Class: MCPClient::ServerStdio

Inherits:
ServerBase show all
Includes:
JsonRpcTransport
Defined in:
lib/mcp_client/server_stdio.rb,
lib/mcp_client/server_stdio/json_rpc_transport.rb

Overview

JSON-RPC implementation of MCP server over stdio.

Defined Under Namespace

Modules: JsonRpcTransport

Constant Summary collapse

READ_TIMEOUT =

Timeout in seconds for responses

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SHUTDOWN_GRACE_PERIOD =

Grace period in seconds allowed at each stage of the shutdown sequence (after closing stdin, then after SIGTERM) before escalating further, per MCP 2025-11-25 basic/lifecycle.mdx (Shutdown / stdio): close stdin, wait for the server to exit, send SIGTERM, then SIGKILL if it still runs.

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STDERR_READ_CHUNK_SIZE =

Chunk size (bytes) used when draining the subprocess stderr pipe

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STDERR_MAX_LINE_SIZE =

Maximum bytes buffered for a single unterminated stderr line before it is flushed. Bounds memory when a server writes to stderr without newlines (e.g. progress output using carriage returns).

64 * 1024

Constants inherited from ServerBase

MCPClient::ServerBase::MAX_LIST_PAGES, MCPClient::ServerBase::RELATED_TASK_META_KEY

Instance Attribute Summary collapse

Attributes inherited from ServerBase

#instructions, #name

Instance Method Summary collapse

Methods included from JsonRpcTransport

#call_tool_streaming, #ensure_initialized, #next_id, #perform_initialize, #rpc_notify, #rpc_request, #send_cancellation_notification, #send_request, #wait_response

Methods included from JsonRpcCommon

#build_jsonrpc_notification, #build_jsonrpc_request, #build_named_request_params, #cancellable_request?, #client_capabilities, #client_info_payload, #declare_sampling_tools, #initialization_params, #ping, #process_jsonrpc_response, #registered_callback?, #sampling_tools_supported?, #split_request_meta, #validate_protocol_version!, #with_retry

Methods inherited from ServerBase

#call_tool_streaming, #capability?, #client_info=, #merge_related_task_meta, #on_notification, #ping, #require_capability!, #rpc_notify, #rpc_request

Constructor Details

#initialize(command:, retries: 0, retry_backoff: 1, read_timeout: READ_TIMEOUT, name: nil, logger: nil, env: {}) ⇒ ServerStdio

Initialize a new ServerStdio instance

Parameters:

  • command (String, Array) —

    the stdio command to launch the MCP JSON-RPC server For improved security, passing an Array is recommended to avoid shell injection issues

  • retries (Integer) (defaults to: 0) —

    number of retry attempts on transient errors

  • retry_backoff (Numeric) (defaults to: 1) —

    base delay in seconds for exponential backoff

  • read_timeout (Numeric) (defaults to: READ_TIMEOUT) —

    timeout in seconds for reading responses

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

    optional name for this server

  • logger (Logger, nil) (defaults to: nil) —

    optional logger

  • env (Hash) (defaults to: {}) —

    optional environment variables for the subprocess



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# File 'lib/mcp_client/server_stdio.rb', line 47

def initialize(command:, retries: 0, retry_backoff: 1, read_timeout: READ_TIMEOUT, name: nil, logger: nil, env: {})
  super(name: name)
  @command_array = command.is_a?(Array) ? command : nil
  @command = command.is_a?(Array) ? command.join(' ') : command
  @mutex = Mutex.new
  @cond = ConditionVariable.new
  @next_id = 1
  @pending = {}
  # Ids of requests awaiting a response; used to drop late/unsolicited
  # responses so @pending cannot grow without bound on a long-lived session
  @awaiting = {}
  @initialized = false
  @server_info = nil
  @capabilities = nil
  initialize_logger(logger)
  @max_retries   = retries
  @retry_backoff = retry_backoff
  @read_timeout  = read_timeout
  @env           = env || {}
  @elicitation_request_callback = nil # MCP 2025-06-18
  @roots_list_request_callback = nil # MCP 2025-06-18
  @sampling_request_callback = nil # MCP 2025-11-25
  @reader_thread = nil
  @stderr_thread = nil
end

