler calls the ASP.NET API with the right credentials. The backend API still owns domain rules, authorization, validation, and data integrity.
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I would keep route handlers thin. They can translate transport concerns and frontend-specific response shapes, but they should not duplicate business logic. If mobile apps, partner clients, or other frontends need the same behavior, it belongs in the backend API. Route handlers also need timeouts, safe error mapping, body size limits, and clear cache behavior, especially when responses are user-specific.

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Putting domain authorization only in the route handler.
Duplicating backend business logic in frontend server code.
Caching user-specific route responses as public data.
Hiding backend errors so debugging becomes impossible.

When would you use a Next.js route handler instead of calling the backend API directly from the browser?

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