GitHub Actions actions/setup-dotnet "Could not find dotnet SDK"

actions/setup-dotnet cannot resolve the requested .NET SDK. The dotnet-version is wrong, a global.json pins an SDK that is not available, or the channel/quality combination yields nothing.

What this error means

The setup-dotnet step fails saying it could not find the requested SDK version, often referencing global.json. The dotnet CLI is not installed, so build/test steps fail afterward.

Actions log

Error: Could not find dotnet SDK version '8.0.999'
# or, from global.json:
A compatible .NET SDK was not found matching version '8.0.100' from global.json

Common causes

Version not in the release index

A non-existent patch (8.0.999) or a too-specific pin that has not shipped cannot be resolved against the .NET release index.

global.json pins an unavailable SDK

If global.json sets sdk.version to an exact build, setup-dotnet must install that exact SDK. A stale or unreleased pin fails the resolution.

How to fix it

Request a channel or band, not a phantom patch

.github/workflows/ci.yml

- uses: actions/setup-dotnet@v4
  with:
    dotnet-version: '8.0.x'   # latest patch in the 8.0 band

Align global.json with rollForward

Allow roll-forward so a slightly newer SDK satisfies the pin instead of requiring an exact, possibly-missing build.

global.json

{
  "sdk": {
    "version": "8.0.100",
    "rollForward": "latestFeature"
  }
}

How to prevent it

Use feature-band specs like 8.0.x rather than exact phantom patches.
Set rollForward in global.json so CI tolerates newer patch SDKs.
Keep setup-dotnet current so newly released SDKs resolve.