From 7e20b2a9f02f09066f59d001d01a3b13f3831dae Mon Sep 17 00:00:00 2001
From: Jan Provaznik <janprovaznik@microsoft.com>
Date: Mon, 4 May 2026 17:36:36 +0200
Subject: [PATCH 01/10] Enable -mt mode in Windows PR CI bootstrapped build
 (#13602)

Mirrors the existing Linux BootstrapMSBuildWithMTMode job for Windows so
that PR CI exercises stage-2 builds with /mt on both platforms.

Two parts:

1. eng/cibuild_bootstrapped_msbuild.ps1
   * Export DOTNET_HOST_PATH (mirrors cibuild_bootstrapped_msbuild.sh:96).
     /mt routes every unmigrated task to a sidecar TaskHost via
     NodeProviderOutOfProcTaskHost.ResolveAppHostOrFallback, which prefers
     the SDK apphost (MSBuild.exe) and needs DOTNET_ROOT in the child env.
     The SDK CLI (`dotnet msbuild`) sets DOTNET_HOST_PATH automatically;
     `dotnet exec MSBuild.dll` (used by this script) does not, so we set
     it here explicitly.
   * Add -stage2Properties parameter (mirrors --stage2Properties in the
     bash script). Lets callers pass /mt to stage 2 only without
     contaminating the stage 1 build that uses the SDK MSBuild.
   * Add -skipTests switch (mirrors --skipTests in the bash script).

2. .vsts-dotnet-ci.yml
   * Add BootstrapMSBuildWithMTModeOnWindows job, mirroring the existing
     BootstrapMSBuildWithMTMode (Linux) job.

Local verification: the Debug stage-2 build with /mt produces shipping
binaries that are byte-for-byte identical to the same build without /mt
across all 1,628 files in artifacts/bin/{Microsoft.Build*, MSBuild*,
StringTools, MSBuild.Bootstrap, MSBuildTaskHost} and across every file
inside the four shipping nupkgs. The C# compiler is invoked the same
242 times with byte-identical command lines.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
---
 .vsts-dotnet-ci.yml                  | 36 ++++++++++++++++++++++++++++
 eng/cibuild_bootstrapped_msbuild.ps1 | 16 ++++++++++---
 2 files changed, 49 insertions(+), 3 deletions(-)

diff --git a/.vsts-dotnet-ci.yml b/.vsts-dotnet-ci.yml
index 5cd79b6af33..dc7aef89585 100644
--- a/.vsts-dotnet-ci.yml
+++ b/.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:
diff --git a/eng/cibuild_bootstrapped_msbuild.ps1 b/eng/cibuild_bootstrapped_msbuild.ps1
index a61ecbe7d4a..cc91e1cd6de 100644
--- a/eng/cibuild_bootstrapped_msbuild.ps1
+++ b/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

From 4cc0d38eba72f05cab23e9e1641abcf22abdd381 Mon Sep 17 00:00:00 2001
From: Jan Provaznik <janprovaznik@microsoft.com>
Date: Mon, 4 May 2026 17:41:10 +0200
Subject: [PATCH 02/10] Enable -mt in experimental insertion pipeline (#13602)

Adds an `additionalBuildArgs` template parameter to
`.vsts-dotnet-build-jobs.yml`, defaulted to empty, and sets it to `/mt`
from `.vsts-dotnet-exp-perf.yml` only. The official `.vsts-dotnet.yml`
pipeline is intentionally untouched in this commit; it will follow as a
stacked change once the experimental run validates parity end-to-end.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
---
 azure-pipelines/.vsts-dotnet-build-jobs.yml | 4 ++++
 azure-pipelines/.vsts-dotnet-exp-perf.yml   | 1 +
 2 files changed, 5 insertions(+)

diff --git a/azure-pipelines/.vsts-dotnet-build-jobs.yml b/azure-pipelines/.vsts-dotnet-build-jobs.yml
index c8fffe6904b..f95fb422203 100644
--- a/azure-pipelines/.vsts-dotnet-build-jobs.yml
+++ b/azure-pipelines/.vsts-dotnet-build-jobs.yml
@@ -12,6 +12,9 @@ parameters:
 - name: enableOptProf
   type: boolean
   default: false
+- name: additionalBuildArgs
+  type: string
+  default: ''
 