Instance Attribute Details

#capabilities ⇒ Hash? (readonly)

Server capabilities from the initialize response

Returns:

  • (Hash, nil) —

    Server capabilities



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# File 'lib/mcp_client/server_stdio.rb', line 79

def capabilities
  @capabilities
end

#command ⇒ String, Array (readonly)

Returns the command used to launch the server.

Returns:

  • (String, Array) —

    the command used to launch the server



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# File 'lib/mcp_client/server_stdio.rb', line 19

def command
  @command
end

#env ⇒ Object (readonly)

Returns the value of attribute env.



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# File 'lib/mcp_client/server_stdio.rb', line 19

attr_reader :command, :env

#server_info ⇒ Hash? (readonly)

Server info from the initialize response

Returns:

  • (Hash, nil) —

    Server information



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# File 'lib/mcp_client/server_stdio.rb', line 75

def server_info
  @server_info
end

Instance Method Details

#call_tool(tool_name, parameters) ⇒ Object

Call a tool with the given parameters

Parameters:

  • tool_name (String) —

    the name of the tool to call

  • parameters (Hash) —

    the parameters to pass to the tool

Returns:

  • (Object) —

    the result of the tool invocation

Raises:



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# File 'lib/mcp_client/server_stdio.rb', line 420

def call_tool(tool_name, parameters)
  ensure_initialized
  req_id = next_id
  # JSON-RPC method for calling a tool
  req = {
    'jsonrpc' => '2.0',
    'id' => req_id,
    'method' => 'tools/call',
    'params' => build_named_request_params(tool_name, parameters)
  }
  send_request(req)
  res = wait_response(req_id)
  if (err = res['error'])
    raise MCPClient::Errors::ServerError, err['message']
  end

  res['result']
rescue StandardError => e
  raise MCPClient::Errors::ToolCallError, "Error calling tool '#{tool_name}': #{e.message}"
end

#cleanup ⇒ void

This method returns an undefined value.

Clean up the server connection Closes all stdio handles and terminates any running processes and threads following the MCP 2025-11-25 stdio shutdown sequence (basic/lifecycle.mdx): close stdin, wait for the server to exit, send SIGTERM if it does not exit within a reasonable time, then SIGKILL if it still does not exit.



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# File 'lib/mcp_client/server_stdio.rb', line 729

def cleanup
  return unless @stdin

  @stdin.close unless @stdin.closed?
  terminate_server_process
  @stdout.close unless @stdout.closed?
  @stderr.close unless @stderr.closed?
  @reader_thread&.kill
  @stderr_thread&.kill
rescue StandardError
  # Clean up resources during unexpected termination
ensure
  # Release any buffered responses / awaiting markers
  @mutex.synchronize do
    @pending.clear
    @awaiting.clear
  end
  @stdin = @stdout = @stderr = @wait_thread = @reader_thread = @stderr_thread = nil
end

#complete(ref:, argument:, context: nil) ⇒ Hash

Request completion suggestions from the server (MCP 2025-06-18)

Parameters:

  • ref (Hash) —

    reference object (e.g., { 'type' => 'ref/prompt', 'name' => 'prompt_name' })

  • argument (Hash) —

    the argument being completed (e.g., { 'name' => 'arg_name', 'value' => 'partial' })

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

    optional context for the completion (MCP 2025-11-25)

Returns:

  • (Hash) —

    completion result with 'values', optional 'total', and 'hasMore' fields

Raises:



