Mtp Hot Reload

MTP Hot Reload for Iterative Test Fixing

Set up and use a long-lived Microsoft Testing Platform host that applies code edits and automatically reruns tests.

When to Use

• User explicitly asks for test hot reload
• User wants a host to stay running and automatically rerun after repeated edits
• 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 asks about Visual Studio or VS Code Test Explorer hot reload or an IDE-integrated rerun experience (different feature, not MTP console-host hot reload)
• User asks whether the editor can auto-rerun affected tests after source edits without using an MTP console host
• User wants one normal run without rebuilding (use run-tests)
• User needs CI/CD pipeline configuration

Inputs

Response sizing

• If setup is already complete and the user asks only which command to use, return one dotnet run --project <path> command and one sentence explaining that it starts the persistent host. Do not repeat package, launch profile, or rude-edit guidance.
• If the package is already installed, show only the remaining enable-and-run steps. Do not suggest reinstalling it or add optional persistence/recovery paths unless requested.
• For a named test, identify the framework and return one runnable command with that framework's filter syntax. Never substitute MSTest/NUnit --filter for xUnit v3 --filter-method or TUnit --treenode-filter.
• A directly launched MTP host is itself the persistent watcher: dotnet run stays alive and the HotReload extension reacts to edits. Do not replace it with dotnet watch run for the normal MTP path; reserve dotnet watch for the explicit restart fallback below.

Workflow

Step 1: Detect the platform before changing anything

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

Follow the complete evaluated-property procedure in the platform-detection skill. Read imported props and package versions as well as the project file. Do this before installing packages, editing files, or returning an MTP launch command.

Hard stop for VSTest: report that MTP hot reload is unavailable for the project as configured and stop the MTP setup path. Do not install the extension, create launchSettings.json, set the environment variable, change runner properties/packages, or return a dotnet run hot-reload command. Never turn a setup request into an implicit VSTest-to-MTP migration.

Offer one valid non-MTP fallback that preserves the project:

dotnet watch --project <project-path> test

This rebuilds and reruns the existing VSTest project when files change; it is not MTP hot reload. Offer an explicit migration as a separate option, but do not perform it unless the user asks. Exact one-shot test commands remain owned by run-tests.

Step 2: Add the hot reload NuGet package

First inspect the effective package references. If Microsoft.Testing.Extensions.HotReload is already installed, preserve its version and skip this step. Otherwise install it:

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:

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