 jobs:
 - job: Windows_NT
@@ -147,6 +150,7 @@ jobs:
               /p:GenerateSbom=true
               /p:SuppressFinalPackageVersion=${{ parameters.isExperimental }}
               /p:IsExperimental=${{ parameters.isExperimental }}
+              ${{ parameters.additionalBuildArgs }}
 
     displayName: Build
     condition: succeeded()
diff --git a/azure-pipelines/.vsts-dotnet-exp-perf.yml b/azure-pipelines/.vsts-dotnet-exp-perf.yml
index ac9aab87104..953c5db4ee3 100644
--- a/azure-pipelines/.vsts-dotnet-exp-perf.yml
+++ b/azure-pipelines/.vsts-dotnet-exp-perf.yml
@@ -94,6 +94,7 @@ extends:
           enableComponentGovernance: false
           signTypeParameter: ${{ parameters.signTypeParameter }}
           enableOptProf: ${{ parameters.enableOptProf }}
+          additionalBuildArgs: '/mt'
 
     - template: /eng/common/templates-official/post-build/post-build.yml@self
       parameters:

From e05c9c00073de4d95b245af83abebd10a70e93d0 Mon Sep 17 00:00:00 2001
From: Jan Provaznik <janprovaznik@microsoft.com>
Date: Mon, 4 May 2026 17:51:14 +0200
Subject: [PATCH 03/10] Switch every PR CI bootstrapped job to -mt in stage 2
 (#13602)
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Follow-up on review feedback: drop the two dedicated
BootstrapMSBuildWithMTMode* jobs and instead append `-stage2Properties /mt`
(`--stage2Properties /mt` for bash) to every existing bootstrapped PR CI
job so each job exercises /mt in its stage 2 (with the test suite still
running, unlike the dedicated MT job which had to skip tests).

Affected jobs:
* BootstrapMSBuildOnFullFrameworkWindows
* BootstrapMSBuildOnCoreWindows
* CoreBootstrappedOnLinux
* CoreOnMac

Removed jobs (now redundant):
* BootstrapMSBuildWithMTMode (Linux dedicated MT mode)
* BootstrapMSBuildWithMTModeOnWindows (added earlier in this PR)

No /p:BuildAnalyzer=true added — the analyzer is independent of /mt and
out of scope for this change.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
---
 .vsts-dotnet-ci.yml | 74 +++------------------------------------------
 1 file changed, 4 insertions(+), 70 deletions(-)

diff --git a/.vsts-dotnet-ci.yml b/.vsts-dotnet-ci.yml
index dc7aef89585..172f05419cd 100644
--- a/.vsts-dotnet-ci.yml
+++ b/.vsts-dotnet-ci.yml
@@ -92,7 +92,7 @@ jobs:
     displayName: cibuild_bootstrapped_msbuild.cmd
     inputs:
       filename: 'eng/cibuild_bootstrapped_msbuild.cmd'
-      arguments: -onlyDocChanged $(onlyDocChanged)
+      arguments: '-onlyDocChanged $(onlyDocChanged) -stage2Properties /mt'
     env:
       ForceUseXCopyMSBuild: 1
 # Task to collect code coverage on Windows. Disabled by default due to being unstable and sometimes it stucks forever
@@ -204,7 +204,7 @@ jobs:
     displayName: cibuild_bootstrapped_msbuild.cmd
     inputs:
       filename: 'eng/cibuild_bootstrapped_msbuild.cmd'
-      arguments: '-msbuildEngine dotnet -onlyDocChanged $(onlyDocChanged)'
+      arguments: '-msbuildEngine dotnet -onlyDocChanged $(onlyDocChanged) -stage2Properties /mt'
     env:
       MSBUILDUSESERVER: "1"
 # Task to collect code coverage on Windows. Disabled by default due to being unstable and sometimes it stucks forever
@@ -261,72 +261,6 @@ jobs:
     continueOnError: true
     condition: eq(variables.onlyDocChanged, 0)
 
