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Ates Goral b832933f9f Parse SSE responses in HTTP client via event_stream_parser

The Streamable HTTP spec allows servers to respond with either
`application/json` or `text/event-stream`. The client previously
rejected the latter. Add SSE parsing via the optional
`event_stream_parser` gem (users add it to their Gemfile when their
server uses SSE), dispatch on the response Content-Type, and scan the
event stream for the first JSON-RPC response message.

Document the new optional dependency in both the README's HTTP
Transport Layer section and docs/building-clients.md.

No other Streamable HTTP features (202 handling, session IDs,
`connect`, DELETE termination) are included here; those will land in
follow-up PRs.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

2026-04-19 11:54:03 -04:00

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layout	title	nav_order
default	Building Clients	4

Building an MCP Client

The MCP::Client class provides an interface for interacting with MCP servers.

Supported operations:

Tool listing (MCP::Client#tools) and invocation (MCP::Client#call_tool)
Resource listing (MCP::Client#resources) and reading (MCP::Client#read_resources)
Resource template listing (MCP::Client#resource_templates)
Prompt listing (MCP::Client#prompts) and retrieval (MCP::Client#get_prompt)
Completion requests (MCP::Client#complete)

Stdio Transport

Use MCP::Client::Stdio to interact with MCP servers running as subprocesses:

stdio_transport = MCP::Client::Stdio.new(
  command: "bundle",
  args: ["exec", "ruby", "path/to/server.rb"],
  env: { "API_KEY" => "my_secret_key" },
  read_timeout: 30
)
client = MCP::Client.new(transport: stdio_transport)

tools = client.tools
tools.each do |tool|
  puts "Tool: #{tool.name} - #{tool.description}"
end

response = client.call_tool(
  tool: tools.first,
  arguments: { message: "Hello, world!" }
)

stdio_transport.close

Parameter	Required	Description
`command:`	Yes	The command to spawn the server process.
`args:`	No	An array of arguments passed to the command. Defaults to `[]`.
`env:`	No	A hash of environment variables for the server process. Defaults to `nil`.
`read_timeout:`	No	Timeout in seconds for waiting for a server response. Defaults to `nil`.

HTTP Transport

Use MCP::Client::HTTP to interact with MCP servers over HTTP. Requires the faraday gem, plus event_stream_parser if the server uses SSE (text/event-stream) responses:

gem 'mcp'
gem 'faraday', '>= 2.0'
gem 'event_stream_parser', '>= 1.0' # optional, required only for SSE responses

http_transport = MCP::Client::HTTP.new(url: "https://api.example.com/mcp")
client = MCP::Client.new(transport: http_transport)

tools = client.tools
tools.each do |tool|
  puts "Tool: #{tool.name} - #{tool.description}"
end

response = client.call_tool(
  tool: tools.first,
  arguments: { message: "Hello, world!" }
)

Authorization

Provide custom headers for authentication:

http_transport = MCP::Client::HTTP.new(
  url: "https://api.example.com/mcp",
  headers: {
    "Authorization" => "Bearer my_token"
  }
)
client = MCP::Client.new(transport: http_transport)

Customizing the Faraday Connection

Pass a block to customize the underlying Faraday connection:

http_transport = MCP::Client::HTTP.new(url: "https://api.example.com/mcp") do |faraday|
  faraday.use MyApp::Middleware::HttpRecorder
  faraday.adapter :typhoeus
end

Custom Transport

If the built-in transports do not fit your needs, you can implement your own:

class CustomTransport
  def send_request(request:)
    # Your transport-specific logic here.
    # Returns a Hash modeling a JSON-RPC response object.
  end
end

client = MCP::Client.new(transport: CustomTransport.new)

For more details, see the full README.

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