Enable -mt in PR CIs with 2 stages

.vsts-dotnet-ci.yml

@@ -291,6 +291,42 @@ jobs:
     continueOnError: true
     condition: always()
 
+- job: BootstrapMSBuildWithMTModeOnWindows
+  displayName: "Windows Core Multithreaded Mode"
+  pool:
+    ${{ if eq(variables['System.TeamProject'], 'public') }}:
+      name: NetCore-Public
+      demands: ImageOverride -equals windows.vs2026preview.scout.amd64.open
+    ${{ if ne(variables['System.TeamProject'], 'public') }}:
+      name: VSEng-MicroBuildVSStable
+      demands: agent.os -equals Windows_NT
+  timeoutInMinutes: 120
+  steps:
+  - template: azure-pipelines/check-documentation-only-change.yml
+  - ${{ if eq(variables['System.TeamProject'], 'DevDiv') }}:
+    - task: PowerShell@2
+      displayName: Setup Private Feeds Credentials
+      inputs:
+        filePath: $(Build.SourcesDirectory)/eng/common/SetupNugetSources.ps1
+        arguments: -ConfigFile $(Build.SourcesDirectory)/NuGet.config -Password $Env:Token
+      env:
+        Token: $(dn-bot-dnceng-artifact-feeds-rw)
+  - task: BatchScript@1
+    displayName: 'Build with bootstrapped MSBuild and -mt mode'
+    inputs:
+      filename: 'eng/cibuild_bootstrapped_msbuild.cmd'
+      arguments: '-msbuildEngine dotnet -onlyDocChanged $(onlyDocChanged) -skipTests -stage2Properties "/mt /p:BuildAnalyzer=true"'
+    condition: eq(variables.onlyDocChanged, 0)
+    env:
+      MSBUILDUSESERVER: "1"
+  - task: PublishBuildArtifacts@1
+    displayName: 'Publish Artifact: logs'
+    inputs:
+      PathtoPublish: 'artifacts/log/Debug'
+      ArtifactName: 'MTModeOnWindows build logs'
+    continueOnError: true
+    condition: always()
+
 - job: FullReleaseOnWindows
   displayName: "Windows Full Release (no bootstrap)"
   variables:

eng/cibuild_bootstrapped_msbuild.ps1

@@ -5,6 +5,8 @@ Param(
   [switch] $prepareMachine,
   [bool] $buildStage1 = $True,
   [bool] $onlyDocChanged = 0,
+  [switch] $skipTests,
+  [string] $stage2Properties = "",
   [Parameter(ValueFromRemainingArguments=$true)][String[]]$properties
 )
 
@@ -117,14 +119,22 @@ try {
   # Opt into performance logging. https://github.com/dotnet/msbuild/issues/5900
   $env:DOTNET_PERFLOG_DIR=$PerfLogDir
 
+  # Mirrors cibuild_bootstrapped_msbuild.sh. Some MSBuild scenarios (notably /mt routing tasks
+  # to a sidecar TaskHost) require this so the apphost-based child can locate the runtime.
+  # The SDK CLI (`dotnet msbuild`) sets it automatically; `dotnet exec MSBuild.dll` does not.
+  $env:DOTNET_HOST_PATH=$dotnetExePath
+
   # When using bootstrapped MSBuild:
   # - Turn off node reuse (so that bootstrapped MSBuild processes don't stay running and lock files)
   # - Create bootstrap environment as it's required when also running tests
-  if ($onlyDocChanged) {
-    & $PSScriptRoot\Common\Build.ps1 -restore -build -ci /p:CreateBootstrap=false /nr:false @properties
+  # - $stage2Properties are appended to the stage 2 build only (matching cibuild_bootstrapped_msbuild.sh).
+  #   Use this for switches like /mt that should not be passed to the SDK MSBuild used in stage 1.
+  $stage2Args = if ($stage2Properties) { $stage2Properties -split '\s+' | Where-Object { $_ } } else { @() }
+  if ($onlyDocChanged -or $skipTests) {
+    & $PSScriptRoot\Common\Build.ps1 -restore -build -ci /p:CreateBootstrap=false /nr:false @properties @stage2Args
   }
   else {
-    & $PSScriptRoot\Common\Build.ps1 -restore -build -test -ci /nr:false @properties
+    & $PSScriptRoot\Common\Build.ps1 -restore -build -test -ci /nr:false @properties @stage2Args
   }
 
   exit $lastExitCode