Add CI job for 2-stage build with -mt multithreaded mode #13124

Copilot · 2026-01-27T09:25:09Z

Context

End-to-end validation is needed for MSBuild's experimental -mt (multithreaded) mode. Currently no CI check verifies that MSBuild can build itself using this mode.

Changes Made

Added new CI job BootstrapMSBuildWithMTMode in .vsts-dotnet-ci.yml:

2-stage bootstrap build with /mt flag passed only to Stage 2 (bootstrapped MSBuild)
Runs on Linux (ubuntu-latest) for faster/cheaper builds
Publishes build logs as artifacts for debugging
Tests are skipped - the job only performs the 2-stage build

Added new parameters to cibuild_bootstrapped_msbuild.sh:

--stage2Properties: Allows passing MSBuild properties only to Stage 2 (the bootstrapped build)
--skipTests: Switch to skip running tests in Stage 2

. 'eng/cibuild_bootstrapped_msbuild.sh' --onlyDocChanged $(onlyDocChanged) --skipTests --stage2Properties '/mt'

Testing

YAML syntax validated. The job itself validates the feature by building MSBuild with -mt mode on each PR.

Notes

The /mt flag is passed via the new --stage2Properties parameter which is only splatted to Stage 2 builds. This is necessary because the /mt switch is not compatible with the SDK MSBuild used in Stage 1.

The --skipTests switch ensures the CI job only performs the 2-stage build and uploads logs, without running the full test suite.

Original prompt

This section details on the original issue you should resolve

<issue_title>Add a CI job that does a 2 stage build of msbuild, the second stage with bootstrapped -mt mode</issue_title>
<issue_description>### Summary

add validating msbuild can build itself with -mt mode

Background and Motivation

The purpose is an an end to end test for validating -mt mode migrations

Proposed Feature

.vsts-dotnet-ci.yml defines required PR checks
Add one that first builds msbuild with build.cmd, and then builds another copy with -mt mode, it should have the similar config to Windows core job

Ideally it would be extensible to be able to include another repository to build with -mt mode

Alternative Designs

No response</issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes Add a CI job that does a 2 stage build of msbuild, the second stage with bootstrapped -mt mode #13123

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.

dotnet-policy-service · 2026-01-27T09:25:39Z

Hello @@copilot, I noticed that you’re changing an .swr file or any file under src/Package/MSBuild.VSSetup.. Please make sure to validate this change by an experimental VS insertion. This is accomplished by pushing to an exp/* branch, which requires write permissions to this repo.

Co-authored-by: JanProvaznik <[email protected]>

JanProvaznik · 2026-01-27T09:39:13Z

the job is failing @copilot

Check failure on line 72 in .dotnet\sdk\10.0.101\Microsoft.Common.CrossTargeting.targets

@azure-pipelines
azure-pipelines
/ msbuild-pr (Windows Core Multithreaded Mode)
.dotnet\sdk\10.0.101\Microsoft.Common.CrossTargeting.targets#L72

.dotnet\sdk\10.0.101\Microsoft.Common.CrossTargeting.targets(72,5): error MSB3202: The project file "Microsoft.Build.Framework.csproj" was not found.
Check failure on line 72 in .dotnet\sdk\10.0.101\Microsoft.Common.CrossTargeting.targets

@azure-pipelines
azure-pipelines
/ msbuild-pr (Windows Core Multithreaded Mode)
.dotnet\sdk\10.0.101\Microsoft.Common.CrossTargeting.targets#L72

.dotnet\sdk\10.0.101\Microsoft.Common.CrossTargeting.targets(72,5): error MSB3202: The project file "Microsoft.Build.Framework.csproj" was not found.
Check failure on line 72 in .dotnet\sdk\10.0.101\Microsoft.Common.CrossTargeting.targets

@azure-pipelines
azure-pipelines
/ msbuild-pr (Windows Core Multithreaded Mode)
.dotnet\sdk\10.0.101\Microsoft.Common.CrossTargeting.targets#L72