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# File 'lib/mcp_client/server_stdio.rb', line 447

def complete(ref:, argument:, context: nil)
  ensure_initialized
  require_capability!('completions', method: 'completion/complete')
  req_id = next_id
  params = { 'ref' => ref, 'argument' => argument }
  params['context'] = context if context
  req = {
    'jsonrpc' => '2.0',
    'id' => req_id,
    'method' => 'completion/complete',
    'params' => params
  }
  send_request(req)
  res = wait_response(req_id)
  if (err = res['error'])
    raise MCPClient::Errors::ServerError, err['message']
  end

  res.dig('result', 'completion') || { 'values' => [] }
rescue MCPClient::Errors::CapabilityError
  raise
rescue StandardError => e
  raise MCPClient::Errors::ServerError, "Error requesting completion: #{e.message}"
end

#connect ⇒ Boolean

Connect to the MCP server by launching the command process via stdin/stdout

Returns:

  • (Boolean) —

    true if connection was successful

Raises:



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# File 'lib/mcp_client/server_stdio.rb', line 84

def connect
  if @command_array
    if @env.any?
      @stdin, @stdout, @stderr, @wait_thread = Open3.popen3(@env, *@command_array)
    else
      @stdin, @stdout, @stderr, @wait_thread = Open3.popen3(*@command_array)
    end
  elsif @env.any?
    @stdin, @stdout, @stderr, @wait_thread = Open3.popen3(@env, @command)
  else
    @stdin, @stdout, @stderr, @wait_thread = Open3.popen3(@command)
  end
  pin_pipe_encodings
  true
rescue StandardError => e
  raise MCPClient::Errors::ConnectionError, "Failed to connect to MCP server: #{e.message}"
end

#flush_stderr_lines(buffer) ⇒ void

This method returns an undefined value.

Emit and remove all newline-terminated lines from the stderr buffer.

Parameters:

  • buffer (String) —

    mutable buffer of accumulated stderr bytes



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# File 'lib/mcp_client/server_stdio.rb', line 705

def flush_stderr_lines(buffer)
  while (newline_index = buffer.index("\n"))
    line = buffer.slice!(0, newline_index + 1)
    @logger.debug("[stderr] #{line.chomp}")
  end
end

#flush_stderr_overflow(buffer) ⇒ void

This method returns an undefined value.

Flush an unterminated stderr fragment that has grown past the size cap, so a newline-less stderr stream cannot buffer without bound.

Parameters:

  • buffer (String) —

    mutable buffer of accumulated stderr bytes



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# File 'lib/mcp_client/server_stdio.rb', line 716

def flush_stderr_overflow(buffer)
  return if buffer.bytesize <= STDERR_MAX_LINE_SIZE

  @logger.debug("[stderr] #{buffer}")
  buffer.clear
end

#get_prompt(prompt_name, parameters) ⇒ Object

Get a prompt with the given parameters

Parameters:

  • prompt_name (String) —

    the name of the prompt to get

  • parameters (Hash) —

    the parameters to pass to the prompt

Returns:

  • (Object) —

    the result of the prompt interpolation

Raises:



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# File 'lib/mcp_client/server_stdio.rb', line 235

def get_prompt(prompt_name, parameters)
  ensure_initialized
  req_id = next_id
  # JSON-RPC method for getting a prompt
  req = {
    'jsonrpc' => '2.0',
    'id' => req_id,
    'method' => 'prompts/get',
    'params' => build_named_request_params(prompt_name, parameters)
  }
  send_request(req)
  res = wait_response(req_id)
  if (err = res['error'])
    raise MCPClient::Errors::ServerError, err['message']
  end

  res['result']
rescue StandardError => e
  raise MCPClient::Errors::PromptGetError, "Error calling prompt '#{prompt_name}': #{e.message}"
end

#handle_elicitation_create(request_id, params) ⇒ void

This method returns an undefined value.

