# `dotnet test --max-parallel-test-assemblies` forwarded to MSBuild as unknown switch when mixing `/p:` properties (regression in SDK 10.0.103)

[TRB7]

Summary

When running dotnet test against a solution file (.sln) with both --max-parallel-test-assemblies and /p: MSBuild properties in the same command, SDK 10.0.103 forwards --max-parallel-test-assemblies verbatim to the underlying MSBuild invocation as a raw command-line switch. MSBuild does not recognize it, causing a hard MSB1001 failure.

This was working correctly in prior SDK versions (confirmed working in .NET 8/9).

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Environment

SDK version	10.0.103 (/usr/share/dotnet/sdk/10.0.103)
OS	Ubuntu (GitHub Actions ubuntu-latest)
Invocation target	Solution file (.sln)

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Steps to reproduce

dotnet test MyApp.sln \
  --configuration Release \
  --no-build \
  --verbosity normal \
  --max-parallel-test-assemblies 1 \
  /p:CollectCoverage=true \
  /p:CoverletOutputFormat=opencover

The presence of at least one /p: property alongside --max-parallel-test-assemblies appears to be required to trigger the issue (the /p: properties cause dotnet test to route through the MSBuild VSTest target).

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Expected behavior

--max-parallel-test-assemblies 1 is consumed by the dotnet test CLI host (as it was in .NET 8/9) and is not forwarded to MSBuild. Tests run with a maximum of 1 parallel assembly.

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Actual behavior

dotnet test forwards --max-parallel-test-assemblies 1 as a raw argument appended to the MSBuild invocation after the solution file path:

MSBUILD : error MSB1001: Unknown switch.
  Full command line: '/usr/share/dotnet/sdk/10.0.103/MSBuild.dll
    -maxcpucount
    --verbosity:m
    -tlp:default=auto
    --property:VSTestLogger="trx%3bLogFilePrefix=test-results"
    --property:VSTestNoBuild=true
    --property:VSTestResultsDirectory="..."
    --property:Configuration=Release
    --property:CollectCoverage=true
    --property:CoverletOutputFormat=opencover
    --property:VSTestVerbosity=normal
    --property:VSTestArtifactsProcessingMode=collect
    --property:VSTestSessionCorrelationId=...
    --target:VSTest
    --verbosity:normal
    -nodereuse:false
    -nologo
    MyApp.sln
    --max-parallel-test-assemblies 1'   ← should never reach MSBuild

Switches appended by response files:
Switch: --max-parallel-test-assemblies

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Regression

This worked correctly in .NET SDK 8.x and 9.x. The --max-parallel-test-assemblies flag was introduced specifically for dotnet test and should be fully consumed before any MSBuild delegation occurs.

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Workaround

Replace the CLI flag with the equivalent MSBuild property that the VSTest target natively understands:

# Remove:  --max-parallel-test-assemblies 1
# Add:     /p:VSTestMaxCpuCount=1

dotnet test MyApp.sln \
  --configuration Release \
  --no-build \
  --verbosity normal \
  /p:VSTestMaxCpuCount=1 \
  /p:CollectCoverage=true \
  /p:CoverletOutputFormat=opencover

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Additional context

The issue appears to be in the argument parsing/forwarding layer of dotnet test when the command is delegated to MSBuild's VSTest target via a solution file. The flag is not being stripped from the argument list before the MSBuild invocation is constructed.