.dotnet\sdk\10.0.101\Microsoft.Common.CrossTargeting.targets(72,5): error MSB3202: The project file "Microsoft.Build.Framework.csproj" was not found.
Check failure on line 1844 in .dotnet\sdk\10.0.101\Microsoft.Common.CurrentVersion.targets

@azure-pipelines
azure-pipelines
/ msbuild-pr (Windows Core Multithreaded Mode)
.dotnet\sdk\10.0.101\Microsoft.Common.CurrentVersion.targets#L1844

.dotnet\sdk\10.0.101\Microsoft.Common.CurrentVersion.targets(1844,5): error MSB3202: (NETCORE_ENGINEERING_TELEMETRY=Build) The project file "..\Framework\Microsoft.Build.Framework.csproj" was not found.
Check failure on line 1844 in .dotnet\sdk\10.0.101\Microsoft.Common.CurrentVersion.targets

@azure-pipelines
azure-pipelines
/ msbuild-pr (Windows Core Multithreaded Mode)
.dotnet\sdk\10.0.101\Microsoft.Common.CurrentVersion.targets#L1844

.dotnet\sdk\10.0.101\Microsoft.Common.CurrentVersion.targets(1844,5): error MSB3202: (NETCORE_ENGINEERING_TELEMETRY=Build) The project file "..\Utilities\Microsoft.Build.Utilities.csproj" was not found.

Co-authored-by: JanProvaznik <[email protected]>

Copilot · 2026-01-27T09:44:44Z

the job is failing @copilot

Check failure on line 72 in .dotnet\sdk\10.0.101\Microsoft.Common.CrossTargeting.targets
...

Fixed in a0f9761. The issue was that /mt was being passed to both Stage 1 and Stage 2, but /mt doesn't work with the SDK MSBuild used in Stage 1.

Added a new stage2Properties parameter to cibuild_bootstrapped_msbuild.ps1 that passes properties only to the Stage 2 bootstrapped build. Now Stage 1 runs normally and Stage 2 runs with /mt.

JanProvaznik · 2026-01-27T10:32:22Z

@copilot I don't want this new CI job to run tests, only do the 2 stage build and upload logs from building

Co-authored-by: JanProvaznik <[email protected]>

Copilot · 2026-01-27T10:35:21Z

@copilot I don't want this new CI job to run tests, only do the 2 stage build and upload logs from building

Fixed in 567faf6. Added a new -skipTests switch parameter to cibuild_bootstrapped_msbuild.ps1 and updated the CI job to use it. The job now only performs the 2-stage build and uploads logs without running tests.

Copilot

Pull request overview

This PR adds CI validation for MSBuild's experimental -mt (multithreaded) mode by introducing a new Windows CI job that performs a 2-stage bootstrap build with the /mt flag applied only to the second stage.

Changes:

Added new CI job BootstrapMSBuildWithMTMode that builds MSBuild using the dotnet engine with /mt flag in Stage 2
Extended cibuild_bootstrapped_msbuild.ps1 with skipTests switch and stage2Properties parameter to support custom Stage 2-only MSBuild properties
Stage 1 intentionally excludes stage2Properties since /mt is incompatible with the SDK MSBuild used in Stage 1

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
`.vsts-dotnet-ci.yml`	Adds new CI job `BootstrapMSBuildWithMTMode` on Windows 2022 that performs 2-stage build with `/mt` flag, publishes build logs, and skips tests
`eng/cibuild_bootstrapped_msbuild.ps1`	Adds `skipTests` switch and `stage2Properties` string array parameter; applies `stage2Properties` only to Stage 2 builds with clear comments explaining the exclusion from Stage 1

rainersigwald · 2026-01-27T21:25:58Z

.vsts-dotnet-ci.yml

+- job: BootstrapMSBuildWithMTMode
+  displayName: "Windows Core Multithreaded Mode"
+  pool:
+    vmImage: 'windows-2022'


Do we care about Windows specifically? If not it might be cheaper or faster to use a Linux queue for this.