Handle elicitation/create request from server (MCP 2025-06-18)

Parameters:

  • request_id (String, Integer) —

    the JSON-RPC request ID

  • params (Hash) —

    the elicitation parameters



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# File 'lib/mcp_client/server_stdio.rb', line 566

def handle_elicitation_create(request_id, params)
  # Without a callback there is no user to interact with: answer with a
  # JSON-RPC error rather than fabricating a user "decline".
  unless @elicitation_request_callback
    @logger.warn('Received elicitation request but no callback registered')
    send_error_response(request_id, -32_601, 'Elicitation not supported: no handler configured')
    return
  end

  # Call the registered callback
  result = @elicitation_request_callback.call(request_id, params)

  # Send the response back to the server (echoing related-task _meta)
  send_elicitation_response(request_id, merge_related_task_meta(result, params))
end

#handle_line(line) ⇒ void

This method returns an undefined value.

Handle a line of output from the stdio server Parses JSON-RPC messages and adds them to pending responses

Parameters:

  • line (String) —

    line of output to parse



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# File 'lib/mcp_client/server_stdio.rb', line 161

def handle_line(line)
  msg = JSON.parse(line)
  @logger.debug("Received line: #{line.chomp}")

  # A JSON-parseable line that is not an object cannot be a JSON-RPC
  # message; skip it rather than raising inside the reader thread
  unless msg.is_a?(Hash)
    @logger.debug("Skipping non-object JSON-RPC line: #{line.chomp}")
    return
  end

  # Dispatch JSON-RPC requests from server (has id AND method) - MCP 2025-06-18
  if msg['method'] && msg.key?('id')
    handle_server_request(msg)
    return
  end

  # Dispatch JSON-RPC notifications (no id, has method)
  if msg['method'] && !msg.key?('id')
    @notification_callback&.call(msg['method'], msg['params'])
    return
  end

  # Handle standard JSON-RPC responses (has id, no method)
  id = msg['id']
  return unless id

  @mutex.synchronize do
    # Only retain a response that corresponds to an outstanding request.
    # Late responses (arriving after the caller timed out) and unsolicited
    # responses are dropped so @pending cannot grow without bound.
    if @awaiting.key?(id)
      @pending[id] = msg
      @cond.broadcast
    else
      @logger.debug("Discarding response for unknown or expired request id=#{id}")
    end
  end
rescue JSON::ParserError, EncodingError
  # Skip non-JSONRPC or undecodable lines in the output stream so a single
  # bad line cannot kill the reader thread
end

#handle_ping(request_id) ⇒ void

This method returns an undefined value.

Handle a server-initiated ping request (MCP ping utility) The receiver MUST respond promptly with an empty result.

Parameters:

  • request_id (String, Integer) —

    the JSON-RPC request ID



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# File 'lib/mcp_client/server_stdio.rb', line 553

def handle_ping(request_id)
  response = {
    'jsonrpc' => '2.0',
    'id' => request_id,
    'result' => {}
  }
  send_message(response)
end

#handle_roots_list(request_id, params) ⇒ void

This method returns an undefined value.

Handle roots/list request from server (MCP 2025-06-18)

Parameters:

  • request_id (String, Integer) —

    the JSON-RPC request ID

  • params (Hash) —

    the request parameters



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# File 'lib/mcp_client/server_stdio.rb', line 586

def handle_roots_list(request_id, params)
  # If no callback is registered, return empty roots list
  unless @roots_list_request_callback
    @logger.debug('Received roots/list request but no callback registered, returning empty list')
    send_roots_list_response(request_id, { 'roots' => [] })
    return
  end

  # Call the registered callback
  result = @roots_list_request_callback.call(request_id, params)

  # Send the response back to the server (echoing related-task _meta)
  send_roots_list_response(request_id, merge_related_task_meta(result, params))
end

#handle_sampling_create_message(request_id, params) ⇒ void

This method returns an undefined value.

Handle sampling/createMessage request from server (MCP 2025-11-25)

Parameters:

  • request_id (String, Integer) —

    the JSON-RPC request ID

  • params (Hash) —

    the sampling parameters



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# File 'lib/mcp_client/server_stdio.rb', line 605

def handle_sampling_create_message(request_id, params)
  # If no callback is registered, return error
  unless @sampling_request_callback
    @logger.warn('Received sampling request but no callback registered, returning error')
    send_error_response(request_id, -1, 'Sampling not supported')
    return
  end

  # Call the registered callback
  result = @sampling_request_callback.call(request_id, params)

  # Send the response back to the server (echoing related-task _meta)
  send_sampling_response(request_id, merge_related_task_meta(result, params))
end

#handle_server_request(msg) ⇒ void

This method returns an undefined value.

