[Evangelink]

Related to #56837.

MTP has the same preparation gap: dotnet test -mt forwards the switch to its main build, but the shared MSBuildSession never applies it to BuildParameters. Restore, device discovery, deployment, and ComputeRunArguments therefore ignore explicit MT settings, including --no-build preparation.

Apply recognized explicit switches using MSBuild's existing parser and environment precedence. Preserve the single-node limit, target-produced project state, and one coherent preparation binlog; do not add response-file discovery or depend on the unmerged run-command PR.

A flag-only fix is insufficient for MTP's long-lived session. The bundled engine's strict-mode scope owns the process current directory until EndBuild and permits only one active strict build per process. Restoring CWD between requests causes MSB4286; interleaving another in-process build causes MSB4289. Keep MT's CWD ownership intact and run intervening build/restore operations out-of-process at the captured invocation directory. Existing execution policy is unchanged when no MT preparation session is active, and relative arguments/log paths keep their original meaning.

Add real-product regressions for aliases, quoted/empty values, duplicate precedence, environment overrides, per-target execution mode, relative project paths, device listing, file-based --no-build, multi-project/multi-TFM binlogs, and restore/build/deployment/run-argument failures. Update CLI contributor guidance with the session invariant.

Validation

Windows x64, built SDK 12.0.100-dev, bootstrap SDK 12.0.100-alpha.1.26480.102, bundled Microsoft.Build 18.13.0-1.26528.111, and MTP 2.6.0-preview.26531.5. No dependency versions changed.

• Focused dotnet.Tests: 169 passed, 0 failed, 0 skipped. These execute the assembled SDK and assert actual target working directories, module results, exit codes, log placement, and binlog structure.
• Microsoft.DotNet.Cli.Utils.Tests: 236 passed, 0 failed, 7 existing platform/issue skips.
• Managed CLI and the dotnet-aot.csproj managed/shared-source compilation passed without warnings.
• Native publishing was not validated: the default Windows build encounters the missing C++ platform linker in the installer finalizer. The managed redist was assembled with native publishing disabled; no product feature flags or strict-mode checks were disabled.

Commands used for the final validation:

$env:DOTNET_ADD_GLOBAL_TOOLS_TO_PATH = '0'
.\eng\build.ps1 -c Debug /p:PublishAot=false /p:_ShouldPublishDotnetAot=false /bl:artifacts\log\Debug\managed-baseline.binlog
.\.dotnet\dotnet.exe build src\Cli\dotnet\dotnet.csproj -c Debug --nologo -v:minimal -bl:artifacts\log\Debug\mtp-mt-lifecycle.binlog
Copy-Item artifacts\bin\Microsoft.DotNet.Cli.Utils\Debug\net12.0\Microsoft.DotNet.Cli.Utils.dll artifacts\bin\redist\Debug\dotnet\sdk\12.0.100-dev\Microsoft.DotNet.Cli.Utils.dll
.\.dotnet\dotnet.exe scripts\RunTests.cs -- --project test\dotnet.Tests\dotnet.Tests.csproj --filter 'FullyQualifiedName~GivenDotnetTestProducesBinaryLog|FullyQualifiedName~GivenDotnetTestRunsFileBasedApps|FullyQualifiedName~LoggerUtilityTests|FullyQualifiedName~GivenCommandServices|FullyQualifiedName~DotnetMsbuildInProcTests|FullyQualifiedName~GivenDotnetTestSelectsDevice.ItFailsWhen'
.\.dotnet\dotnet.exe scripts\RunTests.cs -- --project test\Microsoft.DotNet.Cli.Utils.Tests\Microsoft.DotNet.Cli.Utils.Tests.csproj
.\.dotnet\dotnet.exe build src\Cli\dotnet-aot\dotnet-aot.csproj -c Debug --nologo -v:minimal -bl:artifacts\log\Debug\mtp-mt-aot-managed.binlog

All commands above exited 0. TRX and build binlogs are retained under artifacts\log\test-runs\dotnet.Tests\20261102-093421-154276 and artifacts\log\test-runs\Microsoft.DotNet.Cli.Utils.Tests\20261102-095013-255352.