-- job: BootstrapMSBuildWithMTMode
-  displayName: "Linux Core Multithreaded Mode"
-  pool:
-    vmImage: 'ubuntu-latest'
-  timeoutInMinutes: 120
-  steps:
-  - template: azure-pipelines/check-documentation-only-change.yml
-  - ${{ if eq(variables['System.TeamProject'], 'DevDiv') }}:
-    - task: Bash@3
-      displayName: Setup Private Feeds Credentials
-      inputs:
-        filePath: $(Build.SourcesDirectory)/eng/common/SetupNugetSources.sh
-        arguments: $(Build.SourcesDirectory)/NuGet.config $Token
-      env:
-        Token: $(dn-bot-dnceng-artifact-feeds-rw)
-  - bash: sudo apt-get update
-  - bash: sudo apt-get install -y libxml2
-  - bash: . 'eng/cibuild_bootstrapped_msbuild.sh' --onlyDocChanged $(onlyDocChanged) --skipTests --stage2Properties '/mt /p:BuildAnalyzer=true'
-    displayName: 'Build with bootstrapped MSBuild and -mt mode'
-    condition: eq(variables.onlyDocChanged, 0)
-    env:
-      MSBUILDUSESERVER: "1"
-  - task: PublishBuildArtifacts@1
-    displayName: 'Publish Artifact: logs'
-    inputs:
-      PathtoPublish: 'artifacts/log/Debug'
-      ArtifactName: 'MTModeOnLinux build logs'
-    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:
@@ -436,7 +370,7 @@ jobs:
         Token: $(dn-bot-dnceng-artifact-feeds-rw)
   - bash: sudo apt-get update
   - bash: sudo apt-get install -y libxml2
-  - bash: . 'eng/cibuild_bootstrapped_msbuild.sh' --onlyDocChanged $(onlyDocChanged)
+  - bash: . 'eng/cibuild_bootstrapped_msbuild.sh' --onlyDocChanged $(onlyDocChanged) --stage2Properties '/mt'
     displayName: CI Build
     env:
         MSBUILDUSESERVER: "1"
@@ -524,7 +458,7 @@ jobs:
         arguments: $(Build.SourcesDirectory)/NuGet.config $Token
       env:
         Token: $(dn-bot-dnceng-artifact-feeds-rw)
-  - bash: . 'eng/cibuild_bootstrapped_msbuild.sh' --onlyDocChanged $(onlyDocChanged)
+  - bash: . 'eng/cibuild_bootstrapped_msbuild.sh' --onlyDocChanged $(onlyDocChanged) --stage2Properties '/mt'
     displayName: CI Build
     env:
         MSBUILDUSESERVER: "1"

From a9e4c8681e6988b4f7ff3527df84555b2accee2a Mon Sep 17 00:00:00 2001
From: Jan Provaznik <janprovaznik@microsoft.com>
Date: Mon, 4 May 2026 17:59:06 +0200
Subject: [PATCH 04/10] Trigger PR sync (no-op)


From a414f22bb2f39695eb11e10c8916030ebe6604a1 Mon Sep 17 00:00:00 2001
From: Jan Provaznik <janprovaznik@microsoft.com>
Date: Mon, 4 May 2026 18:07:22 +0200
Subject: [PATCH 05/10] Restore Linux MT-with-analyzer job, add /mt to
 FullReleaseOnWindows, fix -skipTests branch (#13602)
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Three follow-ups from review:

1. Restore the dedicated `BootstrapMSBuildWithMTMode` (Linux Core) job
   that runs `--stage2Properties /mt /p:BuildAnalyzer=true --skipTests`.
   This validates the ThreadSafeTaskAnalyzer in addition to /mt; the
   regular bootstrapped jobs do not enable the analyzer because it is
   more expensive.

2. Add `/mt` to `FullReleaseOnWindows` (both cibuild.cmd invocations).
   Now every PR-CI job that builds the repo runs with /mt, except the
   Source-Build template (which is intentionally untouched).