Handle incoming JSON-RPC request from server (MCP 2025-06-18)

Parameters:

  • msg (Hash) —

    the JSON-RPC request message



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# File 'lib/mcp_client/server_stdio.rb', line 524

def handle_server_request(msg)
  request_id = msg['id']
  method = msg['method']
  params = msg['params'] || {}

  @logger.debug("Received server request: #{method} (id: #{request_id})")

  case method
  when 'ping'
    handle_ping(request_id)
  when 'elicitation/create'
    handle_elicitation_create(request_id, params)
  when 'roots/list'
    handle_roots_list(request_id, params)
  when 'sampling/createMessage'
    handle_sampling_create_message(request_id, params)
  else
    # Unknown request method, send error response
    send_error_response(request_id, -32_601, "Method not found: #{method}")
  end
rescue StandardError => e
  @logger.error("Error handling server request: #{e.message}")
  send_error_response(request_id, -32_603, "Internal error: #{e.message}")
end

#list_prompts ⇒ Array<MCPClient::Prompt>

List all prompts available from the MCP server

Returns:

Raises:



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# File 'lib/mcp_client/server_stdio.rb', line 208

def list_prompts
  ensure_initialized
  collect_paginated('prompts') do |cursor|
    params = {}
    params['cursor'] = cursor if cursor
    req_id = next_id
    req = { 'jsonrpc' => '2.0', 'id' => req_id, 'method' => 'prompts/list', 'params' => params }
    send_request(req)
    res = wait_response(req_id)
    if (err = res['error'])
      raise MCPClient::Errors::ServerError, err['message']
    end

    result = res['result'] || {}
    prompts = (result['prompts'] || []).map { |td| MCPClient::Prompt.from_json(td, server: self) }
    [prompts, result['nextCursor']]
  end
rescue StandardError => e
  raise MCPClient::Errors::PromptGetError, "Error listing prompts: #{e.message}"
end

#list_resource_templates(cursor: nil) ⇒ Hash

List all resource templates available from the MCP server

Parameters:

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

    optional cursor for pagination

Returns:

  • (Hash) —

    result containing resourceTemplates array and optional nextCursor

Raises:



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# File 'lib/mcp_client/server_stdio.rb', line 313

def list_resource_templates(cursor: nil)
  ensure_initialized
  req_id = next_id
  params = {}
  params['cursor'] = cursor if cursor
  req = { 'jsonrpc' => '2.0', 'id' => req_id, 'method' => 'resources/templates/list', 'params' => params }
  send_request(req)
  res = wait_response(req_id)
  if (err = res['error'])
    raise MCPClient::Errors::ServerError, err['message']
  end

  result = res['result'] || {}
  templates = (result['resourceTemplates'] || []).map { |td| MCPClient::ResourceTemplate.from_json(td, server: self) }
  { 'resourceTemplates' => templates, 'nextCursor' => result['nextCursor'] }
rescue StandardError => e
  raise MCPClient::Errors::ResourceReadError, "Error listing resource templates: #{e.message}"
end

#list_resources(cursor: nil) ⇒ Hash

List all resources available from the MCP server

Parameters:

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

    optional cursor for pagination

Returns:

  • (Hash) —

    result containing resources array and optional nextCursor

Raises:



