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: https://github.com/modelcontextprotocol/python-sdk/blob/v1.26.0/src/mcp/client/stdio/init.py#L48 https://github.com/modelcontextprotocol/typescript-sdk/blob/v1.27.1/src/client/stdio.ts#L221

2
STDERR_READ_SIZE = 
4096
MAX_LINE_BYTES =

Default upper bound on a single newline-delimited frame read from the server's stdout. CRuby's IO#gets without a limit accumulates bytes until a newline arrives, so a spawned server that never emits one can grow a single String until the host process is OOM-killed. 4 MiB is large enough for any realistic JSON-RPC frame, including base64-embedded images.

4 * 1024 * 1024

Instance Attribute Summary collapse

Instance Method Summary collapse

Constructor Details

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

Returns a new instance of Stdio.



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# File 'lib/mcp/client/stdio.rb', line 31

def initialize(command:, args: [], env: nil, read_timeout: nil, max_line_bytes: MAX_LINE_BYTES)
  # Reject `nil` or non-positive values: `IO#gets("\n", nil)` and a negative
  # limit read without an upper bound, which would silently disable the
  # protection this option exists to provide.
  unless max_line_bytes.is_a?(Integer) && max_line_bytes > 0
    raise ArgumentError, "max_line_bytes must be a positive Integer"
  end

  @command = command
  @args = args
  @env = env
  @read_timeout = read_timeout
  @max_line_bytes = max_line_bytes
  @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 29

def args
  @args
end

#command ⇒ Object (readonly)

Returns the value of attribute command.



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# File 'lib/mcp/client/stdio.rb', line 29

def command
  @command
end

#env ⇒ Object (readonly)

Returns the value of attribute env.



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# File 'lib/mcp/client/stdio.rb', line 29

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 29

def server_info
  @server_info
end

Instance Method Details

#close ⇒ Object 



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# File 'lib/mcp/client/stdio.rb', line 203

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.

https://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 76

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 has completed the handshake. Returns false before the handshake and after close.

Returns:

  • (Boolean) 


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# File 'lib/mcp/client/stdio.rb', line 147

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 167

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 155

def send_request(request:)
  raise "MCP::Client#connect must be called before sending requests." unless @initialized

  @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 175

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