When would you use ASP.NET Core output caching instead of HTTP response caching or a CDN?

ASP.NET Core output caching vs HTTP response caching

Compares server-controlled output caching with HTTP cache semantics and defines safe keys, variation, invalidation, identity, and multi-instance behavior.

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HTTP response caching is driven by headers such as Cache-Control, validators, and Vary, so browsers and shared proxies can reuse responses according to standard semantics. ASP.NET Core output caching is a server policy that stores generated output even when client request headers would make response-caching middleware impractical. I use it for expensive, repeatable endpoints where the cache key can represent every input that changes the response. A CDN is useful when public content should be served near users and the edge can own the variation and purge rules.

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ASP.NET Core output caching, response caching, Cache-Control, CDN caching, cache policies, vary-by rules, authorization, invalidation, and scale-out.

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Why this question matters
Caching an entire response can save substantial application work, but the owner of the cache and its policy changes the correctness model. Confusing browser, proxy, middleware, and server output caches can leak data or produce stale behavior no layer understands.
HTTP response caching is driven by headers such as Cache-Control, validators, and Vary, so browsers and shared proxies can reuse responses according to standard semantics. ASP.NET Core output caching is a server policy that stores generated output even when client request headers would make response-caching middleware impractical. I use it for expensive, repeatable endpoints where the cache key can represent every input that changes the response. A CDN is useful when public content should be served near users and the edge can own the variation and purge rules.
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Treating output caching as the same mechanism as client HTTP caching.
Caching authenticated output without identity or permission isolation.
Varying on unbounded arbitrary input.
Adding a TTL without deciding what happens after a write.

When would you use ASP.NET Core output caching instead of HTTP response caching or a CDN?

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