Stryker crashes dotnet runtime with MTP and concurrent testing on MacOS

[richardwerkman]

Describe the bug
On MacOS dotnet crashes when concurrency is high during MTP testing with Stryker. On my machine it crashes consistently when running with MTP against Polly.Specs. This seems related to dotnet/runtime#132581

I've used Claude to analyze this and it seems unrelated to the current Stryker implementation. It is a genuine bug in the dotnet runtime that surfaces in this specific usecase.

There are a few workarounds available:

• Setting the concurrency to 1 seems to stop the issue from happening. But that costs a lot of performance. I've tried with -c 2, -c 4, -c 7 and they all fail at some point, it just takes a while. With -c 20 the issue happens very quick.
• Set env variable DOTNET_INTERNAL_ThreadSuspendInjection=0 before starting Stryker. This seems to prevent the issue from happening.
• I have a working workaround in code that wraps Stryker in a second process on MacOS and restarts the process when it crashes. It also sets the DOTNET_INTERNAL_ThreadSuspendInjection variable. It seems to work, but is a weird piece of code to include in stryker...

Logs
There are no useful Stryker logs for this. It is the dotnet runtime that is crashing. The stryker logging just stops at a random point without any exception being thrown. Below I attached a Mac crash report.

dotnet-2026-08-28-114857.txt

Expected behavior
I expect to be able to use concurrency on MacOS.

Desktop (please complete the following information):

• OS: MacOS 26.6.2
• Type of project: Core
• Framework Version: Core 6.0 (runtime 10.0)
• Stryker Version: latest main

Additional context
Add any other context about the problem here.

[dupdob]

The report states it is a segfault, which is not possible from Stryker code, but could be due to unsafe code in some dependency.

You should try another run time version and see where it goes