.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)