diff --git a/eng/build-utils.ps1 b/eng/build-utils.ps1
index e40cf36fd9f..644171f1eb4 100644
--- a/eng/build-utils.ps1
+++ b/eng/build-utils.ps1
@@ -241,6 +241,8 @@ function Make-BootstrapBuild() {
 
     # prepare FsLex and Fsyacc and AssemblyCheck
     $dotnetPath = InitializeDotNetCli
+    # Ensure apphosts spawned by this publish see the repo-local runtime even if the caller forgot DOTNET_ROOT.
+    $env:DOTNET_ROOT = $dotnetPath
     $dotnetExe = Join-Path $dotnetPath "dotnet.exe"
 
     # prepare compiler
diff --git a/eng/build.sh b/eng/build.sh
index e404208df89..0ae1c1b766d 100755
--- a/eng/build.sh
+++ b/eng/build.sh
@@ -384,6 +384,9 @@ trap TrapAndReportError EXIT
 
 InitializeDotNetCli $restore
 
+# Apphosts (bootstrap fsc, testhost, etc.) resolve runtimes via DOTNET_ROOT, not PATH.
+export DOTNET_ROOT="$DOTNET_INSTALL_DIR"
+
 # Resolve product TFM from centralized source of truth if not overridden via --tfm
 if [[ "$tfm" == "" ]]; then
   tfm=$("$DOTNET_INSTALL_DIR/dotnet" msbuild "$scriptroot/TargetFrameworks.props" -getProperty:FSharpNetCoreProductTargetFramework 2>/dev/null | tr -d '[:space:]')