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!" }
)

Sessions

After a successful initialize request, the transport captures the Mcp-Session-Id header and protocolVersion from the response and includes the session ID on subsequent requests. Both are exposed on the transport:

http_transport.session_id       # => "abc123..."
http_transport.protocol_version # => "2025-11-25"

If the server terminates the session, subsequent requests return HTTP 404 and the transport raises MCP::Client::SessionExpiredError (a subclass of RequestHandlerError). Session state is cleared automatically; callers should start a new session by sending a fresh initialize request.

To explicitly terminate a session (e.g., when the client application is shutting down), call close. The transport sends an HTTP DELETE to the MCP endpoint with the session header and clears local session state. A 405 Method Not Allowed response (server doesn’t support client-initiated termination) or 404 Not Found (session already terminated server-side) is treated as success. Other errors — 5xx, authentication failures, connection errors — propagate to the caller. Local session state is cleared either way. Calling close without an active session is a no-op.

http_transport.close

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.