diff --git a/Documentation/TestRunnerConsolidation.md b/Documentation/TestRunnerConsolidation.md
new file mode 100644
index 00000000000..6785ae853e4
--- /dev/null
+++ b/Documentation/TestRunnerConsolidation.md
@@ -0,0 +1,142 @@
+# Test runner consolidation on Microsoft.Testing.Platform
+
+## Why
+
+Arcade currently maintains five parallel test-runner code paths in
+`src/Microsoft.DotNet.Arcade.Sdk/tools/` (`XUnit`, `XUnitV3`, `MSTest`,
+`VSTest`, `Microsoft.Testing.Platform`) plus two parallel Helix work-item
+generators in `src/Microsoft.DotNet.Helix/Sdk/` (`xunit-runner` /
+`CreateXUnitWorkItems` and `mtp-runner` / `CreateMTPWorkItems`).
+
+Most of that surface area predates [Microsoft.Testing.Platform][mtp] (MTP):
+
+* xUnit v3 [defaults to MTP][xunitv3-mtp] and produces a self-hosting test
+  executable when paired with `Microsoft.Testing.Platform.MSBuild`.
+* MSTest 4.x [defaults to MTP][mstest-runner] via `MSTest.Sdk` /
+  `MSTest` metapackage.
+* NUnit, TUnit, and other frameworks ship MTP runners.
+* `dotnet test` is now MTP-aware (`TestingPlatformDotnetTestSupport`) and in
+  .NET 10 the [`dotnet test --use-testing-platform`][dotnet-test-mtp]
+  experience is the official orchestrator for MTP-based projects.
+* VSTest is in maintenance mode; the [testfx team has stated][vstest-status]
+  that future investment is on MTP.
+
+The legacy paths therefore exist purely as compatibility surface. Each one
+adds property names that have to be documented and supported, complicates
+the `Test` target dispatch, and ships duplicated logger / TRX wiring that
+MTP gives us for free.
+
+## Goal
+
+Reduce the Arcade test-execution surface to:
+
+* **One** runner targets file in the Arcade SDK that invokes MTP exes
+  directly (already present:
+  [`tools/Microsoft.Testing.Platform/Microsoft.Testing.Platform.targets`][mtp-targets]).
+* **One** thin `dotnet test` wrapper for projects that have not yet
+  migrated to MTP — used only when the project is not a Testing Platform
+  application.
+* **One** Helix work-item generator
+  ([`CreateMTPWorkItems`][create-mtp]) replacing both the legacy
+  `CreateXUnitWorkItems` flow and any framework-specific work-item
+  generators.
+
+Helix orchestration itself (`SendHelixJob`, queue selection, correlation
+payloads, `EnableHelixJobMonitor`, XHarness for iOS/Android/WASM, test
+retry, AzDO test-run lifecycle) is **out of scope** — those are Arcade's
+genuine value-add and are independent of which runner runs the test.
+
+## Plan
+
+This is staged so each phase is independently revertable.
+
+### Phase 1 — Signal direction (this PR)
+
+* Add this document.
+* Emit MSBuild warnings when a project opts into a path that we intend to
+  remove:
+  * Setting `UsingToolXUnit` (any value) — property is already documented
+    as deprecated in [`DefaultVersions.props`][defaultversions] but emits
+    no warning.
+  * Setting `UseVSTestRunner=true`.
+  * Including `XUnitProject` items in a Helix project (use `MTPProject`
+    instead — see the
+    [Helix Sdk readme](../src/Microsoft.DotNet.Helix/Sdk/Readme.md)).
+* Delete the orphaned `xunit-reporter.py` Helix Python script which is
+  already self-marked as deprecated and is no longer invoked from any
+  Arcade target.
+
+No behavior changes for repos that do not set the deprecated properties.
+
+### Phase 2 — Flip defaults
+
+* Default `UseMicrosoftTestingPlatformRunner=true` is already in place for
+  xUnit v3 ([`XUnitV3.targets`][xunitv3-targets]). Audit downstream repos
+  via arcade-validation to confirm there are no remaining consumers
+  relying on the VSTest path.
+* Default `EnableMSTestRunner=true` for MSTest projects, with an opt-out
+  for repos that still depend on VSTest-specific loggers / runsettings
+  features.
+* Remove `Microsoft.NET.Test.Sdk` from the implicit reference set for MTP
+  projects (already conditional on `IsTestingPlatformApplication != true`
+  in [`Tests.props`][tests-props]; ensure that flag is reliably set for
+  every MTP path).
+
+### Phase 3 — Drop xUnit v2
+
+* Delete `tools/XUnit/` (`XUnit.targets` and `XUnit.Runner.targets`).
+  xUnit v2 is in maintenance upstream; v3 has been the Arcade
+  recommendation since [#15671][pr-15671].
+* Repos still on v2 can pin to an older Arcade SDK or set
+  `TestRunnerName=XUnit` and import their own targets.
