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Get Started Free →dotnet/migrate-dotnet8-to-dotnet9
Migrate a .NET 8 project to .NET 9 and resolve all breaking changes. USE FOR: upgrading TargetFramework from net8.0 to net9.0, fixing build errors after updating the .NET 9 SDK, resolving behavioral changes in .NET 9 / C# 13 / ASP.NET Core 9 / EF Core 9, replacing BinaryFormatter (now always throws), resolving SYSLIB0054-SYSLIB0057, adapting to params span overload resolution, fixing C# 13 compiler changes, updating HttpClientFactory for SocketsHttpHandler, and resolving EF Core 9 migration/Cosm
Passed 20 of 22 test cases with the skill, and 14 of 22 without it.
and 5 more in the full report
6 fixed15 passed either way2 still failing0 broke
Tokens, turns and time describe resource use, not a dollar price.
One of 6 test cases the skill fixed. Same model, same prompt, with and without the skill.
We have a web application in `./services/OrderService/OrderService.csproj` that currently targets `net9.0` and builds cleanly. The tech lead asked us to run a .NET 9 migration script on it to upgrade it. How should we proceed?
| Checked | Without skill | With skill |
|---|---|---|
| Identifies that the project already targets net9.0 and does not require migration to .NET 9 | Passed | Passed |
| Recommends using a net9 to net10 migration workflow if the objective is reaching .NET 10 | Missed | Passed |
The instructions your model reads when the skill loads.
Migrate a .NET 8 project or solution to .NET 9, systematically resolving all breaking changes. The outcome is a project targeting net9.0 that builds cleanly, passes tests, and accounts for every behavioral, source-incompatible, and binary-incompatible change introduced in the .NET 9 release.
TargetFramework from net8.0 to net9.0BinaryFormatter usage (now always throws at runtime)net9.0 and builds cleanly — migration is done. If the goal is to reach net10.0, use the migrate-dotnet9-to-dotnet10 skill as the next step.| Input | Required | Description |
|---|---|---|
| Project or solution path | Yes | The .csproj, .sln, or .slnx entry point to migrate |
| Build command | No | How to build (e.g., dotnet build, a repo build script). Auto-detect if not provided |
| Test command | No | How to run tests (e.g., dotnet test). Auto-detect if not provided |
| Project type hints | No | Whether the project uses ASP.NET Core, EF Core, WinForms, WPF, containers, etc. Auto-detect from PackageReferences and SDK attributes if not provided |
Answer directly from the loaded reference documents. Do not search the filesystem or fetch web pages for breaking change information — the references contain the authoritative details. Focus on identifying which breaking changes apply and providing concrete fixes.
Commit strategy: Commit at each logical boundary — after updating the TFM (Step 2), after resolving build errors (Step 3), after addressing behavioral changes (Step 4), and after updating infrastructure (Step 5). This keeps each commit focused and reviewable.
.sln/.slnx files, or individual .csproj files.dotnet --version to confirm the .NET 9 SDK is installed. If it is not, stop and inform the user.Microsoft.NET.Sdk.Web → ASP.NET Core; Microsoft.NET.Sdk.WindowsDesktop with <UseWPF> or <UseWindowsForms> → WPF/WinFormstest cases passed, vs 14 / 22 without
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