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# File 'lib/mcp_client/server_stdio.rb', line 261

def list_resources(cursor: nil)
  ensure_initialized
  req_id = next_id
  params = {}
  params['cursor'] = cursor if cursor
  req = { 'jsonrpc' => '2.0', 'id' => req_id, 'method' => 'resources/list', 'params' => params }
  send_request(req)
  res = wait_response(req_id)
  if (err = res['error'])
    raise MCPClient::Errors::ServerError, err['message']
  end

  result = res['result'] || {}
  resources = (result['resources'] || []).map { |td| MCPClient::Resource.from_json(td, server: self) }
  { 'resources' => resources, 'nextCursor' => result['nextCursor'] }
rescue StandardError => e
  raise MCPClient::Errors::ResourceReadError, "Error listing resources: #{e.message}"
end

#list_tools ⇒ Array<MCPClient::Tool>

List all tools available from the MCP server

Returns:

Raises:



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# File 'lib/mcp_client/server_stdio.rb', line 392

def list_tools
  ensure_initialized
  collect_paginated('tools') do |cursor|
    params = {}
    params['cursor'] = cursor if cursor
    req_id = next_id
    # JSON-RPC method for listing tools
    req = { 'jsonrpc' => '2.0', 'id' => req_id, 'method' => 'tools/list', 'params' => params }
    send_request(req)
    res = wait_response(req_id)
    if (err = res['error'])
      raise MCPClient::Errors::ServerError, err['message']
    end

    result = res['result'] || {}
    tools = (result['tools'] || []).map { |td| MCPClient::Tool.from_json(td, server: self) }
    [tools, result['nextCursor']]
  end
rescue StandardError => e
  raise MCPClient::Errors::ToolCallError, "Error listing tools: #{e.message}"
end

#log_level=(level) ⇒ Hash

Set the logging level on the server (MCP 2025-06-18)

Parameters:

  • level (String) —

    the log level ('debug', 'info', 'notice', 'warning', 'error', 'critical', 'alert', 'emergency')

Returns:

  • (Hash) —

    empty result on success

Raises:



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# File 'lib/mcp_client/server_stdio.rb', line 477

def log_level=(level)
  ensure_initialized
  require_capability!('logging', method: 'logging/setLevel')
  req_id = next_id
  req = {
    'jsonrpc' => '2.0',
    'id' => req_id,
    'method' => 'logging/setLevel',
    'params' => { 'level' => level }
  }
  send_request(req)
  res = wait_response(req_id)
  if (err = res['error'])
    raise MCPClient::Errors::ServerError, err['message']
  end

  res['result'] || {}
rescue MCPClient::Errors::CapabilityError
  raise
rescue StandardError => e
  raise MCPClient::Errors::ServerError, "Error setting log level: #{e.message}"
end

#on_elicitation_request(&block) ⇒ void

This method returns an undefined value.

Register a callback for elicitation requests (MCP 2025-06-18)

Parameters:

  • block (Proc) —

    callback that receives (request_id, params) and returns response hash



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# File 'lib/mcp_client/server_stdio.rb', line 503

def on_elicitation_request(&block)
  @elicitation_request_callback = block
end

#on_roots_list_request(&block) ⇒ void

This method returns an undefined value.

Register a callback for roots/list requests (MCP 2025-06-18)

Parameters:

  • block (Proc) —

    callback that receives (request_id, params) and returns response hash



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# File 'lib/mcp_client/server_stdio.rb', line 510

def on_roots_list_request(&block)
  @roots_list_request_callback = block
end

#on_sampling_request(&block) ⇒ void

This method returns an undefined value.

Register a callback for sampling requests (MCP 2025-11-25)

Parameters:

  • block (Proc) —

    callback that receives (request_id, params) and returns response hash



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# File 'lib/mcp_client/server_stdio.rb', line 517

def on_sampling_request(&block)
  @sampling_request_callback = block
end

#pin_pipe_encodings ⇒ void

This method returns an undefined value.

Pin the subprocess pipe encodings to UTF-8 instead of inheriting the process locale (Encoding.default_external). JSON-RPC messages MUST be UTF-8 encoded (MCP 2025-11-25 basic/transports.mdx); under a non-UTF-8 locale (e.g. LANG=C) a valid UTF-8 message would otherwise fail to decode and kill the reader thread. The server MAY also write UTF-8 to stderr, so that pipe is pinned as well.



