Issue when using `@hono/mcp`, content-length is included when chunked encoding is used.

[ciamshrek]

I was trying to use @hono/mcp, when using Inspector it returns a precarious error.

[cause]: HTTPParserError: Response does not match the HTTP/1.1 protocol (Content-Length can't be present with Transfer-Encoding)
      at Parser.execute (node:internal/deps/undici/undici:5998:19)
      at Parser.readMore (node:internal/deps/undici/undici:5957:16)
      at Socket.<anonymous> (node:internal/deps/undici/undici:6285:18)
      at Socket.emit (node:events:518:28)
      at emitReadable_ (node:internal/streams/readable:834:12)
      at process.processTicksAndRejections (node:internal/process/task_queues:89:21) {
    code: 'HPE_UNEXPECTED_CONTENT_LENGTH',
    data: 'c9\r\n' +
      'event: message\n' +
      'data: {"result":{"protocolVersion":"2025-06-18","capabilities":{"tools":{"listChanged":true}},"serverInfo":{"name":"example-mcp-server","version":"1.0.0"}},"jsonrpc":"2.0","id":0}\n' +
      '\n' +
      '\r\n' +
      '0\r\n' +
      '\r\n'
  }

Upon further debugging I traced that down to this line, https://github.com/honojs/node-server/blob/main/src/listener.ts#L167-L169, to me it sees more reasonable to expand the logic to check for chunked perhaps expand the code to shouldAddContentLength(headers): true/false and then derive it.

Happy to open PR if ya'll think that's the best way :)

[usualoma]

Hi @ciamshrek

Thanks for the feedback. I see. Normally, clients ignore the combination of transfer-encoding:chunked and content-length, but I see how that could cause an error in certain environments.

This could potentially occur in other areas as well, and we had omitted the check for optimization purposes, but we will look into a solution.
https://github.com/honojs/node-server/blob/main/src/listener.ts#L77-L83