Upgrade .NET 8 to .NET 10 Without Breaking Your API Contract

If I need to upgrade .NET 8 to .NET 10 , I treat the work as an API contract migration, not a project-file edit. A service can compile, pass unit tests, and still surprise consumers with a changed JSON shape, status code, authentication response, or OpenAPI document. That risk matters now because Microsoft has confirmed that .NET 8 and .NET 9 reach end of support on November 10, 2026 . .NET 10 and C# 14 are the current stable releases, and .NET 10 is the supported LTS destination. Why the deadline changes my upgrade order My first step is inventory, not retargeting. I list every deployable project, test project, global.json , container base image, CI SDK pin, and Microsoft package reference. dotnet --list-sdks shows what a machine can build; dotnet --info shows what the current environment actually resolves. If that inventory needs more detail, my older guide to dotnet sdk check is a useful starting point. For APIs still on .NET 8, the broader Web API setup and security checklist can help identify behavior worth protecting before the move. I then separate the migration into three changes: SDK and target framework, NuGet dependencies, and runtime infrastructure. Keeping those changes visible makes a failure easier to locate. A giant dependency-refresh commit may be quick to create, but it is hard to diagnose. Upgrade .NET 8 to .NET 10 behind contract tests Before changing net8.0 , I add a small set of tests around the endpoints consumers cannot tolerate changing. I care about observable behavior: status codes, content types, required JSON names, and authentication boundaries. I avoid asserting an entire serialized string because harmless property ordering can make that test noisy. Here is a focused xUnit test for a Minimal API: using System.Net ; using System.Text.Json ; using Microsoft.AspNetCore.Mvc.Testing ; using Xunit ; public sealed class ProductContractTests ( WebApplicationFactory < Program > factory ) : IClassFixture < WebApplicationFactory < Program >> { [