Synthesis doesn't even try to run "External Program" type patcher

[lerarosalene]

What happens:

1. I have "External Program" patcher that I want to run through Synthesis
2. I add it to Synthesis, specifying full path to the program
3. I press run
4. After several seconds I get "Errored" state and this log:

("C:\Users\HOME\projects\SmallSaltPile.Patcher\bin\Debug\net9.0"): "C:\Users\HOME\projects\SmallSaltPile.Patcher\bin\Debug\net9.0\SmallSaltPile.Patcher.exe" "run-patcher --DataFolderPath \"C:\Program Files (x86)\Steam\steamapps\common\Skyrim Special Edition\Data\" --ExtraDataFolder \"C:\env\Synthesis\Data\Skyrim Special Edition\SmallSaltPile.Patcher\" --FormIDRangeMode Auto --GameRelease SkyrimSE --ModKey Synthesis.esp --LoadOrderFilePath C:\Users\HOME\AppData\Local\Temp\Synthesis\0cbkqdi3.xeu\Workspace\Synthesis\Plugins.txt --LoadOrderIncludesCreationClub --OutputPath \"C:\Users\HOME\AppData\Local\Temp\Synthesis\0cbkqdi3.xeu\Workspace\Synthesis\1 - SmallSaltPile.Patcher\Synthesis.esp\" --PatcherName SmallSaltPile.Patcher --SourcePath \"C:\Users\HOME\AppData\Local\Temp\Synthesis\0cbkqdi3.xeu\Workspace\Seed Mod\Synthesis.esp\""

However, I know for a fact that Synthesis didn't even manage to run my patcher, because I added MessageBox there before any other code. If I run it manually via Terminal or Win+R, it appears. Via Synthesis? Nothing, zilch. Furthermore, Procmon shows that even long after patch attempt it just continues to spam these two operations on my patcher:

Furthermore, I can't simply change my patcher's type to "Local Solution" because .NET 10 doesn't work with Synthesis+MO2 setup, and I am not uninstalling it just to run a patch.

What do? How to make Synthesis work without making modifications to my system (e.g. renaming/uninstalling .NET 10) or migrating my whole modlist from MO2 to something worse?

[lerarosalene]

However, I know for a fact that Synthesis didn't even manage to run my patcher

This was bold and, ironically, unfactual claim. Several facts for everyone else who will encounter the same problem:

1. It's not that Synthesis + MO2 setup just can't compile .NET 10 patches, it seems that no .NET 10 OR .NET 9 program can run through MO2 at all.
2. You can and you will forget filters that you had set up in Procmon, such is life.

TL;DR: Synthesis was running my program. Program, however, was crashing even before managed code was able to run. Procmon had filters that prevented "Process Create" event from showing up, leading me to believe it was Synthesis error. If you want your patches to run in Synthesis under MO2, set target framework to .NET 8 or earlier.

I am closing this issue since it's a user error and not Synthesis problem.

[Noggog]

Heya @lerarosalene

I think Mo2 can run DotNet 10 apps. I think they just need some tweaks to overcome some issues Mo2 has, ala this issue #521

Can you try running your app with that build variable active?

Also, what error did you see? The one you posted just seems to be a printout of the args being sent to the patcher. Not actually an error. If we can distill the error out again, I can probably add a new error pane UI. The new prereleases of Synth have a nicer way to show various error readouts. Might add one for the CET issue to alert others in the future better

[lerarosalene]

Also, what error did you see? The one you posted just seems to be a printout of the args being sent to the patcher. Not actually an error.

That's the thing — it was the only line in the log. Nothing besides that. When I was trying to run my patcher later directly through MO2 I also saw no error — 2-3 second lag and then crash without errors in the stderr/stdout.

[lerarosalene]

Can you try running your app with that build variable active?

Yup, I will check tomorrow, thanks. If it's really CET compat in my case, which seems likely, the exit code should be consistent between program runs, different programs, and probably target frameworks. If that's helpful for error handling on your side, I can verify that tomorrow as well.

