UEFN crash: EXCEPTION_ACCESS_VIOLATION in python311.dll right after a successful Verse Build All

General Issues and Bug Reporting
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1

Summary

The editor was driven through the built-in Model Context Protocol server (editor toolsets, call_tool):

  1. 08:15:44 to 08:15:48: 128 calls of ValkyrieToolset.VerseToolset.WriteFile (writing .verse files).
  2. 08:15:51: ValkyrieToolset.VerseToolset.BuildAll.
  3. 08:15:59: “Global Verse compile (incremental, 2 packages compiled in 8691.5 ms) finished: SUCCESS”,
    then “VerseBuild: SUCCESS – Build complete.” The digests were written.
  4. 08:16:01, about 1 s later, on the same frame (848): the editor crashed.

So the build itself succeeded. The crash happens while the result returns to Python (the toolset / MCP dispatch).

Error

Unhandled Exception: EXCEPTION_ACCESS_VIOLATION reading address 0x00007465736c6f18
[Callstack] python311.dll!UnknownFunction  (x4, top of stack)
[Callstack] UnrealEditorFortnite-Engine-Win64-Shipping.dll!UnknownFunction  (x10)
[Callstack] UnrealEditorFortnite-Common-Win64-Shipping.dll!UnknownFunction
[Callstack] UnrealEditorFortnite-Win64-Shipping.exe!UnknownFunction  (x6)
[Callstack] KERNEL32.DLL!UnknownFunction
LogExit: Executing StaticShutdownAfterError

The bad address 0x00007465736c6f18 holds ASCII bytes (“olset”, as in “toolset”). That looks like string data used as a
pointer: a freed or overwritten Python object in the toolset’s Python layer.

Please select what you are reporting on:

Unreal Editor for Fortnite

What Type of Bug are you experiencing?

Verse

Steps to Reproduce

Repro (not yet confirmed as 100%)

  • Open a big Verse project.
  • Through the editor’s MCP server, write many Verse files in a burst (VerseToolset.WriteFile), then call
    VerseToolset.BuildAll from the same Python-driven call.
  • The editor crashes about 1 s after “Build complete”.

Expected Result

no crash

Observed Result

crash

Platform(s)

PC

Additional Notes

  • The log has repeated LogIas: LatencyTest Error ... x509 verification error (20) for the Epic CDN endpoints
    throughout. They were already present before the crash, so they are probably unrelated.
  • The last UI focus was the Message Log tab (UserActivityHint).
  • The editor relaunched fine at 11:23.
2

hi @Wertandrew

On my PC with UEFN 42.40 running my Python version is 3.14 as below

Screenshot (3)
Screenshot (3)387×174 7.51 KB

Possibly something has downgraded your Python from version 3.14.6 to version 3.11

Thanks

3

@Wertandrew,

If you can try updating Python back to 3.14.6 and let us know if that fixes the crashing
From there I’ll post the findings internally

4

My PC already has Python 3.14.8 installed system-wide, which is newer than 3.14.6. As far as I can tell, though, UEFN doesn’t use the system Python: it runs its own bundled interpreter, Python 3.11.8 under Fortnite\Engine\Binaries\ThirdParty\Python3\Win64, which ships with the Fortnite install.

Could you confirm which Python installation you mean and how it relates to the crash we reported? If the editor needs a different bundled version, is there a supported way to update it, or will it come with a UEFN update? I’d rather not replace files inside the Fortnite install myself.

6

hi @Wertandrew , @rhyan.smith

You cannot update to any newer version than 3.11 because its used as described in the Epic Doc

Scripting the Unreal Editor Using Python | Unreal Engine 5.8 Documentation - Why use Python 3.11| Epic Developer Community

Therefore upgrading to latest would probably crash Fortnite/UEFN.

7

Got it

I’ve sent this issue in internally to the team
Should hear back from them soon

8

Are you able to provide us with the CRASHGUID?
(This also applies to future crash reports)

It should look something like this (example):

Session CrashGUID >   UECC-Windows-8E82580948B6B9C0FA978DA4E9F4610B
9

hi

The path to find these crashes is in Windows File Explorer paste the file path

%USERPROFILE%\AppData\Local\UnrealEditorFortnite\Saved\Crashes

There seems a problem that only 32 are stored on my pc so the most recent may not have stored the current crash. 

Check the most recent crash date
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UECC-Windows-33A0E16843081E2E242FBB9DC85B5106_0000

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FORT-1174293 is ‘Closed’ as ‘Fixed’. The issue will be addressed in .