+
+### Phase 4 — Collapse runner targets
+
+* Replace `XUnitV3.Runner.targets`, `VSTest.targets`, and the runner
+  portion of `MSTest.targets` with a single `RunTests` target that:
+  * for MTP projects (`IsTestingPlatformApplication=true`), execs the
+    self-hosting test app with MTP reporter flags; and
+  * for non-MTP projects, invokes `dotnet test` with standard
+    `--logger trx;...` arguments.
+* Framework-specific `.targets` shrink to package-reference lists only.
+
+### Phase 5 — Helix-side cleanup
+
+* Delete `tools/xunit-runner/` (`XUnitRunner.props`, `XUnitRunner.targets`,
+  `XUnitPublish.targets` — the publish helper has already been generalized
+  to `_MTPPublishTargetsPath`) and `CreateXUnitWorkItems.cs`. `MTPProject`
+  already covers xUnit v3, MSTest 4, NUnit, and TUnit.
+* Drop the related `XUnitProject`, `XUnitPublishTargetFramework`,
+  `XUnitRuntimeTargetFramework`, `XUnitRunnerVersion`, and
+  `XUnitArguments` properties from the SDK and the
+  [SendingJobsToHelix doc][send-helix-doc].
+
+### Phase 6 — `TestArchitectures` loop
+
+* `Tests.targets`' inner/outer loops over `TestArchitectures` ×
+  `TargetFrameworks` exist to feed runner CLIs. Once `RunTests` is just
+  `dotnet test`, the .NET SDK's per-TFM dispatch covers the TFM axis. The
+  architecture axis (x86/x64 multiplexing of a single binary on .NET
+  Framework) is Arcade-specific and stays — but the wrapper shrinks to
+  roughly 20 lines.
+
+## Estimated impact
+
+| Area | Files removed | LoC removed | Public properties / items removed |
+|---|---|---|---|
+| Arcade SDK runner targets | XUnit v2 dir, `VSTest.targets`, two `*.Runner.targets` | ~400 | `UseVSTestRunner`, `UsingToolXUnit`, `TestRunnerName=XUnit\|VSTest` |
+| Helix Sdk legacy xUnit | `xunit-runner/`, `CreateXUnitWorkItems.cs`, `xunit-reporter/` | ~600 | `XUnitProject`, `XUnitPublishTargetFramework`, `XUnitRuntimeTargetFramework`, `XUnitRunnerVersion`, `XUnitArguments`, `EnableXUnitReporter` |
+| **Total** | **~10 files** | **~1000 LoC** | **8 properties + 1 item group** |
+
+[mtp]: https://learn.microsoft.com/dotnet/core/testing/microsoft-testing-platform-intro
+[mtp-targets]: ../src/Microsoft.DotNet.Arcade.Sdk/tools/Microsoft.Testing.Platform/Microsoft.Testing.Platform.targets
+[create-mtp]: ../src/Microsoft.DotNet.Helix/Sdk/CreateMTPWorkItems.cs
+[xunitv3-targets]: ../src/Microsoft.DotNet.Arcade.Sdk/tools/XUnitV3/XUnitV3.targets
+[defaultversions]: ../src/Microsoft.DotNet.Arcade.Sdk/tools/DefaultVersions.props
+[tests-props]: ../src/Microsoft.DotNet.Arcade.Sdk/tools/Tests.props
+[send-helix-doc]: ./AzureDevOps/SendingJobsToHelix.md
+[xunitv3-mtp]: https://xunit.net/docs/getting-started/v3/microsoft-testing-platform
+[mstest-runner]: https://learn.microsoft.com/dotnet/core/testing/unit-testing-mstest-runner-intro
+[dotnet-test-mtp]: https://learn.microsoft.com/dotnet/core/testing/unit-testing-platform-integration-dotnet-test
+[vstest-status]: https://github.com/microsoft/vstest/blob/main/README.md
+[pr-15671]: https://github.com/dotnet/arcade/pull/15671
diff --git a/src/Microsoft.DotNet.Arcade.Sdk/tools/DefaultVersions.props b/src/Microsoft.DotNet.Arcade.Sdk/tools/DefaultVersions.props
index c053822a4ff..c228f120fa3 100644
--- a/src/Microsoft.DotNet.Arcade.Sdk/tools/DefaultVersions.props
+++ b/src/Microsoft.DotNet.Arcade.Sdk/tools/DefaultVersions.props
@@ -38,7 +38,10 @@
     <!--
       The UsingToolXUnit property is deprecated,
       use the TestRunnerName property to choose which test runner to use.
+      A deprecation warning is emitted from Tests.targets when this property
+      is set explicitly. See Documentation/TestRunnerConsolidation.md.
     -->
+    <_UsingToolXUnitExplicitlySet Condition="'$(UsingToolXUnit)' != ''">true</_UsingToolXUnitExplicitlySet>
     <UsingToolXUnit Condition="'$(UsingToolXUnit)' == ''">true</UsingToolXUnit>
 
     <!--
diff --git a/src/Microsoft.DotNet.Arcade.Sdk/tools/Tests.targets b/src/Microsoft.DotNet.Arcade.Sdk/tools/Tests.targets
index 1bd2527c060..b03ad7fc173 100644
--- a/src/Microsoft.DotNet.Arcade.Sdk/tools/Tests.targets
+++ b/src/Microsoft.DotNet.Arcade.Sdk/tools/Tests.targets
@@ -27,6 +27,27 @@
     <Error Condition="!Exists($(TestRunnerTargets))" Text="Test runner $(TestRunnerName) is invalid."/>
   </Target>
 
+  <!--
+    Phase 1 of the test-runner consolidation effort. See
+    Documentation/TestRunnerConsolidation.md for the full plan.