[Noggog]

Cool! Thanks for the testing. And yeah, any info you can give will help add the error pane. I'll probably test it myself too when i get to it, but good to get all the info we can

[lerarosalene]

1. <CETCompat>false</CETCompat> actually helped and allowed my patcher to run under MO2 seamlessly. So, this one works.
2. Sadly, error code that I got from trying to run .NET 9/10 executable with CETCompat set to true through MO2 (the original problem) via a batch script is just generic 0xc0000005 access violation. I am not sure you can judge anything just by that. I also wasn't able to find anything more specific in Windows Event Log, but I guess parsing it on patcher crash is a bit overkill anyways.

[Noggog]

I think even a 0xc0000005 code is enough to show a panel saying "It's probably CETCompat, read more". Does this error show up in the logs/display of Synthesis? If so, we can probably just sniff for it there and add the panel

[lerarosalene]

Does this error show up in the logs/display of Synthesis?

2026-04-02 22:51:51.652 +03:00 [INF] ================= Starting Patcher SmallSaltPile.Patcher Run =================
2026-04-02 22:51:51.654 +03:00 [INF] run-patcher --DataFolderPath "C:\Program Files (x86)\Steam\steamapps\common\Skyrim Special Edition\Data" --FormIDRangeMode Auto --GameRelease SkyrimSE --ModKey Synthesis.esp --LoadOrderFilePath C:\Users\HOME\AppData\Local\Temp\Synthesis\0cbkqdi3.xeu\Workspace\Synthesis\Plugins.txt --LoadOrderIncludesCreationClub --OutputPath "C:\Users\HOME\AppData\Local\Temp\Synthesis\0cbkqdi3.xeu\Workspace\Synthesis\1 - SmallSaltPile.Patcher\Synthesis.esp" --PatcherName SmallSaltPile.Patcher --SourcePath "C:\Users\HOME\AppData\Local\Temp\Synthesis\0cbkqdi3.xeu\Workspace\Seed Mod\Synthesis.esp" --TargetLanguage English
2026-04-02 22:51:51.654 +03:00 [INF] Starting
2026-04-02 22:51:51.655 +03:00 [INF] (C:\Users\HOME\projects\SmallSaltPile.Patcher\bin\Debug\net9.0): C:\Users\HOME\projects\SmallSaltPile.Patcher\bin\Debug\net9.0\SmallSaltPile.Patcher.exe run-patcher --DataFolderPath "C:\Program Files (x86)\Steam\steamapps\common\Skyrim Special Edition\Data" --ExtraDataFolder "C:\env\Synthesis\Data\Skyrim Special Edition\SmallSaltPile.Patcher" --FormIDRangeMode Auto --GameRelease SkyrimSE --ModKey Synthesis.esp --LoadOrderFilePath C:\Users\HOME\AppData\Local\Temp\Synthesis\0cbkqdi3.xeu\Workspace\Synthesis\Plugins.txt --LoadOrderIncludesCreationClub --OutputPath "C:\Users\HOME\AppData\Local\Temp\Synthesis\0cbkqdi3.xeu\Workspace\Synthesis\1 - SmallSaltPile.Patcher\Synthesis.esp" --PatcherName SmallSaltPile.Patcher --SourcePath "C:\Users\HOME\AppData\Local\Temp\Synthesis\0cbkqdi3.xeu\Workspace\Seed Mod\Synthesis.esp"
2026-04-02 22:51:54.868 +03:00 [ERR] Error while running: 
2026-04-02 22:51:54.869 +03:00 [INF] Finished patcher run
2026-04-02 23:03:34.946 +03:00 [INF] Executing shutdown
2026-04-02 23:03:34.988 +03:00 [INF] Disposing container
2026-04-02 23:03:35.044 +03:00 [INF] Disposing repository jobs
2026-04-02 23:03:35.048 +03:00 [INF] No in flight repository jobs
2026-04-02 23:03:35.049 +03:00 [INF] Finished disposing repository jobs

I think even a 0xc0000005 code is enough to show a panel saying "It's probably CETCompat, read more"

I guess, it depends? For "External Program" type patcher you generally don't know if it's CET-related or regular segfault from sloppy memory-related code. For normal "Local Solution"/"Git Repository" patchers, I guess, if 0xc0000005 shows up during compilation it says a lot.