600 Filters and a 414: The New QUERY Method in .NET 10

A product search, a filter list that kept growing, and a status code I hadn't seen in years. Filters went in the query string, the way they always do. That held up fine until someone saved a "filter set" with a few hundred SKUs in it and the endpoint started answering with 414. I rebuilt a small version of it to find the exact wall. Same search, filters as repeated ?sku= values, count going up in steps of a hundred: 1) GET with filters in the URL 100 filters | request line 1534 bytes | 200 OK 200 filters | request line 3034 bytes | 200 OK 300 filters | request line 4534 bytes | 200 OK 400 filters | request line 6034 bytes | 200 OK 500 filters | request line 7534 bytes | 200 OK 600 filters | request line 9034 bytes | 414 RequestUriTooLong Kestrel's default max request line is 8 KB, and the request line is the method plus the URL plus the HTTP version. Somewhere between 500 and 600 filters, my URL stopped being a URL. Every fix I knew was a compromise. A body on GET is undefined by spec and some proxies quietly drop it. POST works, but POST announces "this might change something", so caches skip it, gateways won't auto-retry it, and anyone reading your API docs has to guess whether POST /search is actually a search. Cramming the filters into a header is the kind of idea that sounds clever for about a day. The method that was missing RFC 10008 defines QUERY , and it's exactly the thing that spot in the matrix was waiting for. The body carries the query. The method is safe and idempotent, so it can be retried after a dropped connection without anyone panicking. Responses are cacheable, and the spec is explicit that the cache key has to be built from "the request content and related metadata". There's also a nice touch on the response side: Content-Location can point at a URL where those exact results can be fetched with a plain GET. The one-line version I keep giving people: it's a GET with a body, and that's the entire point. Wiring it up in ASP.NET Core 10 .NET 10 shi