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, greenfie
npx skills add dotnet/skills --skill migrate-dotnet8-to-dotnet9 --agent claude-code
Same command for any agent — swap --agent for codex, cursor, copilot.
Weekly change comes from our own snapshots, not the repository page — it measures attention, not adoption.
# .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) ## Inputs | Input | Required | Description | |-------|----------|-------------| | Project or soluti
- When to Use
- When Not to Use
- Inputs
- Workflow
- Step 1: Assess the project
- Step 2: Update the Target Framework
- Step 3: Resolve build errors and source-incompatible changes
- Step 4: Address behavioral changes
- Step 5: Update infrastructure
- Step 6: Verify
- Reference Documents
What does the migrate-dotnet8-to-dotnet9 skill do?
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, greenfie
How do I install it?
Run `npx skills add dotnet/skills --skill migrate-dotnet8-to-dotnet9 --agent claude-code` — it drops the skill into your project so the agent can pick it up. Swap the --agent value for codex, cursor or copilot if you use one of those.
Where does this skill come from?
From dotnet/skills, a repository with 4,927 stars. We read it straight from the repository tree rather than a submitted listing, so what you see here is what is actually published.
Is a popular skill a good skill?
Not necessarily. Stars measure attention, not adoption — a repository can trend for a week and be abandoned. That is why we show the weekly change from our own snapshots next to the total, instead of a single flattering number.