Falcon and FastMCP

[tleish]

Anyone gotten fast-mcp gem to work with Falcon?

# config.ru
require 'bundler/setup'
require 'rack'
require 'fast_mcp'           # keep both requires
# require 'fast_mcp/rack'    # (not always needed, but harmless if present)

# If you keep tools under app/tools/*.rb:
$LOAD_PATH.unshift File.expand_path('app', __dir__)

# Minimal tool so something is registered:
class EchoTool < FastMcp::Tool
  description 'Echo text back'
  arguments { required(:text).filled(:string) }
  def call(text:) = "echo: #{text}"
end

base_app = ->(_env) { [200, {'Content-Type'=>'text/plain'}, ['Hello World!']] }

app = FastMcp.rack_middleware(
  base_app,
  name: 'my-ai-server',
  version: '1.0.0',
  path_prefix: '/mcp'
) do |server|
  server.register_tool(EchoTool)
end

run app

curl -i -N http://127.0.0.1:9000/mcp/sse

# falcon logs

error: Protocol::Rack::Adapter::Rack31: Reading HTTP/1.1 requests for Async::HTTP::Protocol::HTTP1::Server. [oid=0xfc8] [ec=0xfdc] [pid=73011] [2025-08-15 14:55:51 -0600]
       | undefined method `write' for nil:NilClass
       |   NoMethodError: undefined method `write' for nil:NilClass (NoMethodError)
       |   
       |             io.write("HTTP/1.1 200 OK\r\n")
       |               ^^^^^^
       |   → gems/fast-mcp-1.5.0/lib/mcp/transports/rack_transport.rb:403 in `block in setup_sse_connection'
       |     gems/fast-mcp-1.5.0/lib/mcp/transports/rack_transport.rb:402 in `synchronize'
       |     gems/fast-mcp-1.5.0/lib/mcp/transports/rack_transport.rb:402 in `setup_sse_connection'
       |     gems/fast-mcp-1.5.0/lib/mcp/transports/rack_transport.rb:387 in `handle_rack_hijack_sse'
       |     gems/fast-mcp-1.5.0/lib/mcp/transports/rack_transport.rb:287 in `handle_streaming'
       |     gems/fast-mcp-1.5.0/lib/mcp/transports/rack_transport.rb:278 in `handle_sse_request'
       |     gems/fast-mcp-1.5.0/lib/mcp/transports/rack_transport.rb:226 in `handle_mcp_request'
       |     gems/fast-mcp-1.5.0/lib/mcp/transports/rack_transport.rb:130 in `call'

[ioquatix]

The implementation of FastMCP is buggy: yjacquin/fast-mcp#144

(It's also incompatible with HTTP/2).

[tleish]

I haven't tried it yet, but would the official MCP Ruby SDK work better with falcon?

GitHub - modelcontextprotocol/ruby-sdk: The official Ruby SDK for the Model Context Protocol. Maintained in collaboration with Shopify.