Several clients have issues connecting through Streamable HTTP protocol

[thisismydesign]

Context: SSE is deprecated and doesn't handle multi-process environment.

So I've been trying to use transport: :streamable_http from the branch mcp-2025-06-18.

This works well for some clients: claude code, libre chat, inspector

But it fails with various errors for other clients: n8n, elevenlabs, vapi.

For example elevenlabs:

Internal error: Unexpected ExceptionGroup occurred while connecting to MCP server at https://xxx/mcp using TransportType.STREAMABLE_HTTP transport. Please try again or contact support if the issue persists.

I don't see any error on server-side, so I'm suspicious of the clients, but it happens on too many popular providers to be a one-off error on their side. Any ideas about what the issue can be? Can it be the MCP version they support?

Server logs when I try to connect elevenlabs:

2025-11-27T14:44:27.514868+00:00 app[web.1]: [245fdda7-613a-fc85-38d2-7e1f3a02319c] Started POST "/mcp" for 34.59.11.47 at 2025-11-27 14:44:27 +0000
2025-11-27T14:44:27.515594+00:00 app[web.1]: [245fdda7-613a-fc85-38d2-7e1f3a02319c] Client connected: mcp v0.1.0
2025-11-27T14:44:27.515622+00:00 app[web.1]: [245fdda7-613a-fc85-38d2-7e1f3a02319c] Server response: {:protocolVersion=>"2025-06-18", :capabilities=>{:resources=>{:subscribe=>true, :listChanged=>true}, :tools=>{:listChanged=>true}}, :serverInfo=>{:name=>"app", :version=>"1.0.0"}}
2025-11-27T14:44:27.515646+00:00 app[web.1]: [245fdda7-613a-fc85-38d2-7e1f3a02319c] Sending result: {:jsonrpc=>"2.0", :id=>0, :result=>{:protocolVersion=>"2025-06-18", :capabilities=>{:resources=>{:subscribe=>true, :listChanged=>true}, :tools=>{:listChanged=>true}}, :serverInfo=>{:name=>"app", :version=>"1.0.0"}}}
2025-11-27T14:44:27.625993+00:00 app[web.1]: [693445df-045a-6280-29fe-14bb8518a6b8] Started DELETE "/mcp" for 34.59.11.47 at 2025-11-27 14:44:27 +0000

[thisismydesign]

Looks like this is related to the version of the MCP protocol used. Some clients only support 2025-03-26. Presumably the gem uses 2025-06-18? This should be documented. Ideally different version should be supported. The official ruby sdk (https://github.com/modelcontextprotocol/ruby-sdk) allows you to switch between versions.

[theclunkerjunker]

I had this issue with ElevenLabs. I had to monkey patch fast MCP to echo back whatever protocol version it's using (I think it's currently 2025-03-26) when eleven labs requests it. Hacky, but the only way I could get it to work. Elevenlabs needs to fix this.

This is LLM generated but works for me in a fast mcp initializer (rails)

# Monkey-patch FastMCP to echo back the client's requested protocol version
# instead of forcing 2025-06-18. This fixes compatibility with clients like
# ElevenLabs that use older protocol versions (e.g., 2025-03-26).
module FastMcp
  class Server
    alias_method :original_handle_initialize, :handle_initialize

    def handle_initialize(params, id)
      # Store client capabilities for later use
      @client_capabilities = params['capabilities'] || {}
      client_info = params['clientInfo'] || {}

      # Log client information
      @logger.info("Client connected: #{client_info['name']} v#{client_info['version']}")

      # Echo back the client's requested protocol version for compatibility
      client_version = params['protocolVersion'] || FastMcp::Protocol::VERSION

      # Prepare server response
      response = {
        protocolVersion: client_version,
        capabilities: @capabilities,
        serverInfo: {
          name: @name,
          version: @version
        }
      }

      @logger.info("Server response: #{response.inspect}")

      send_result(response, id)
    end
  end
end

[theclunkerjunker]

Additionally, fast MCP ruby does not seem to handle CORS correctly, or at all.
It also doesn't handle DELETE which is in the MCP spec...
It also seems to require authentication on the OPTIONS preflight which isn't correct.

I had to use this additional monkey patch to get my server to work with eleven labs.

# frozen_string_literal: true

# Middleware to handle CORS and other MCP protocol gaps.
#
# This is a workaround for FastMCP's AuthenticatedStreamableHttpTransport:
# 1. It checks auth before allowing OPTIONS preflight requests
# 2. It doesn't add CORS headers to responses
# 3. It doesn't implement DELETE for session termination (MCP spec)
#
# This middleware:
# - Handles OPTIONS preflight without auth (per CORS spec)
# - Handles DELETE for session termination (FastMCP doesn't support it)
# - Adds CORS headers to all MCP responses
class McpCors
  CORS_HEADERS = {
    'Access-Control-Allow-Methods' => 'GET, POST, DELETE, OPTIONS',
    'Access-Control-Allow-Headers' => 'Content-Type, Accept, Authorization, MCP-Protocol-Version, MCP-Session-Id',
    'Access-Control-Allow-Credentials' => 'true',
    'Access-Control-Max-Age' => '86400',
    'Access-Control-Expose-Headers' => 'MCP-Session-Id'
  }.freeze

  def initialize(app)
    @app = app
  end

  def call(env)
    return @app.call(env) unless env['PATH_INFO']&.start_with?('/mcp')

    origin = env['HTTP_ORIGIN'] || '*'

    # Handle OPTIONS preflight
    if env['REQUEST_METHOD'] == 'OPTIONS'
      return [200, cors_headers(origin).merge('Content-Type' => 'text/plain', 'Content-Length' => '0'), ['']]
    end

    # Handle DELETE for session termination (FastMCP doesn't implement this)
    # Per MCP spec, DELETE terminates the session. Return 204 No Content.
    if env['REQUEST_METHOD'] == 'DELETE'
      return [204, cors_headers(origin), []]
    end

    # For all other requests, add CORS headers to the response
    status, headers, body = @app.call(env)
    [status, headers.merge(cors_headers(origin)), body]
  end

  private

  def cors_headers(origin)
    CORS_HEADERS.merge('Access-Control-Allow-Origin' => origin)
  end
end

Lastly, if you're using the mcp-2025-06-18 branch and need authentication, be sure to change the transport from "http_streamable" to "authenticated". Otherwise it will silently let anyone and everyone connect without checking any authentication.