Class: MCP::Client::Stdio

Inherits:
Object
  • Object
show all
Defined in:
lib/mcp/client/stdio.rb

Constant Summary collapse

CLOSE_TIMEOUT =

Seconds to wait for the server process to exit before sending SIGTERM. Matches the Python and TypeScript SDKs’ shutdown timeout: github.com/modelcontextprotocol/python-sdk/blob/v1.26.0/src/mcp/client/stdio/__init__.py#L48 github.com/modelcontextprotocol/typescript-sdk/blob/v1.27.1/src/client/stdio.ts#L221

2
STDERR_READ_SIZE = 
4096

Instance Attribute Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize(command:, args: [], env: nil, read_timeout: nil) ⇒ Stdio

Returns a new instance of Stdio.



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# File 'lib/mcp/client/stdio.rb', line 24

def initialize(command:, args: [], env: nil, read_timeout: nil)
  @command = command
  @args = args
  @env = env
  @read_timeout = read_timeout
  @stdin = nil
  @stdout = nil
  @stderr = nil
  @wait_thread = nil
  @stderr_thread = nil
  @started = false
  @initialized = false
  @server_info = nil
  # Serializes writes to `@stdin` so a request line and a notification line emitted from
  # different threads (e.g. cancellation) cannot interleave on the wire.
  @write_mutex = Mutex.new
end

Instance Attribute Details

#args ⇒ Object (readonly)

Returns the value of attribute args.



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# File 'lib/mcp/client/stdio.rb', line 22

def args
  @args
end

#command ⇒ Object (readonly)

Returns the value of attribute command.



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# File 'lib/mcp/client/stdio.rb', line 22

def command
  @command
end

#env ⇒ Object (readonly)

Returns the value of attribute env.



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# File 'lib/mcp/client/stdio.rb', line 22

def env
  @env
end

#server_info ⇒ Object (readonly)

Returns the value of attribute server_info.



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# File 'lib/mcp/client/stdio.rb', line 22

def server_info
  @server_info
end

Instance Method Details

#close ⇒ Object 



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# File 'lib/mcp/client/stdio.rb', line 194

def close
  return unless @started

  @stdin.close
  @stdout.close
  @stderr.close

  begin
    Timeout.timeout(CLOSE_TIMEOUT) { @wait_thread.value }
  rescue Timeout::Error
    begin
      Process.kill("TERM", @wait_thread.pid)
      Timeout.timeout(CLOSE_TIMEOUT) { @wait_thread.value }
    rescue Timeout::Error
      begin
        Process.kill("KILL", @wait_thread.pid)
      rescue Errno::ESRCH
        nil
      end
    rescue Errno::ESRCH
      nil
    end
  end

  @stderr_thread.join(CLOSE_TIMEOUT)
  @started = false
  @initialized = false
  @server_info = nil
end

#connect(client_info: nil, protocol_version: nil, capabilities: {}) ⇒ Hash

Performs the MCP ‘initialize` handshake: sends an `initialize` request followed by the required `notifications/initialized` notification. The server’s ‘InitializeResult` (protocol version, capabilities, server info, instructions) is cached on the transport and returned.

Idempotent: a second call returns the cached ‘InitializeResult` without contacting the server. After `close`, state is cleared and `connect` will handshake again. Spawns the subprocess via `start` if it has not been started yet.

modelcontextprotocol.io/specification/2025-11-25/basic/lifecycle#initialization

Parameters:

  • client_info (Hash, nil) (defaults to: nil) —

    ‘{ name:, version: }` identifying the client. Defaults to `{ name: “mcp-ruby-client”, version: MCP::VERSION }`.

  • protocol_version (String, nil) (defaults to: nil) —

    Protocol version to offer. Defaults to ‘MCP::Configuration::LATEST_STABLE_PROTOCOL_VERSION`.

  • capabilities (Hash) (defaults to: {}) —

    Capabilities advertised by the client. Defaults to ‘{}`.

Returns:

  • (Hash) —

    The server’s ‘InitializeResult`.

Raises:

  • (RequestHandlerError) —

    If the server responds with a JSON-RPC error, a malformed result, or an unsupported protocol version.



