dotnet SDK / Runtime Mismatch - TargetFramework Not Supported

Your project targets a framework newer than the installed SDK can build. The build stops with NETSDK1045 because an older SDK has no knowledge of a newer TargetFramework.

What this error means

Build fails with NETSDK1045 stating the current SDK does not support targeting the requested net8.0 (or newer). It reproduces every run on that runner because the SDK simply predates the framework.

dotnet build output

error NETSDK1045: The current .NET SDK does not support targeting .NET 8.0.
Either target .NET 6.0 or lower, or use a version of the .NET SDK that supports .NET 8.0.

Common causes

SDK older than the TargetFramework

A net8.0 project needs the 8.0 SDK (or newer). On a runner with only the 6.0 SDK, the build cannot target 8.0.

Multiple SDKs but the wrong one is selected

A global.json or PATH order selects an older SDK even though a newer one is installed, so the newer framework is rejected.

How to fix it

Install an SDK that supports the framework

Pin an SDK at least as new as the framework you target.

.github/workflows/ci.yml

- uses: actions/setup-dotnet@v4
  with:
    dotnet-version: '8.0.x'   # supports net8.0 and earlier

Pin the right SDK with global.json

When several SDKs are installed, pin the one that supports your target framework.

global.json

{
  "sdk": { "version": "8.0.401", "rollForward": "latestFeature" }
}

Or lower the TargetFramework

If you must stay on an older SDK, set <TargetFramework> to a version it supports.
Confirm the SDK with dotnet --version matches the framework requirement.
Prefer upgrading the SDK over downgrading the framework when possible.

How to prevent it

Keep the CI SDK at or above every project’s TargetFramework.
Pin the SDK with global.json so selection is deterministic.
Upgrade SDKs and target frameworks together across the solution.