Server crashes with "Unexpected end of JSON input" when connected from an SSE client via ngrok

[Geefer1985]

Describe the bug
When running the server via the npx command and connecting to it from a standard SSE client through an ngrok tunnel, the server process crashes instantly upon receiving the first connection request. The crash appears to be caused by the server trying to JSON.parse a raw HTTP request.

Steps to Reproduce

1. Start an ngrok tunnel: ngrok http 8008 --request-header-add "ngrok-skip-browser-warning: true"
2. Start the server with a valid API key: LIMITLESS_API_KEY="sk-..." npx 199bio-mcp-limitless-server
3. The server logs: Limitless MCP Server starting...
4. Connect a client (e.g., an app that connects to a custom SSE URL) to the ngrok forwarding address.
5. The ngrok console shows a 502 Bad Gateway error.
6. The server terminal shows no immediate output, but after pressing Enter, the following stack trace is revealed:

Error Log
MCP Server Error: SyntaxError: Unexpected end of JSON input
at JSON.parse ()
at deserializeMessage (file:///Users/someuser/.npm/_npx/17fdf017b1962859/node_modules/@modelcontextprotocol/sdk/dist/esm/shared/stdio.js:26:44)
at ReadBuffer.readMessage (file:///Users/someuser/.npm/_npx/17fdf017b1962859/node_modules/@modelcontextprotocol/sdk/dist/esm/shared/stdio.js:19:16)
at StdioServerTransport.processReadBuffer (file:///Users/someuser/.npm/_npx/17fdf017b1962859/node_modules/@modelcontextprotocol/sdk/dist/esm/server/stdio.js:39:50)
at StdioServerTransport._ondata (file:///Users/someuser/.npm/_npx/17fdf017b1962859/node_modules/@modelcontextprotocol/sdk/dist/esm/server/stdio.js:17:18)
at ReadStream.emit (node:events:507:28)
at addChunk (node:internal/streams/readable:559:12)
at readableAddChunkPushByteMode (node:internal/streams/readable:510:3)
at Readable.push (node:internal/streams/readable:390:5)
at TTY.onStreamRead (node:internal/stream_base_commons:189:23)

Expected behavior
The server should be able to handle an incoming HTTP/SSE connection from a standard client without crashing.

[199-bio]

Thank you for reporting this. I've investigated the issue and updated our documentation to clarify what's happening.

The Issue:
MCP (Model Context Protocol) servers are fundamentally different from HTTP servers. They communicate using JSON-RPC messages over stdio (stdin/stdout), not HTTP. When you connect an SSE client via ngrok, it sends HTTP requests like:

GET /events HTTP/1.1
Host: example.ngrok.io
...

The MCP server tries to parse this as JSON-RPC, which fails because it's plaintext HTTP headers, not JSON, causing the "Unexpected end of JSON input" error.

What I've Done:

1. Added a comprehensive troubleshooting section to the README explaining this common misconception
2. Enhanced the server startup to detect and warn when run directly in a terminal
3. Improved error messages to specifically identify when HTTP requests are sent to the JSON-RPC server

MCP servers are designed to work with:

• Claude Desktop (primary use case - configure in Settings → Developer → MCP)
• Other MCP-compatible clients that implement the JSON-RPC protocol
• NOT web browsers, curl, or HTTP/SSE clients

If you need HTTP access to Limitless data:

1. Use the Limitless API directly from your web application
2. Create a separate HTTP server that wraps this MCP functionality
3. Consider building an HTTP-to-MCP bridge service (we might add this as a separate package if there's enough demand)

This is working as intended - the server is functioning correctly for its designed purpose as an MCP server for Claude Desktop.