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# File 'lib/mcp/client/stdio.rb', line 61

def connect(client_info: nil, protocol_version: nil, capabilities: {})
  return @server_info if @initialized

  start unless @started

  client_info ||= { name: "mcp-ruby-client", version: MCP::VERSION }
  protocol_version ||= MCP::Configuration::LATEST_STABLE_PROTOCOL_VERSION

  init_request = {
    jsonrpc: JsonRpcHandler::Version::V2_0,
    id: SecureRandom.uuid,
    method: MCP::Methods::INITIALIZE,
    params: {
      protocolVersion: protocol_version,
      capabilities: capabilities,
      clientInfo: client_info,
    },
  }

  write_message(init_request)
  response = read_response(init_request)

  if response.key?("error")
    error = response["error"]
    raise RequestHandlerError.new(
      "Server initialization failed: #{error["message"]}",
      { method: MCP::Methods::INITIALIZE },
      error_type: :internal_error,
    )
  end

  unless response["result"].is_a?(Hash)
    raise RequestHandlerError.new(
      "Server initialization failed: missing result in response",
      { method: MCP::Methods::INITIALIZE },
      error_type: :internal_error,
    )
  end

  @server_info = response["result"]

  negotiated_protocol_version = @server_info["protocolVersion"]
  unless MCP::Configuration::SUPPORTED_STABLE_PROTOCOL_VERSIONS.include?(negotiated_protocol_version)
    # Per spec, if the client does not support the server's returned protocol version,
    # the client SHOULD disconnect. Roll back the cached `InitializeResult` before
    # raising so a retry starts without a stale `server_info`.
    # https://modelcontextprotocol.io/specification/2025-11-25/basic/lifecycle#version-negotiation
    @server_info = nil
    raise RequestHandlerError.new(
      "Server initialization failed: unsupported protocol version #{negotiated_protocol_version.inspect}",
      { method: MCP::Methods::INITIALIZE },
      error_type: :internal_error,
    )
  end

  begin
    notification = {
      jsonrpc: JsonRpcHandler::Version::V2_0,
      method: MCP::Methods::NOTIFICATIONS_INITIALIZED,
    }
    write_message(notification)
  rescue StandardError
    @server_info = nil
    raise
  end

  @initialized = true
  @server_info
end

#connected? ⇒ Boolean

Returns true once ‘connect` (or the implicit handshake on the first `send_request`) has completed. Returns false before the handshake and after `close`.

Returns:

  • (Boolean) 


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# File 'lib/mcp/client/stdio.rb', line 134

def connected?
  @initialized
end

#send_notification(notification:) ⇒ Object

Sends a JSON-RPC notification (no response expected). Used by ‘Client#cancel` to deliver `notifications/cancelled` for an in-flight request.



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# File 'lib/mcp/client/stdio.rb', line 158

def send_notification(notification:)
  start unless @started
  connect unless @initialized

  @write_mutex.synchronize { write_message(notification) }
  nil
end

#send_request(request:) ⇒ Object

Transports may yield once the request line has been written to ‘@stdin`. `MCP::Client#dispatch_with_cancellation` uses this signal to ensure a `notifications/cancelled` write does not race ahead of the request write on the wire. The yield happens inside `@write_mutex`, so any subsequent `send_notification` write waits for the mutex and is guaranteed to land after the request.



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# File 'lib/mcp/client/stdio.rb', line 142

def send_request(request:)
  start unless @started
  unless @initialized
    warn("Calling `MCP::Client::Stdio#send_request` without calling `MCP::Client#connect` is deprecated. Use `MCP::Client#connect` before sending requests instead.", uplevel: 1)
    connect
  end

  @write_mutex.synchronize do
    write_message(request)
    yield if block_given?
  end
  read_response(request)
end

#start ⇒ Object 



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# File 'lib/mcp/client/stdio.rb', line 166

def start
  raise "MCP::Client::Stdio already started" if @started

  spawn_env = @env || {}
  @stdin, @stdout, @stderr, @wait_thread = Open3.popen3(spawn_env, @command, *@args)
  @stdout.set_encoding("UTF-8")
  @stdin.set_encoding("UTF-8")

  # Drain stderr in the background to prevent the pipe buffer from filling up,
  # which would cause the server process to block and deadlock.
  @stderr_thread = Thread.new do
    loop do
      @stderr.readpartial(STDERR_READ_SIZE)
    end
  rescue IOError
    nil
  end

  @started = true
rescue Errno::ENOENT, Errno::EACCES, Errno::ENOEXEC => e
  raise RequestHandlerError.new(
    "Failed to spawn server process: #{e.message}",
    {},
    error_type: :internal_error,
    original_error: e,
  )
end