3. Fix a regression in `cibuild_bootstrapped_msbuild.ps1` where
   `-skipTests` accidentally also passed `/p:CreateBootstrap=false`,
   diverging from the bash reference. Restored the bash semantics:
     - onlyDocChanged=1 → bootstrap not created
     - skipTests       → bootstrap IS created, tests omitted
     - default         → bootstrap created, tests run

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
---
 .vsts-dotnet-ci.yml                  | 37 ++++++++++++++++++++++++++--
 eng/cibuild_bootstrapped_msbuild.ps1 |  9 ++++++-
 2 files changed, 43 insertions(+), 3 deletions(-)

diff --git a/.vsts-dotnet-ci.yml b/.vsts-dotnet-ci.yml
index 172f05419cd..6d827738ce1 100644
--- a/.vsts-dotnet-ci.yml
+++ b/.vsts-dotnet-ci.yml
@@ -261,6 +261,39 @@ jobs:
     continueOnError: true
     condition: eq(variables.onlyDocChanged, 0)
 
+# Dedicated /mt validation that turns on the ThreadSafeTaskAnalyzer (BuildAnalyzer=true),
+# kept as a separate Linux Core job because the analyzer is more expensive and the regular
+# bootstrapped CI jobs (which all run /mt now too) do not need it.
+- job: BootstrapMSBuildWithMTMode
+  displayName: "Linux Core Multithreaded Mode"
+  pool:
+    vmImage: 'ubuntu-latest'
+  timeoutInMinutes: 120
+  steps:
+  - template: azure-pipelines/check-documentation-only-change.yml
+  - ${{ if eq(variables['System.TeamProject'], 'DevDiv') }}:
+    - task: Bash@3
+      displayName: Setup Private Feeds Credentials
+      inputs:
+        filePath: $(Build.SourcesDirectory)/eng/common/SetupNugetSources.sh
+        arguments: $(Build.SourcesDirectory)/NuGet.config $Token
+      env:
+        Token: $(dn-bot-dnceng-artifact-feeds-rw)
+  - bash: sudo apt-get update
+  - bash: sudo apt-get install -y libxml2
+  - bash: . 'eng/cibuild_bootstrapped_msbuild.sh' --onlyDocChanged $(onlyDocChanged) --skipTests --stage2Properties '/mt /p:BuildAnalyzer=true'
+    displayName: 'Build with bootstrapped MSBuild and -mt mode'
+    condition: eq(variables.onlyDocChanged, 0)
+    env:
+      MSBUILDUSESERVER: "1"
+  - task: PublishBuildArtifacts@1
+    displayName: 'Publish Artifact: logs'
+    inputs:
+      PathtoPublish: 'artifacts/log/Debug'
+      ArtifactName: 'MTModeOnLinux build logs'
+    continueOnError: true
+    condition: always()
+
 - job: FullReleaseOnWindows
   displayName: "Windows Full Release (no bootstrap)"
   variables:
@@ -288,13 +321,13 @@ jobs:
     displayName: cibuild.cmd
     inputs:
       filename: 'eng/cibuild.cmd'
-      arguments: '-configuration Release -test'
+      arguments: '-configuration Release -test /mt'
     condition: eq(variables.onlyDocChanged, 0)
   - task: BatchScript@1
     displayName: cibuild.cmd without test
     inputs:
       filename: 'eng/cibuild.cmd'
-      arguments: '-configuration Release'
+      arguments: '-configuration Release /mt'
     condition: eq(variables.onlyDocChanged, 1)
 # Task to collect code coverage on Windows. Disabled by default due to being unstable and sometimes it stucks forever
 #   - task: PowerShell@2
diff --git a/eng/cibuild_bootstrapped_msbuild.ps1 b/eng/cibuild_bootstrapped_msbuild.ps1
index cc91e1cd6de..250304335f4 100644
--- a/eng/cibuild_bootstrapped_msbuild.ps1
+++ b/eng/cibuild_bootstrapped_msbuild.ps1
@@ -129,10 +129,17 @@ try {
   # - Create bootstrap environment as it's required when also running tests
   # - $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.
+  # Branches mirror cibuild_bootstrapped_msbuild.sh exactly:
+  #   onlyDocChanged=1 → bootstrap not created (artifacts not needed downstream)
+  #   skipTests        → bootstrap IS created (downstream MAY consume it), tests omitted
+  #   default          → bootstrap created, tests run
   $stage2Args = if ($stage2Properties) { $stage2Properties -split '\s+' | Where-Object { $_ } } else { @() }
-  if ($onlyDocChanged -or $skipTests) {
+  if ($onlyDocChanged) {
     & $PSScriptRoot\Common\Build.ps1 -restore -build -ci /p:CreateBootstrap=false /nr:false @properties @stage2Args
   }
+  elseif ($skipTests) {
+    & $PSScriptRoot\Common\Build.ps1 -restore -build -ci /nr:false @properties @stage2Args
+  }
   else {
     & $PSScriptRoot\Common\Build.ps1 -restore -build -test -ci /nr:false @properties @stage2Args
   }