JanProvaznik · 2026-01-29T17:50:30Z

@copilot make the job run on linux instead

Co-authored-by: JanProvaznik <[email protected]>

Copilot · 2026-01-29T17:54:37Z

@copilot make the job run on linux instead

Done in d2e4f7e. Changed the CI job to run on Linux (ubuntu-latest) instead of Windows. Also added --skipTests and --stage2Properties support to cibuild_bootstrapped_msbuild.sh to match the PowerShell script functionality.

JanProvaznik · 2026-01-29T18:16:03Z

eng/cibuild_bootstrapped_msbuild.ps1

  [bool] $buildStage1 = $True,
  [bool] $onlyDocChanged = 0,
+  [switch] $skipTests,
+  [string[]] $stage2Properties = @(),


revert @copilot yagni

Reverted in e0f78f1. The PowerShell script is back to its original state since the job now runs on Linux.

Co-authored-by: JanProvaznik <[email protected]>

Initial plan

562e011

Copilot AI assigned Copilot and JanProvaznik Jan 27, 2026

Copilot started work on behalf of JanProvaznik January 27, 2026 09:25 View session

Add CI job for 2-stage build with -mt multithreaded mode

7471a3f

Co-authored-by: JanProvaznik <[email protected]>

Copilot AI changed the title ~~[WIP] Add CI job for two-stage msbuild with -mt mode~~ Add CI job for 2-stage build with -mt multithreaded mode Jan 27, 2026

Copilot AI requested a review from JanProvaznik January 27, 2026 09:30

Copilot finished work on behalf of JanProvaznik January 27, 2026 09:30

Copilot started work on behalf of JanProvaznik January 27, 2026 09:39 View session

Fix CI job: pass /mt only to Stage 2 via new stage2Properties parameter

a0f9761

Co-authored-by: JanProvaznik <[email protected]>

Copilot finished work on behalf of JanProvaznik January 27, 2026 09:45

Copilot started work on behalf of JanProvaznik January 27, 2026 10:32 View session

Add -skipTests switch to skip running tests in Stage 2 build

567faf6

Co-authored-by: JanProvaznik <[email protected]>

Copilot finished work on behalf of JanProvaznik January 27, 2026 10:36

JanProvaznik approved these changes Jan 27, 2026

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JanProvaznik marked this pull request as ready for review January 27, 2026 11:22

JanProvaznik requested a review from a team as a code owner January 27, 2026 11:22

Copilot AI review requested due to automatic review settings January 27, 2026 11:22

Copilot started reviewing on behalf of JanProvaznik January 27, 2026 11:23 View session

Copilot AI reviewed Jan 27, 2026

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This was referenced Jan 27, 2026

sdk tasks migration to the ThreadSafe attribute, simple tasks, part 1 dotnet/sdk#52554

Open

Create a CI job that builds composite repos with MSBuild's -mt mode dotnet/dotnet#4451

Open

rainersigwald reviewed Jan 29, 2026

View reviewed changes

Copilot started work on behalf of JanProvaznik January 29, 2026 17:50 View session

Switch CI job from Windows to Linux for faster/cheaper builds

d2e4f7e

Co-authored-by: JanProvaznik <[email protected]>

Copilot AI requested a review from JanProvaznik January 29, 2026 17:55

Copilot finished work on behalf of JanProvaznik January 29, 2026 17:55

JanProvaznik reviewed Jan 29, 2026

View reviewed changes

Copilot started work on behalf of JanProvaznik January 29, 2026 18:16 View session

Revert cibuild_bootstrapped_msbuild.ps1 to original state (YAGNI)

e0f78f1

Co-authored-by: JanProvaznik <[email protected]>

Copilot AI requested a review from JanProvaznik January 29, 2026 18:18

Copilot finished work on behalf of JanProvaznik January 29, 2026 18:18

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Add CI job for 2-stage build with -mt multithreaded mode #13124

Add CI job for 2-stage build with -mt multithreaded mode #13124

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Context

Changes Made

Testing

Notes

Background and Motivation

Proposed Feature

Alternative Designs

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