migrate-dotnet8-to-dotnet9

SKILL.md

.NET 8 → .NET 9 Migration

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.

When to Use

Upgrading TargetFramework from net8.0 to net9.0

Resolving build errors or new warnings after updating the .NET 9 SDK

Adapting to behavioral changes in .NET 9 runtime, ASP.NET Core 9, or EF Core 9

Replacing BinaryFormatter usage (now always throws at runtime)

Updating CI/CD pipelines, Dockerfiles, or deployment scripts for .NET 9

When Not to Use

The project already targets 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.

Upgrading from .NET 7 or earlier — address the prior version breaking changes first

Migrating from .NET Framework — that is a separate, larger effort

Greenfield projects that start on .NET 9 (no migration needed)