From abbb981e15eed2e9cbf74c709f197bbdeb5e842f Mon Sep 17 00:00:00 2001
From: Jan Provaznik <janprovaznik@microsoft.com>
Date: Mon, 4 May 2026 18:30:21 +0200
Subject: [PATCH 06/10] Address Rainer's review: only enable -mt in 2-stage
 bootstrapped jobs (#13602)
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Revert /mt from pipelines that run a single MSBuild invocation against the
agent's stable VS MSBuild (no bootstrap). Those would test whatever /mt
support the agent's pre-installed MSBuild has, not the freshly built one.

Reverted:
* .vsts-dotnet-ci.yml: FullReleaseOnWindows (cibuild.cmd, single-stage with VS MSBuild)
* azure-pipelines/.vsts-dotnet-exp-perf.yml: experimental insertion (CIBuild.cmd, single-stage with VS MSBuild)
* azure-pipelines/.vsts-dotnet-build-jobs.yml: drop the now-unused additionalBuildArgs template parameter

Kept:
* All 4 bootstrapped PR-CI jobs (Linux/macOS/Windows Core/Windows Full) — these
  do stage 1 with VS/SDK MSBuild then stage 2 with the freshly built bootstrapped
  MSBuild, which is the correct surface to exercise /mt.
* Dedicated BootstrapMSBuildWithMTMode (Linux + BuildAnalyzer=true) — unchanged.

Also clarify the script comment to call out that the apphost path requires
DOTNET_HOST_PATH, and explain why we don't switch to `dotnet msbuild` (the SDK
CLI mangles `/mt` argument splitting).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
---
 .vsts-dotnet-ci.yml                         |  4 ++--
 azure-pipelines/.vsts-dotnet-build-jobs.yml |  4 ----
 azure-pipelines/.vsts-dotnet-exp-perf.yml   |  1 -
 eng/cibuild_bootstrapped_msbuild.ps1        | 13 +++++++++----
 4 files changed, 11 insertions(+), 11 deletions(-)

diff --git a/.vsts-dotnet-ci.yml b/.vsts-dotnet-ci.yml
index 5a894ca144f..518e53c89a3 100644
--- a/.vsts-dotnet-ci.yml
+++ b/.vsts-dotnet-ci.yml
@@ -327,13 +327,13 @@ jobs:
     displayName: cibuild.cmd
     inputs:
       filename: 'eng/cibuild.cmd'
-      arguments: '-configuration Release -test /mt'
+      arguments: '-configuration Release -test'
     condition: eq(variables.onlyDocChanged, 0)
   - task: BatchScript@1
     displayName: cibuild.cmd without test
     inputs:
       filename: 'eng/cibuild.cmd'
-      arguments: '-configuration Release /mt'
+      arguments: '-configuration Release'
     condition: eq(variables.onlyDocChanged, 1)
 # Task to collect code coverage on Windows. Disabled by default due to being unstable and sometimes it stucks forever
 #   - task: PowerShell@2
diff --git a/azure-pipelines/.vsts-dotnet-build-jobs.yml b/azure-pipelines/.vsts-dotnet-build-jobs.yml
index f95fb422203..c8fffe6904b 100644
--- a/azure-pipelines/.vsts-dotnet-build-jobs.yml
+++ b/azure-pipelines/.vsts-dotnet-build-jobs.yml
@@ -12,9 +12,6 @@ parameters:
 - name: enableOptProf
   type: boolean
   default: false
-- name: additionalBuildArgs
-  type: string
-  default: ''
 