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[Copilot]

🟢 Approval recommended

No active review issues block approval of the MTP preparation changes.

No open issues 🎉

What changed in this PR

This PR makes Microsoft.Testing.Platform (MTP) preparation honor MSBuild’s explicit -mt settings and environment precedence, including --no-build, while preserving the shared preparation session and binlog.

Changes:

• Applies multithreading settings to preparation without adding automatic response-file discovery.
• Preserves strict-mode working-directory ownership by running intervening builds and restores out-of-process at the captured invocation directory.
• Adds regression coverage for switch precedence, device operations, file-based apps, failures, and binlogs, and documents the session invariant.

File	Description
test/​dotnet.Tests/​CommandTests/​Test/​GivenDotnetTestSelectsDevice.cs	Covers deployment and run-argument failures with MT enabled and disabled.
test/​dotnet.Tests/​CommandTests/​Test/​GivenDotnetTestRunsFileBasedApps.cs	Verifies MT preparation for file-based --no-build runs.
test/​dotnet.Tests/​CommandTests/​Test/​GivenDotnetTestProducesBinaryLog.cs	Tests switch precedence, preparation execution mode, failures, and coherent binlogs.
src/​Cli/​Microsoft.DotNet.Cli.Utils/​MSBuildForwardingAppWithoutLogging.cs	Supports out-of-process execution in a specified working directory.
src/​Cli/​dotnet/​MSBuildSession.cs	Applies MT settings and preserves strict-mode directory ownership.
src/​Cli/​dotnet/​Commands/​Test/​MTP/​MSBuildUtility.cs	Resolves input paths and routes intervening builds through session-aware execution.
src/​Cli/​dotnet/​Commands/​Restore/​RestoringCommand.cs	Propagates out-of-process execution to separate restore and main build operations.
src/​Cli/​dotnet/​Commands/​MSBuild/​MSBuildForwardingApp.cs	Exposes working-directory-aware out-of-process forwarding.
src/​Cli/​AGENTS.md	Documents MTP preparation lifecycle and regression requirements.

_{Review effort: Balanced}

[JanProvaznik]

overall reasonable

should we get this also for release/11.0.2xx branch ?

[Evangelink]

/backport to release/11.0.1xx

[github-actions]

Started backporting to release/11.0.1xx (link to workflow run)

[github-actions]

@Evangelink backporting to release/11.0.1xx failed, the patch most likely resulted in conflicts. Please backport manually!

git am output

$ git cherry-pick 2f0da079350a63d165b990e0cbfb3ead6bd09a58

Auto-merging src/Cli/AGENTS.md
CONFLICT (content): Merge conflict in src/Cli/AGENTS.md
Auto-merging src/Cli/Microsoft.DotNet.Cli.Utils/MSBuildForwardingAppWithoutLogging.cs
CONFLICT (content): Merge conflict in src/Cli/Microsoft.DotNet.Cli.Utils/MSBuildForwardingAppWithoutLogging.cs
Auto-merging src/Cli/dotnet/Commands/MSBuild/MSBuildForwardingApp.cs
Auto-merging src/Cli/dotnet/Commands/Restore/RestoringCommand.cs
Auto-merging src/Cli/dotnet/Commands/Test/MTP/MSBuildUtility.cs
CONFLICT (content): Merge conflict in src/Cli/dotnet/Commands/Test/MTP/MSBuildUtility.cs
Auto-merging src/Cli/dotnet/MSBuildSession.cs
Auto-merging test/dotnet.Tests/CommandTests/Test/GivenDotnetTestProducesBinaryLog.cs
CONFLICT (modify/delete): test/dotnet.Tests/CommandTests/Test/GivenDotnetTestRunsFileBasedApps.cs deleted in HEAD and modified in 2f0da07935 (Honor MSBuild -mt in MTP test preparation (#56893)).  Version 2f0da07935 (Honor MSBuild -mt in MTP test preparation (#56893)) of test/dotnet.Tests/CommandTests/Test/GivenDotnetTestRunsFileBasedApps.cs left in tree.
Auto-merging test/dotnet.Tests/CommandTests/Test/GivenDotnetTestSelectsDevice.cs
error: could not apply 2f0da07935... Honor MSBuild -mt in MTP test preparation (#56893)
hint: After resolving the conflicts, mark them with
hint: "git add/rm <pathspec>", then run
hint: "git cherry-pick --continue".
hint: You can instead skip this commit with "git cherry-pick --skip".
hint: To abort and get back to the state before "git cherry-pick",
hint: run "git cherry-pick --abort".
hint: Disable this message with "git config set advice.mergeConflict false"