+
+    We warn when a project opts into a code path that is scheduled for
+    removal so that downstream repos can migrate before the next major
+    Arcade release. Suppress by setting:
+      <NoWarn>$(NoWarn);ARCADE0001;ARCADE0002</NoWarn>
+  -->
+  <Target Name="_WarnDeprecatedTestRunnerOptions"
+          Condition="'$(IsTestProject)' == 'true' AND '$(SuppressArcadeTestRunnerDeprecationWarnings)' != 'true'"
+          BeforeTargets="Test;IntegrationTest;Build">
+    <Warning Code="ARCADE0001"
+             Text="The 'UsingToolXUnit' property is deprecated and will be removed in a future Arcade release. Use the 'TestRunnerName' property (e.g. TestRunnerName=XUnitV3) to select a test runner. See Documentation/TestRunnerConsolidation.md."
+             Condition="'$(_UsingToolXUnitExplicitlySet)' == 'true'" />
+
+    <Warning Code="ARCADE0002"
+             Text="The 'UseVSTestRunner' property is deprecated and the VSTest-based runner path will be removed in a future Arcade release. Migrate to Microsoft.Testing.Platform (the default for xUnit v3 and MSTest 4.x). See Documentation/TestRunnerConsolidation.md."
+             Condition="'$(UseVSTestRunner)' == 'true'" />
+  </Target>
+
   <PropertyGroup Condition="'$(IsTestProject)' == 'true' and '$(TestArchitectures)' == ''">
     <TestArchitectures>$(PlatformTarget)</TestArchitectures>
     <TestArchitectures Condition="'$(PlatformTarget)' == '' or '$(PlatformTarget)' == 'AnyCpu'">x64</TestArchitectures>
diff --git a/src/Microsoft.DotNet.Helix/Sdk/tools/xunit-reporter/xunit-reporter.py b/src/Microsoft.DotNet.Helix/Sdk/tools/xunit-reporter/xunit-reporter.py
deleted file mode 100644
index 750909a815a..00000000000
--- a/src/Microsoft.DotNet.Helix/Sdk/tools/xunit-reporter/xunit-reporter.py
+++ /dev/null
@@ -1,10 +0,0 @@
-#!/usr/bin/env python
-
-import helix.event
-import helix.settings
-
-if __name__ == '__main__':
-    settings = helix.settings.settings_from_env()
-    event_client = helix.event.create_from_uri(settings.event_uri)
-    event_client.warning(settings, "Obsolete", "xunit-reporter.py is deprecated, please remove 'EnableXUnitReporter' from your build and use 'EnableAzurePipelinesReporter' instead.")
-    print("This reporter is deprecated")
diff --git a/src/Microsoft.DotNet.Helix/Sdk/tools/xunit-runner/XUnitRunner.targets b/src/Microsoft.DotNet.Helix/Sdk/tools/xunit-runner/XUnitRunner.targets
index 5c2f250360e..3b4baf4a509 100644
--- a/src/Microsoft.DotNet.Helix/Sdk/tools/xunit-runner/XUnitRunner.targets
+++ b/src/Microsoft.DotNet.Helix/Sdk/tools/xunit-runner/XUnitRunner.targets
@@ -19,6 +19,23 @@
     </HelixCorrelationPayload>
   </ItemGroup>
 
+  <!--
+    Phase 1 of the test-runner consolidation effort. XUnitProject is
+    scheduled for removal in favour of MTPProject + CreateMTPWorkItems,
+    which covers xUnit v3, MSTest 4.x, NUnit, TUnit and any other
+    Microsoft.Testing.Platform based framework. See
+    Documentation/TestRunnerConsolidation.md.
+
+    Suppress by setting:
+      <SuppressArcadeXUnitProjectDeprecationWarning>true</SuppressArcadeXUnitProjectDeprecationWarning>
+  -->
+  <Target Name="_WarnXUnitProjectDeprecated"
+          Condition="'@(XUnitProject)' != '' AND '$(SuppressArcadeXUnitProjectDeprecationWarning)' != 'true'"
+          BeforeTargets="Restore;Build">
+    <Warning Code="ARCADE0010"
+             Text="The 'XUnitProject' item and the xunit.runner.console-based Helix work-item generator are deprecated and will be removed in a future Arcade release. Migrate to 'MTPProject' (see src/Microsoft.DotNet.Helix/Sdk/Readme.md and Documentation/TestRunnerConsolidation.md)." />
+  </Target>
+
   <Target Name="RestoreXUnitProjects"
           Condition="'@(XUnitProject)' != ''"
           BeforeTargets="Restore"