 jobs:
 - job: Windows_NT
@@ -150,7 +147,6 @@ jobs:
               /p:GenerateSbom=true
               /p:SuppressFinalPackageVersion=${{ parameters.isExperimental }}
               /p:IsExperimental=${{ parameters.isExperimental }}
-              ${{ parameters.additionalBuildArgs }}
 
     displayName: Build
     condition: succeeded()
diff --git a/azure-pipelines/.vsts-dotnet-exp-perf.yml b/azure-pipelines/.vsts-dotnet-exp-perf.yml
index 953c5db4ee3..ac9aab87104 100644
--- a/azure-pipelines/.vsts-dotnet-exp-perf.yml
+++ b/azure-pipelines/.vsts-dotnet-exp-perf.yml
@@ -94,7 +94,6 @@ extends:
           enableComponentGovernance: false
           signTypeParameter: ${{ parameters.signTypeParameter }}
           enableOptProf: ${{ parameters.enableOptProf }}
-          additionalBuildArgs: '/mt'
 
     - template: /eng/common/templates-official/post-build/post-build.yml@self
       parameters:
diff --git a/eng/cibuild_bootstrapped_msbuild.ps1 b/eng/cibuild_bootstrapped_msbuild.ps1
index 250304335f4..ed290aec5cf 100644
--- a/eng/cibuild_bootstrapped_msbuild.ps1
+++ b/eng/cibuild_bootstrapped_msbuild.ps1
@@ -119,16 +119,21 @@ 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.
+  # Mirrors cibuild_bootstrapped_msbuild.sh:96. Required so the apphost-based child task host
+  # (NodeProviderOutOfProcTaskHost.ResolveAppHostOrFallback) can locate the runtime when the
+  # parent MSBuild is launched as `dotnet exec MSBuild.dll` (which, unlike the SDK CLI
+  # `dotnet msbuild`, does not set DOTNET_HOST_PATH automatically). Trying to switch the
+  # invocation to `dotnet msbuild` here would be cleaner but the SDK CLI's argument
+  # translation mangles `/mt` into `/m` + `t`, so we keep the explicit `MSBuild.dll` path
+  # and set DOTNET_HOST_PATH ourselves.
   $env:DOTNET_HOST_PATH=$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
   # - $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.
+  #   Use this for switches like /mt that should not be passed to the stable MSBuild used in stage 1
+  #   until a stable version of MT is available in the images.
   # Branches mirror cibuild_bootstrapped_msbuild.sh exactly:
   #   onlyDocChanged=1 → bootstrap not created (artifacts not needed downstream)
   #   skipTests        → bootstrap IS created (downstream MAY consume it), tests omitted

From 73362b3a85785bb92ebd3b0957e2ac1b0bdc921f Mon Sep 17 00:00:00 2001
From: Jan Provaznik <janprovaznik@microsoft.com>
Date: Mon, 4 May 2026 18:38:48 +0200
Subject: [PATCH 07/10] Fix /mt mangling: wrap -split result in @() to keep
 array even for one token (#13602)
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

`stage2Properties -split '\s+'` returns a string when there is exactly one
token, and `& cmd @stringVar` then splats that string's characters one per
argument — so `-stage2Properties "/mt"` reaches MSBuild as `/`, `m`, `t`.
Wrapping the result in @() keeps it an array (length 0/1/N all behave the
same), and `/mt` flows through unchanged.

Found by interactive repro:
  $a = "/mt" -split '\s+' | Where-Object { $_ }     # → string "/mt"
  & some.ps1 @a                                     # → arg list: "/", "m", "t"
  $a = @("/mt" -split '\s+' | Where-Object { $_ })  # → array ["/mt"]
  & some.ps1 @a                                     # → arg list: "/mt"

