MTP Hot Reload for Iterative Test Fixing

Set up and use Microsoft Testing Platform hot reload to rapidly iterate fixes on failing tests without rebuilding between each change.

When to Use

• User has one or more failing tests and wants to iterate fixes quickly
• User wants to avoid rebuild overhead while fixing test code or production code
• User asks about hot reload for tests or speeding up the test-fix loop
• User needs to set up MTP hot reload in their project

When Not to Use

• User needs to write new tests from scratch (use general coding assistance)
• User needs to diagnose why a test is failing (use diagnostic skills)
• User wants Visual Studio Test Explorer hot reload (different feature, built into VS)
• Project uses VSTest -- hot reload requires Microsoft Testing Platform (MTP)
• User needs CI/CD pipeline configuration

Inputs

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

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

Step 2: Add the hot reload NuGet package

Install the

Microsoft.Testing.Extensions.HotReload

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

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

Create or update

Properties/launchSettings.json

{
  "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

dotnet run --project <project-path>

To filter to specific failing tests, pass the filter after

--

filter-syntax

dotnet run --project <path> -- --filter "FullyQualifiedName~TestMethodName"

dotnet run --project <path> -- --filter "FullyQualifiedName~TestMethodName"

dotnet run --project <path> -- --filter-method "*TestMethodName"

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

Validation

• Project uses Microsoft Testing Platform (not VSTest)

• Microsoft.Testing.Extensions.HotReload

  package is installed

• TESTINGPLATFORM_HOTRELOAD_ENABLED

  environment variable is set to

  1

• Tests run and the host remains active waiting for changes
• Code changes are picked up without manual restart

Common Pitfalls

dotnet test

dotnet run

dotnet run --project <path>

TESTINGPLATFORM_HOTRELOAD_ENABLED=1