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Troubleshooting

A client cannot connect

The server runs inside the editor, so the project has to be open. The editor's Output Log says whether the server started:

  • UEAgentBridge: MCP HTTP server started on http://127.0.0.1:55561/mcp (...): it is up. Check the URL in the client, port included.
  • UEAgentBridge: MCP HTTP server NOT started: could not open 127.0.0.1:55561 -- is another editor or process using the port?: another editor, or another program, holds the port. Close it, or pick another HTTP port in Editor Preferences > Plugins > UEAgent (or start the editor with -McpHttpPort=N), then restart the server from the status-bar menu or with mcp.http.restart.

mcp.http.status in the editor console prints the server's state at any time.

401 Unauthorized

An Auth token is set and the request did not carry Authorization: Bearer <token>. Give the client the header (see Clients), or clear the token.

403 Forbidden

The request's Host was not the loopback, or it came from a web page (its Origin). The server refuses both, so that no website can drive your editor. Connect to 127.0.0.1 or localhost.

The client warns about too many tools, or slows down

Some clients cap how many tools a server may list, and the full list has more than 2,000. Give them the compact tool list: ?tools=compact at the end of the URL.

A call is refused while Play-in-Editor runs

Calls that compile a Blueprint or write to disk are refused during PIE: a compile can keep the play world alive and the engine then stops the editor, and the engine refuses to save while PIE runs. Stop PIE and retry. force: true runs the call anyway, at your own risk.

A call is refused before it runs

The call is one known to crash or freeze the editor in that situation. The refusal says why, and what to do instead.

"Unknown tool"

The client called a tool this server does not have, often from a tool list it cached from another version: reconnect it so it lists the tools again. The error names the closest tools.

The precompiled plugin asks to be rebuilt

The release is built for Unreal Engine 5.8 on Windows. With another engine version, build the plugin from source (Visual Studio 2022 with the C++ toolchain).

Still stuck

Ask in Discussions, or, if it looks like a bug, open an issue with the Bug report template and the lines around the call from Saved/Logs/<Project>.log.