x86 testhost fails to load runtime (hostfxr 0x800700C1) in 18.9.0-preview via vstest.console.exe — regression of #15235 / dotnet/sdk#49436 #16151

Summary

An x86 test project still fails to launch its testhost with hostfxr error 0x800700C1 (ERROR_BAD_EXE_FORMAT) when using Microsoft.TestPlatform.TestHost / Microsoft.NET.Test.Sdk 18.9.0-preview-26319-02, run via vstest.console.exe.

The x86 testhost.x86.exe resolves the runtime from a 64-bit .NET install (the one DOTNET_ROOT points at) and then tries to load the x64 hostfxr.dll into the x86 process, which fails with 0x800700C1 (bad image format / architecture mismatch).

This looks like the same regression as the now-closed #15235, which was closed as a duplicate of dotnet/sdk#49436. That SDK issue is marked completed (closed 2025-10-27), but we are still hitting it with the 18.9.0-preview-26319-02 package and via the vstest.console.exe path (not only dotnet test --arch x86). So either the fix has not flowed into this preview of the TestPlatform packages, or the console-host x86 runtime resolution path is not covered by the fix.

Versions

• Microsoft.TestPlatform.TestHost = 18.9.0-preview-26319-02
• Microsoft.NET.Test.Sdk = 18.9.0-preview-26319-02
• Last known-good for the same project: Microsoft.TestPlatform.TestHost 17.6.1 / Microsoft.NET.Test.Sdk 17.2.0
• Runner: vstest.console.exe
• OS: Windows x64; DOTNET_ROOT points to an x64 .NET install (no separate x86 runtime on DOTNET_ROOT(x86))

Affected project shape

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <TargetFramework>net8.0</TargetFramework>
    <PlatformTarget>x86</PlatformTarget>
    <IsTestProject>true</IsTestProject>
  </PropertyGroup>
  <ItemGroup>
    <PackageReference Include="Microsoft.NET.Test.Sdk" />
    <PackageReference Include="xunit" />
  </ItemGroup>
</Project>

The test assembly is x86 (PlatformTarget x86) on a net8.0 TFM, executed through vstest.console.exe.

Error

Testhost process for source(s) '<path>\Tests.dll' exited with error: Failed to load the dll from [<dotnet-x64-root>\host\fxr\10.0.8\hostfxr.dll], HRESULT: 0x800700C1
The library hostfxr.dll was found, but loading it from <dotnet-x64-root>\host\fxr\10.0.8\hostfxr.dll failed
  - Installing .NET prerequisites might help resolve this problem.
     https://go.microsoft.com/fwlink/?linkid=798306
Failed to resolve hostfxr.dll [<dotnet-x64-root>\host\fxr\10.0.8\hostfxr.dll]. Error code: 0x80008082
. Please check the diagnostic logs for more information.
Test Run Aborted.

Note the x86 testhost rolls forward to a .NET 10 runtime and resolves the x64 hostfxr.dll (the only one under DOTNET_ROOT), then fails to load it into the x86 process.

Expected

The x86 testhost should resolve and load an x86 runtime/hostfxr.dll (e.g. via DOTNET_ROOT(x86), registry, or default x86 install location), or fail with a clear, actionable diagnostic — not attempt to load the x64 hostfxr.dll into an x86 process.

Notes / ask

• Reverting the test SDK/host to 17.x makes x86 execution work again, which is how we are unblocking our build for now.
• Please confirm whether the [NetTestingE2E][Regression] Error "The library hostfxr.dll was found..." will throw when executing "dotnet test --arch x86 --diag:log1.txt". dotnet/sdk#49436 fix is included in 18.9.0-preview-26319-02, and whether the vstest.console.exe (not just dotnet test) x86 host-resolution path is covered. If not, this is worth investigating as a continuation/incomplete-fix of x86 builds no longer work from 17.14.0 #15235 / [NetTestingE2E][Regression] Error "The library hostfxr.dll was found..." will throw when executing "dotnet test --arch x86 --diag:log1.txt". dotnet/sdk#49436.

Related

• x86 builds no longer work from 17.14.0 #15235 (closed as duplicate)
• [NetTestingE2E][Regression] Error "The library hostfxr.dll was found..." will throw when executing "dotnet test --arch x86 --diag:log1.txt". dotnet/sdk#49436 (closed as completed 2025-10-27)