Verified end-to-end with `cibuild_bootstrapped_msbuild.ps1 -skipTests
-stage2Properties "/mt"` succeeding (Build succeeded, 0 errors).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
---
 eng/cibuild_bootstrapped_msbuild.ps1 | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/eng/cibuild_bootstrapped_msbuild.ps1 b/eng/cibuild_bootstrapped_msbuild.ps1
index ed290aec5cf..cc76dcece12 100644
--- a/eng/cibuild_bootstrapped_msbuild.ps1
+++ b/eng/cibuild_bootstrapped_msbuild.ps1
@@ -138,7 +138,11 @@ try {
   #   onlyDocChanged=1 → bootstrap not created (artifacts not needed downstream)
   #   skipTests        → bootstrap IS created (downstream MAY consume it), tests omitted
   #   default          → bootstrap created, tests run
-  $stage2Args = if ($stage2Properties) { $stage2Properties -split '\s+' | Where-Object { $_ } } else { @() }
+  # The @(...) wrapper is important: when -split returns exactly one element PowerShell
+  # gives back a string, and `& cmd @stringVar` splats its characters one-per-argument
+  # (so "/mt" becomes "/", "m", "t"). Wrapping with @() forces an array even for a
+  # single token.
+  $stage2Args = @(if ($stage2Properties) { $stage2Properties -split '\s+' | Where-Object { $_ } } else { @() })
   if ($onlyDocChanged) {
     & $PSScriptRoot\Common\Build.ps1 -restore -build -ci /p:CreateBootstrap=false /nr:false @properties @stage2Args
   }

From d4c72fc703b7f7e61061410586346de52542f62e Mon Sep 17 00:00:00 2001
From: Jan Provaznik <janprovaznik@microsoft.com>
Date: Mon, 4 May 2026 19:04:39 +0200
Subject: [PATCH 08/10] Use `dotnet msbuild` instead of `dotnet exec
 MSBuild.dll` (#13602)
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Per Rainer's suggestion. Cleaner: the SDK CLI sets DOTNET_HOST_PATH for the
MSBuild process automatically, so we don't need to export it ourselves and
can drop the Versions.props lookup for the bootstrap SDK MSBuild.dll path.

Verified locally that /mt flows through unmangled (no MSB1001, no
`/m t` argument mangling) and the build produces the same outputs:
binlog shows 243 Csc invocations, no errors. The earlier suspicion that
`dotnet msbuild` mangled /mt was a separate bug — fixed in 73362b3a85
(stage2Args splat).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
---
 eng/cibuild_bootstrapped_msbuild.ps1 | 13 +------------
 1 file changed, 1 insertion(+), 12 deletions(-)

diff --git a/eng/cibuild_bootstrapped_msbuild.ps1 b/eng/cibuild_bootstrapped_msbuild.ps1
index cc76dcece12..21ab6927d3e 100644
--- a/eng/cibuild_bootstrapped_msbuild.ps1
+++ b/eng/cibuild_bootstrapped_msbuild.ps1
@@ -87,9 +87,7 @@ try {
   else
   {
     $buildToolPath = "$bootstrapRoot\core\dotnet.exe"
-    $propsFile = Join-Path $PSScriptRoot "Versions.props"
-    $bootstrapSdkVersion = ([xml](Get-Content $propsFile)).SelectSingleNode("//PropertyGroup/BootstrapSdkVersion").InnerText
-    $buildToolCommand = "$bootstrapRoot\core\sdk\$bootstrapSdkVersion\MSBuild.dll"
+    $buildToolCommand = "msbuild"
     $buildToolFramework = "net"
 
     $env:DOTNET_ROOT="$bootstrapRoot\core"
@@ -119,15 +117,6 @@ try {
   # Opt into performance logging. https://github.com/dotnet/msbuild/issues/5900
   $env:DOTNET_PERFLOG_DIR=$PerfLogDir
 
-  # Mirrors cibuild_bootstrapped_msbuild.sh:96. Required so the apphost-based child task host
-  # (NodeProviderOutOfProcTaskHost.ResolveAppHostOrFallback) can locate the runtime when the
-  # parent MSBuild is launched as `dotnet exec MSBuild.dll` (which, unlike the SDK CLI
-  # `dotnet msbuild`, does not set DOTNET_HOST_PATH automatically). Trying to switch the
-  # invocation to `dotnet msbuild` here would be cleaner but the SDK CLI's argument
-  # translation mangles `/mt` into `/m` + `t`, so we keep the explicit `MSBuild.dll` path
-  # and set DOTNET_HOST_PATH ourselves.
-  $env:DOTNET_HOST_PATH=$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

From 96fff5dfb2e31ca732f4a4512e30d049c2a0c393 Mon Sep 17 00:00:00 2001
From: Jan Provaznik <janprovaznik@microsoft.com>
Date: Thu, 7 May 2026 17:56:13 +0200
Subject: [PATCH 09/10] Restore DOTNET_HOST_PATH script-env export (#13602)
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

CI on the previous commit revealed Windows test failures (MSB4216 launching .NET
task hosts, mostly net472 x86 testhosts spawning .NET Core MSBuild grandchildren
under /mt). Linux/macOS pass because cibuild_bootstrapped_msbuild.sh has been
exporting DOTNET_HOST_PATH at line 96 all along.

