MTP Hot Reload for Iterative Test Fixing

Workflow

Step 1: Verify the project uses Microsoft Testing Platform

Hot reload requires MTP. It does not work with VSTest.

Follow the detection procedure in the platform-detection skill to determine the test platform.

If the project uses VSTest, inform the user that MTP hot reload is not available and suggest migrating to MTP first (see migrate-vstest-to-mtp), or using Visual Studio's built-in Test Explorer hot reload feature instead.

Step 2: Add the hot reload NuGet package

Install the Microsoft.Testing.Extensions.HotReload package:

dotnet add <project-path> package Microsoft.Testing.Extensions.HotReload

> Note: When using Microsoft.Testing.Platform.MSBuild (included transitively by MSTest, NUnit, and xUnit runners), the extension is auto-registered when you install its NuGet package -- no code changes needed.

Step 3: Enable hot reload

Hot reload is activated by setting the TESTINGPLATFORM_HOTRELOAD_ENABLED environment variable to 1.

Option A -- Set it in the shell before running tests:

# PowerShell
$env:TESTINGPLATFORM_HOTRELOAD_ENABLED = "1"

# bash/zsh
export TESTINGPLATFORM_HOTRELOAD_ENABLED=1

Option B -- Add it to `launchSettings.json` (recommended for repeatable use):

Create or update Properties/launchSettings.json in the test project:

{
  "profiles": {
    "<ProjectName>": {
      "commandName": "Project",
      "environmentVariables": {
        "TESTINGPLATFORM_HOTRELOAD_ENABLED": "1"
      }
    }
  }
}

Step 4: Run the tests with hot reload

Run the test project directly (not through dotnet test) to use hot reload in console mode:

dotnet run --project <project-path>

To filter to specific failing tests, pass the filter after --. The syntax depends on the test framework -- see the filter-syntax skill for full details. Quick examples:

| Framework | Filter syntax | |-----------|--------------| | MSTest | dotnet run --project <path> -- --filter "FullyQualifiedName~TestMethodName" | | NUnit | dotnet run --project <path> -- --filter "FullyQualifiedName~TestMethodName" | | xUnit v3 | dotnet run --project <path> -- --filter-method "*TestMethodName" | | TUnit | dotnet run --project <path> -- --treenode-filter "/*/*/ClassName/TestMethodName" |

The test host will start, run the tests, and remain running waiting for code changes.

Step 5: Iterate on the fix

1. Edit the source code (test code or production code) in your editor
2. The test host detects the changes and re-runs the affected tests automatically
3. Review the updated results in the console
4. Repeat until all targeted tests pass

> Important: Hot reload currently works in console mode only. There is no support for hot reload in Test Explorer for Visual Studio or Visual Studio Code.

Step 6: Finalize

Once all tests pass:

1. Stop the test host (Ctrl+C)
2. Run a full dotnet test to confirm all tests pass with a clean build
3. Optionally remove TESTINGPLATFORM_HOTRELOAD_ENABLED from the environment or keep launchSettings.json for future use

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