[AR-May]

Context

 -multiThreaded  /  -mt  was a presence-only switch: any occurrence turned multi-threaded mode on, and there was no way to turn it back off. That blocks turning MT on by default later, because users would have no opt-out — including anyone who inherits  -mt  from an auto-response file or a repo's  Directory.Build.rsp . It also meant  -mt:false  silently enabled MT, the opposite of what it reads like.

 MSBUILDFORCEMULTITHREADED=1  was the only environment opt-in, and it was documented as "equivalent to passing  -mt ". Once the switch can express  false , "equivalent to  -mt " and "force" are two different things and need two different knobs.

Changes Made

1. -mt  is now a real boolean switch: -mt  and  -mt:true  enable;  -mt:false  disables. Repeated occurrences take the last value, matching  -nodeReuse  and  -m . This holds across response files as well: the command line beats the auto-response file and  Directory.Build.rsp .

2. Now we have two environment variables with distinct meanings
   MSBUILDFORCEMULTITHREADED=1 is Authoritative — forces MT on
   MSBUILDENABLEMULTITHREADED=1 is Soft default — MT on unless told otherwise

Resulting precedence: force env var → last  -mt  value → enable env var → off.

Testing

New unit tests

[Copilot]

Pull request overview

Updates MSBuild’s experimental multi-threaded mode switch (-multiThreaded / -mt) to behave like a real boolean switch (including response-file precedence), and splits environment configuration into “force” vs “soft default” knobs to enable a future default-on story without removing opt-out.

Changes:

• Make -mt/-multithreaded support :true|false (including last-one-wins semantics and correct response-file vs command-line precedence).
• Add MSBUILDENABLEMULTITHREADED=1 as a soft default while keeping MSBUILDFORCEMULTITHREADED=1 authoritative.
• Add resources + unit tests + documentation updates for the new parsing/precedence behavior.

Reviewed changes

Copilot reviewed 20 out of 20 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/MSBuild/XMake.cs	Implements precedence logic (force env var → last -mt value → enable env var → off) and adds invalid-value handling via ProcessBooleanSwitch.
src/MSBuild/CommandLine/CommandLineParser.cs	Treats -mt/-multithreaded as having an optional parameter by defaulting missing values to :true.
src/MSBuild.UnitTests/XMake_Tests.cs	Adds unit tests for boolean parsing, last-one-wins behavior, env-var precedence, and invalid-value rejection.
src/Framework/Traits.cs	Introduces EnableMultiThreaded trait backed by MSBUILDENABLEMULTITHREADED.
src/MSBuild/Resources/Strings.resx	Updates help text to show `-multithreaded[:true
src/MSBuild/Resources/xlf/Strings.cs.xlf	Localized help/source update and adds InvalidMultiThreadedValue trans-unit.
src/MSBuild/Resources/xlf/Strings.de.xlf	Localized help/source update and adds InvalidMultiThreadedValue trans-unit.
src/MSBuild/Resources/xlf/Strings.es.xlf	Localized help/source update and adds InvalidMultiThreadedValue trans-unit.
src/MSBuild/Resources/xlf/Strings.fr.xlf	Localized help/source update and adds InvalidMultiThreadedValue trans-unit.
src/MSBuild/Resources/xlf/Strings.it.xlf	Localized help/source update and adds InvalidMultiThreadedValue trans-unit.
src/MSBuild/Resources/xlf/Strings.ja.xlf	Localized help/source update and adds InvalidMultiThreadedValue trans-unit.
src/MSBuild/Resources/xlf/Strings.ko.xlf	Localized help/source update and adds InvalidMultiThreadedValue trans-unit.
src/MSBuild/Resources/xlf/Strings.pl.xlf	Localized help/source update and adds InvalidMultiThreadedValue trans-unit.
src/MSBuild/Resources/xlf/Strings.pt-BR.xlf	Localized help/source update and adds InvalidMultiThreadedValue trans-unit.
src/MSBuild/Resources/xlf/Strings.ru.xlf	Localized help/source update and adds InvalidMultiThreadedValue trans-unit.
src/MSBuild/Resources/xlf/Strings.tr.xlf	Localized help/source update and adds InvalidMultiThreadedValue trans-unit.
src/MSBuild/Resources/xlf/Strings.zh-Hans.xlf	Localized help/source update and adds InvalidMultiThreadedValue trans-unit.
src/MSBuild/Resources/xlf/Strings.zh-Hant.xlf	Localized help/source update and adds InvalidMultiThreadedValue trans-unit.
documentation/wiki/MSBuild-Environment-Variables.md	Documents the split between force (MSBUILDFORCEMULTITHREADED) and soft default (MSBUILDENABLEMULTITHREADED).
documentation/specs/multithreading/multithreaded-msbuild.md	Updates the multithreading spec to reflect new switch + env-var semantics and precedence.

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[JanProvaznik]

[AR-May]

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