Restore the export in the .ps1 to match. Keep the `dotnet msbuild` invocation
from the prior cleanup — it's orthogonal: the SDK CLI sets DOTNET_HOST_PATH for
the MSBuild process it spawns, but does not propagate it into the parent script
environment, which is what test grandchildren inherit.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
---
 eng/cibuild_bootstrapped_msbuild.ps1 | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/eng/cibuild_bootstrapped_msbuild.ps1 b/eng/cibuild_bootstrapped_msbuild.ps1
index 21ab6927d3e..b30cb2f772f 100644
--- a/eng/cibuild_bootstrapped_msbuild.ps1
+++ b/eng/cibuild_bootstrapped_msbuild.ps1
@@ -117,6 +117,14 @@ try {
   # Opt into performance logging. https://github.com/dotnet/msbuild/issues/5900
   $env:DOTNET_PERFLOG_DIR=$PerfLogDir
 
+  # Mirrors cibuild_bootstrapped_msbuild.sh:96. Required for some test scenarios that spawn
+  # MSBuild grandchildren which need to launch .NET task hosts (notably net472 x86 testhosts
+  # invoking .NET Core MSBuild → /mt → sidecar TaskHost). The SDK CLI `dotnet msbuild` sets
+  # DOTNET_HOST_PATH for the MSBuild process it spawns, but does not propagate it into the
+  # parent script's environment, so we set it here for child processes (tests, their MSBuild
+  # grandchildren) to inherit.
+  $env:DOTNET_HOST_PATH=$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

From bfc4a0328953bdb0a01b752b97d2d84d43d9110d Mon Sep 17 00:00:00 2001
From: Jan Provaznik <janprovaznik@microsoft.com>
Date: Thu, 7 May 2026 18:45:19 +0200
Subject: [PATCH 10/10] Apply Option B: revert dotnet msbuild back to dotnet
 exec MSBuild.dll (#13602)
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Option A (restoring DOTNET_HOST_PATH script-env export) didn't fix the
Windows Core / Windows Full failures. Both jobs still fail in the same
SDK-style EndToEnd tests (net10.0 x64) under /mt, while the same tests
pass on Linux Core. The non-SDK variants and net472 x86 testhost all
pass.

Reverting `dotnet msbuild` → `dotnet exec MSBuild.dll` brings the .ps1
back to the bash script's exact shape (which works on Linux/macOS).
This is the lowest-risk way to determine whether the SDK CLI's argument
forwarding contributes to the test process tree breakage.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
---
 eng/cibuild_bootstrapped_msbuild.ps1 | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/eng/cibuild_bootstrapped_msbuild.ps1 b/eng/cibuild_bootstrapped_msbuild.ps1
index b30cb2f772f..871ca5b741a 100644
--- a/eng/cibuild_bootstrapped_msbuild.ps1
+++ b/eng/cibuild_bootstrapped_msbuild.ps1
@@ -87,7 +87,9 @@ try {
   else
   {
     $buildToolPath = "$bootstrapRoot\core\dotnet.exe"
-    $buildToolCommand = "msbuild"
+    $propsFile = Join-Path $PSScriptRoot "Versions.props"
+    $bootstrapSdkVersion = ([xml](Get-Content $propsFile)).SelectSingleNode("//PropertyGroup/BootstrapSdkVersion").InnerText
+    $buildToolCommand = "$bootstrapRoot\core\sdk\$bootstrapSdkVersion\MSBuild.dll"
     $buildToolFramework = "net"
 
     $env:DOTNET_ROOT="$bootstrapRoot\core"