Summary
The built-in Unreal MCP server in UEFN 42.00 accepts connections, processes MCP requests correctly server-side (confirmed via editor log), and sends valid HTTP response headers — including a correct content-length and a fresh Mcp-Session-Id — but never actually writes the response body before the connection is reset. This means no MCP client (Claude Code, Cursor, or a raw HTTP client) can ever complete an initialize handshake or receive results from tools/list, even though the server-side logic runs successfully.
Please select what you are reporting on:
Unreal Editor for Fortnite
What Type of Bug are you experiencing?
Other
Steps to Reproduce
Enable Python Editor Scripting and UEFN MCP Toolsets in Project Settings.
Open a project in UEFN with Auto Start Server enabled (or run ModelContextProtocol.StartServer).
From an external client, send a valid JSON-RPC initialize request via HTTP POST to http://127.0.0.1:8000/mcp.
Observe the response.
Expected Result
The client receives the full JSON-RPC response body (matching the advertised content-length), gets a valid Mcp-Session-Id, and can proceed to send the initialized notification and tools/list.
Observed Result
The server immediately replies HTTP/1.1 200, with headers including Mcp-Session-Id: and content-length: 279.
No body bytes arrive — confirmed by holding the raw TCP socket open and reading for 60+ seconds.
The next write attempt on that same connection is rejected — the server has reset it (RST).
This reproduces identically for tools/list sent on an already-established session: the editor log shows the request was handled (Listing tools (3) logged within ~2 seconds), but the client again receives only headers with no body.
The editor’s output log contains: LogHttpConnection: Error: errors.com.epicgames.httpserver.socket_send_failure
Error responses (400/404/405) show the same pattern — headers arrive, body does not.
Platform(s)
Environment
UEFN version: 42.00
OS: Windows x64
MCP configuration: built-in Model Context Protocol plugin, Auto Start Server enabled, default endpoint 127.0.0.1:8000/mcp
Project Settings: Python Editor Scripting and UEFN MCP Toolsets both enabled
A project is loaded in the editor (confirmed via log) at the time of testing
Additional Notes
This may be related to an earlier bug reported against the UE 5.8 experimental MCP plugin (server starts on port 8000 but instantly drops connections with external tools), suggesting this could be a regression or incomplete fix carried into the UEFN 42.00 release rather than something new/UEFN-specific.