Summary
In Unreal Engine 5.8.1, the experimental Unreal MCP ObjectTools.get_properties tool can crash the Unreal Editor if a JSON Schema object returned by ObjectTools.list_properties is mistakenly passed to the properties argument.
Instead of rejecting the invalid argument and returning a tool/MCP error, the conversion path attempts to convert the entire JSON Schema to an Unreal FName.
In my case, the schema was 71,624 characters long, causing the following assertion:
FName’s 1023 max length exceeded.
Got 71624 characters excluding null-terminator.
The crash occurred while querying the WorldSettings object in Lvl_ThirdPerson.
ObjectTools.list_properties itself did not crash the Editor. The crash occurred on the subsequent ObjectTools.get_properties call.
Expected behavior:
get_properties should validate its properties argument and return an invalid-argument/tool error when it receives a schema object instead of property-name strings. Invalid MCP input should not crash the Unreal Editor.
What Type of Bug are you experiencing?
Editor
Steps to Reproduce
-
Launch Unreal Engine 5.8.1 on Windows with the experimental ModelContextProtocol, EditorToolset, and ToolsetRegistry plugins enabled.
-
Start the Unreal MCP server and connect an MCP client.
-
Open Lvl_ThirdPerson and obtain its WorldSettings object.
-
Call:
editor_toolset.toolsets.object.ObjectTools.list_properties
for the WorldSettings object. -
list_properties returns a JSON Schema object similar to:
{
“returnValue”: {
“visibilityCellSize”: {
“type”: “integer”,
“description”: “…”
},
“defaultGameMode”: {
“type”: “object”,
“description”: “…”
}
}
}
-
Pass the returned schema object itself to the “properties” argument of:
editor_toolset.toolsets.object.ObjectTools.get_properties
through the MCP call_tool interface, instead of passing an array/list of property-name strings. -
Unreal Editor asserts and crashes.
Expected Result
ObjectTools.get_properties should validate the “properties” argument before attempting Unreal type conversion.
If a JSON Schema object or any other invalid type is supplied instead of valid property-name strings, the MCP tool should return an invalid-argument/tool-call error.
Invalid MCP input should not crash the Unreal Editor process.
Observed Result
Unreal Editor crashes with an assertion while ObjectTools.get_properties is processing the invalid input.
Assertion failed: false
FName’s 1023 max length exceeded.
Got 71624 characters excluding null-terminator:
{“visibilityCellSize”:{“type”:“integer”,“description”:…
The Script Stack identifies:
/EditorToolset/Python/editor_toolset/toolsets/object_PY.ObjectTools.get_properties
ObjectTools.list_properties itself did not crash the Editor. The crash occurred on the subsequent get_properties call after the returned JSON Schema object was incorrectly passed as the properties argument.
Affects Versions
5.8
Platform(s)
Windows
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For crash reports, include your callstack
Assertion failed: false
FName’s 1023 max length exceeded.
Got 71624 characters excluding null-terminator.
UnrealEditor_Core
UnrealEditor_Core
UnrealEditor_Core
UnrealEditor_Core
UnrealEditor_CoreUObject
UnrealEditor_PythonScriptPlugin
UnrealEditor_PythonScriptPlugin
UnrealEditor_PythonScriptPlugin
UnrealEditor_PythonScriptPlugin
UnrealEditor_PythonScriptPlugin
UnrealEditor_PythonScriptPlugin
UnrealEditor_PythonScriptPlugin
python311
python311
python311
python311
python311
python311
python311
python311
python311
UnrealEditor_PythonScriptPlugin
UnrealEditor_PythonScriptPlugin
UnrealEditor_CoreUObject
UnrealEditor_ToolsetRegistry
UnrealEditor_ToolsetRegistry
UnrealEditor_ToolsetRegistry
UnrealEditor_ToolsetRegistry
UnrealEditor_ModelContextProtocolEditor
UnrealEditor_ModelContextProtocolEditor
UnrealEditor_ModelContextProtocol
UnrealEditor_ModelContextProtocol
UnrealEditor_ModelContextProtocol
UnrealEditor_ModelContextProtocol
UnrealEditor_HTTPServer
UnrealEditor_HTTPServer
UnrealEditor_HTTPServer
UnrealEditor_HTTPServer
UnrealEditor_HTTPServer
UnrealEditor_HTTPServer
UnrealEditor_HTTPServer
UnrealEditor_Core
UnrealEditor_Core
UnrealEditor_Core
UnrealEditor
UnrealEditor
UnrealEditor
UnrealEditor
UnrealEditor
UnrealEditor
kernel32
ntdll
Additional Notes
Environment:
- Unreal Engine 5.8.1-56057345
- Windows 11
- Unreal MCP (Experimental)
- MCP Protocol: 2025-11-25
- MCP endpoint: http://127.0.0.1:8000/mcp
- Toolset: editor_toolset.toolsets.object.ObjectTools
- Target object: WorldSettings in Lvl_ThirdPerson
Log analysis showed the following sequence:
- ObjectTools.list_properties was executed on WorldSettings.
- list_properties returned a large JSON Schema object.
- Some JSON Schema generation warnings were logged for unsupported delegate/property types.
- The client incorrectly treated the returned schema object as if it were a list of property names.
- That schema object was then passed to ObjectTools.get_properties through call_tool.
- ObjectTools.get_properties did not reject the invalid argument.
- The Python/Unreal conversion path attempted to convert the JSON Schema to FName.
- The schema was 71,624 characters long, exceeding the FName maximum length of 1023 characters.
- Unreal Editor asserted and crashed.
The relevant Script Stack points to:
/EditorToolset/Python/editor_toolset/toolsets/object_PY.ObjectTools.get_properties
ObjectTools.list_properties itself did not crash the Editor. The crash occurred during the subsequent get_properties call.
I understand that the immediate trigger involved incorrect client-side handling of the list_properties return value. However, I am reporting this because invalid MCP/tool input should be rejected safely with an argument validation error instead of causing the entire Unreal Editor process to assert and crash.
I also found related community reports about very large list_properties responses and other ObjectTools issues, but I could not find an existing report for this specific get_properties → FName max-length Editor crash.
CallStack Hash:
88D15CF8406991E96208CCD391A5FA32543F1E9D