diff --git a/eng/Build.ps1 b/eng/Build.ps1
index d7053f7da5a..a81b55acdad 100644
--- a/eng/Build.ps1
+++ b/eng/Build.ps1
@@ -89,6 +89,7 @@ if ($ci -and -not $PSBoundParameters.ContainsKey('msbuildMultiThreaded')) {
 }
 $BuildCategory = ""
 $BuildMessage = ""
+$bootstrapMultiThreaded = !$PSBoundParameters.ContainsKey('msbuildMultiThreaded') -or $msbuildMultiThreaded
 
 $desktopTargetFramework = "net472"
 # Read product TFM from centralized source of truth via MSBuild
@@ -827,7 +828,7 @@ try {
 
     if ($bootstrap) {
         $script:BuildMessage = "Failure building bootstrap compiler"
-        $bootstrapDir = Make-BootstrapBuild
+        $bootstrapDir = Make-BootstrapBuild -msbuildMultiThreaded $bootstrapMultiThreaded
     }
 
     $script:BuildMessage = "Failure building product"
diff --git a/eng/build-utils.ps1 b/eng/build-utils.ps1
index 644171f1eb4..42c4a879247 100644
--- a/eng/build-utils.ps1
+++ b/eng/build-utils.ps1
@@ -232,7 +232,7 @@ function Run-MSBuild([string]$projectFilePath, [string]$buildArgs = "", [string]
 #
 # Important to not set $script:bootstrapDir here yet as we're actually in the process of
 # building the bootstrap.
-function Make-BootstrapBuild() {
+function Make-BootstrapBuild([bool]$msbuildMultiThreaded = $true) {
     Write-Host "Building bootstrap '$bootstrapTfm' compiler with '$fsharpNetCoreProductTfm' .NET Core product TFM"
 
     $dir = Join-Path $ArtifactsDir "Bootstrap"
@@ -248,6 +248,9 @@ function Make-BootstrapBuild() {
     # prepare compiler
     $projectpath = "$RepoRoot" + "proto.proj"
     $args = "publish `"$projectpath`" -c $bootstrapConfiguration"
+    if ($msbuildMultiThreaded) {
+        $args += " -mt"
+    }
     if ($binaryLog) {
         $logFilePath = Join-Path $LogDir "bootstrap.binlog"
         $args += " /bl:`"$logFilePath`""
diff --git a/eng/build.sh b/eng/build.sh
index 2af459b621e..e10dbe95923 100755
--- a/eng/build.sh
+++ b/eng/build.sh
@@ -220,6 +220,7 @@ while [[ $# > 0 ]]; do
 done
 
 # Import Arcade functions
+bootstrap_multi_threaded=${msbuild_multi_threaded:-true}
 . "$scriptroot/common/tools.sh"
 
 function Test() {
@@ -334,6 +335,9 @@ function BuildSolution {
 
     BuildMessage="Error building tools"
     local args=("publish" "$repo_root/proto.proj" "$blrestore" "$bltools" "/p:Configuration=Proto" "/p:DotNetBuild=$product_build" "/p:DotNetBuildSourceOnly=$source_build" "/p:DotNetBuildFromVMR=$from_vmr" ${properties[@]+"${properties[@]}"})
+    if [[ "$(NormalizeBoolArg "$bootstrap_multi_threaded")" == true ]]; then
+      args+=("-mt")
+    fi
     echo $args
     "$DOTNET_INSTALL_DIR/dotnet" "${args[@]}" || exit $?
   fi
diff --git a/eng/test-determinism.ps1 b/eng/test-determinism.ps1
index 1984acc01a7..2450adeae61 100644
--- a/eng/test-determinism.ps1
+++ b/eng/test-determinism.ps1
@@ -406,6 +406,7 @@ function TryDownloadDotnetFrameworkSdk() {
 }
 
 try {
+  $msbuildMultiThreaded = $true
   . (Join-Path $PSScriptRoot "build-utils.ps1")
 
   # Create all of the logging directories
diff --git a/tests/EndToEndBuildTests/MultithreadedTasks/README.md b/tests/EndToEndBuildTests/MultithreadedTasks/README.md
index e8340336c8a..3f6a070e53a 100644
--- a/tests/EndToEndBuildTests/MultithreadedTasks/README.md
+++ b/tests/EndToEndBuildTests/MultithreadedTasks/README.md
@@ -34,7 +34,8 @@ Both reuse their built products and publish the SDK evidence. There are no separ
 The existing Linux and macOS build jobs use Arcade's `--mt true` option.
 The three `Plain_Build_*` SDK jobs pass `-mt` directly to MSBuild.
 Those plain builds use SDK-shipped tasks; the redirected product builds and this harness exercise the migrated tasks.
-VS/MSBuild.exe jobs and Arcade-managed source-build configuration are unchanged.
+Bootstrap publishing uses MT by default on Windows and Unix; explicit `-mt 0` / `--mt false` build options disable it. ILVerify and both compiler determinism modes explicitly opt into MSBuild MT. The determinism modes still compare the same F# compiler flags or sequential versus parallel compilation, independently of MSBuild scheduling.
+Other VS/MSBuild.exe product builds and Arcade-managed source-build configuration are unchanged.
 No repository-wide environment override or replacement for Arcade's MT controls is added.
 
 Use `--repetitions 10` for local stress runs.
diff --git a/tests/ILVerify/ilverify.ps1 b/tests/ILVerify/ilverify.ps1
index 70e331b06c3..8be69f41b2e 100644
--- a/tests/ILVerify/ilverify.ps1
+++ b/tests/ILVerify/ilverify.ps1
@@ -39,7 +39,7 @@ Write-Host "Checking whether running on Windows: $IsWindows"
 Write-Host "Repository path: $repo_path"
 
 [string] $script = if ($IsWindows) { Join-Path $repo_path "build.cmd" } else { Join-Path $repo_path "build.sh" }
-[string] $additional_arguments = if ($IsWindows) { "-noVisualStudio -ci -bootstrap" } else { "" }
+[string[]] $additional_arguments = if ($IsWindows) { "-noVisualStudio", "-ci", "-bootstrap", "-mt", "1" } else { "--mt", "true" }
 
 # Set environment variable to disable UpdateXlf target (not needed for IL verification)
 $env:UpdateXlfOnBuild = "false"
@@ -76,11 +76,7 @@ if ($LASTEXITCODE -ne 0) {
 # Run build script for each configuration (NOTE: We don't build Proto)
 foreach ($configuration in $configurations) {
     Write-Host "Building $configuration configuration..."
-    if ($additional_arguments) {
-        & $script -c $configuration $additional_arguments
-    } else {
-        & $script -c $configuration
-    }
+    & $script -c $configuration @additional_arguments
     if ($LASTEXITCODE -ne 0 -And $LASTEXITCODE -ne '') {
         Write-Host "Build failed for $configuration configuration (last exit code: $LASTEXITCODE)."
         exit 1