.NET 10: Using the QUERY Method to Fix 414 Errors

Hitting a 414 Request-URI Too Long error is a classic sign that your query string has grown too large for the server to handle. I ran into this when a product search feature allowed users to save "filter sets" containing hundreds of SKUs. Once the URL hit about 9KB, Kestrel (which defaults to an 8KB max request line) started rejecting requests.

The typical "fixes" are all compromises. Putting a body in a GET request is technically undefined and risky with certain proxies. Using POST works, but it breaks caching and tells the client the operation might change server state.

The Solution: RFC 10008 QUERY

The QUERY method is essentially a GET request that allows a body. It's safe, idempotent, and explicitly cacheable. It solves the "too many filters" problem without sacrificing the semantic meaning of a read operation.

Deployment in ASP.NET Core 10

.NET 10 introduces the primitives for this, though it doesn't provide high-level "sugar" like a [HttpQuery] attribute yet. You have to use MapMethods to wire it up.

Here is a practical tutorial on how to implement this from scratch:

app.MapMethods("/products/search", [HttpMethods.Query], async (HttpContext ctx) =>
{
    // Manually deserialize the body since there's no built-in model binding for QUERY yet
    var filter = await JsonSerializer.DeserializeAsync(ctx.Request.Body);
    var result = RunSearch(filter!.Skus, filter.MaxPrice);

    // Content-Location allows clients to fetch these results via a standard GET later
    ctx.Response.Headers.ContentLocation = $"/products/search-results/{result.Execution}";
    return Results.Ok(result);
});

On the client side, you can now use HttpMethod.Query:

using var req = new HttpRequestMessage(HttpMethod.Query, "/products/search");
req.Content = new StringContent(json, Encoding.UTF8, "application/json");
using var resp = await http.SendAsync(req);

Critical Implementation Detail

One "gotcha" is that ASP.NET Core doesn't automatically enforce the Content-Type requirement specified in the RFC. If you use MapMethods, you must manually validate the header to stay spec-compliant:

if (string.IsNullOrEmpty(ctx.Request.ContentType))
    return Results.Problem("QUERY requires a Content-Type.", statusCode: 400);

if (!ctx.Request.ContentType.StartsWith("application/json", StringComparison.OrdinalIgnoreCase))
    return Results.StatusCode(StatusCodes.Status415UnsupportedMediaType);

By moving filters from the URL to the body via QUERY, I was able to jump from a 600-filter limit to 5,000+ SKUs (roughly 65KB of data) without a single 414 error. It's a much cleaner AI workflow for handling complex search parameters in a real-world API.