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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/Cosmos DB changes. DO NOT USE FOR: .NET Framework migrations, upgrading from .NET 7 or earlier, greenfield .NET 9 projects, or cosmetic modernization unrelated to the upgrade.

plugins/dotnet-upgrade/skills/migrate-dotnet8-to-dotnet9/SKILL.md

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# .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