$ git am --3way --empty=keep --ignore-whitespace --keep-non-patch changes.patch

Applying: Honor MSBuild -mt in MTP test preparation
Using index info to reconstruct a base tree...
M	src/Cli/AGENTS.md
M	src/Cli/Microsoft.DotNet.Cli.Utils/MSBuildForwardingAppWithoutLogging.cs
M	src/Cli/dotnet/Commands/MSBuild/MSBuildForwardingApp.cs
M	src/Cli/dotnet/Commands/Restore/RestoringCommand.cs
M	src/Cli/dotnet/Commands/Test/MTP/MSBuildUtility.cs
M	src/Cli/dotnet/MSBuildSession.cs
M	test/dotnet.Tests/CommandTests/Test/GivenDotnetTestProducesBinaryLog.cs
A	test/dotnet.Tests/CommandTests/Test/GivenDotnetTestRunsFileBasedApps.cs
M	test/dotnet.Tests/CommandTests/Test/GivenDotnetTestSelectsDevice.cs
Falling back to patching base and 3-way merge...
Auto-merging src/Cli/AGENTS.md
CONFLICT (content): Merge conflict in src/Cli/AGENTS.md
Auto-merging src/Cli/Microsoft.DotNet.Cli.Utils/MSBuildForwardingAppWithoutLogging.cs
CONFLICT (content): Merge conflict in src/Cli/Microsoft.DotNet.Cli.Utils/MSBuildForwardingAppWithoutLogging.cs
Auto-merging src/Cli/dotnet/Commands/MSBuild/MSBuildForwardingApp.cs
Auto-merging src/Cli/dotnet/Commands/Restore/RestoringCommand.cs
Auto-merging src/Cli/dotnet/Commands/Test/MTP/MSBuildUtility.cs
CONFLICT (content): Merge conflict in src/Cli/dotnet/Commands/Test/MTP/MSBuildUtility.cs
Auto-merging src/Cli/dotnet/MSBuildSession.cs
Auto-merging test/dotnet.Tests/CommandTests/Test/GivenDotnetTestProducesBinaryLog.cs
CONFLICT (modify/delete): test/dotnet.Tests/CommandTests/Test/GivenDotnetTestRunsFileBasedApps.cs deleted in HEAD and modified in Honor MSBuild -mt in MTP test preparation.  Version Honor MSBuild -mt in MTP test preparation of test/dotnet.Tests/CommandTests/Test/GivenDotnetTestRunsFileBasedApps.cs left in tree.
Auto-merging test/dotnet.Tests/CommandTests/Test/GivenDotnetTestSelectsDevice.cs
error: Failed to merge in the changes.
hint: Use 'git am --show-current-patch=diff' to see the failed patch
hint: When you have resolved this problem, run "git am --continue".
hint: If you prefer to skip this patch, run "git am --skip" instead.
hint: To restore the original branch and stop patching, run "git am --abort".
hint: Disable this message with "git config set advice.mergeConflict false"
Patch failed at 0001 Honor MSBuild -mt in MTP test preparation
Error: The process '/usr/bin/git' failed with exit code 128

Link to workflow output