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# File 'lib/mcp_client/server_stdio.rb', line 109

def pin_pipe_encodings
  [@stdin, @stdout, @stderr].each do |io|
    io&.set_encoding(Encoding::UTF_8)
  end
end

#read_resource(uri) ⇒ Array<MCPClient::ResourceContent>

Read a resource by its URI

Parameters:

  • uri (String) —

    the URI of the resource to read

Returns:

Raises:



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# File 'lib/mcp_client/server_stdio.rb', line 285

def read_resource(uri)
  ensure_initialized
  req_id = next_id
  # JSON-RPC method for reading a resource
  req = {
    'jsonrpc' => '2.0',
    'id' => req_id,
    'method' => 'resources/read',
    'params' => { 'uri' => uri }
  }
  send_request(req)
  res = wait_response(req_id)
  if (err = res['error'])
    raise MCPClient::Errors::ServerError, err['message']
  end

  result = res['result'] || {}
  contents = result['contents'] || []
  contents.map { |content| MCPClient::ResourceContent.from_json(content) }
rescue StandardError => e
  raise MCPClient::Errors::ResourceReadError, "Error reading resource '#{uri}': #{e.message}"
end

#send_elicitation_response(request_id, result) ⇒ void

This method returns an undefined value.

Send elicitation response back to server (MCP 2025-06-18)

Parameters:

  • request_id (String, Integer) —

    the JSON-RPC request ID

  • result (Hash) —

    the elicitation result (action and optional content)



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# File 'lib/mcp_client/server_stdio.rb', line 656

def send_elicitation_response(request_id, result)
  # Error-shaped results become JSON-RPC error responses (e.g. -32602 for
  # an undeclared elicitation mode), mirroring the sampling error path.
  if result.is_a?(Hash) && result['error']
    send_error_response(request_id, result['error']['code'] || -32_603,
                        result['error']['message'] || 'Elicitation error')
    return
  end

  response = {
    'jsonrpc' => '2.0',
    'id' => request_id,
    'result' => result
  }
  send_message(response)
end

#send_error_response(request_id, code, message) ⇒ void

This method returns an undefined value.

Send error response back to server (MCP 2025-06-18)

Parameters:

  • request_id (String, Integer) —

    the JSON-RPC request ID

  • code (Integer) —

    the error code

  • message (String) —

    the error message



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# File 'lib/mcp_client/server_stdio.rb', line 678

def send_error_response(request_id, code, message)
  response = {
    'jsonrpc' => '2.0',
    'id' => request_id,
    'error' => {
      'code' => code,
      'message' => message
    }
  }
  send_message(response)
end

#send_message(message) ⇒ void

This method returns an undefined value.

Send a JSON-RPC message to the server

Parameters:

  • message (Hash) —

    the message to send



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# File 'lib/mcp_client/server_stdio.rb', line 693

def send_message(message)
  json = JSON.generate(message)
  @stdin.puts(json)
  @stdin.flush
  @logger.debug("Sent message: #{json}")
rescue StandardError => e
  @logger.error("Error sending message: #{e.message}")
end

#send_roots_list_response(request_id, result) ⇒ void

This method returns an undefined value.

Send roots/list response back to server (MCP 2025-06-18)

Parameters:

  • request_id (String, Integer) —

    the JSON-RPC request ID

  • result (Hash) —

    the roots list result



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# File 'lib/mcp_client/server_stdio.rb', line 624

def send_roots_list_response(request_id, result)
  response = {
    'jsonrpc' => '2.0',
    'id' => request_id,
    'result' => result
  }
  send_message(response)
end

#send_sampling_response(request_id, result) ⇒ void

This method returns an undefined value.

Send sampling response back to server (MCP 2025-11-25)

Parameters:

  • request_id (String, Integer) —

    the JSON-RPC request ID

  • result (Hash) —

    the sampling result (role, content, model, stopReason)



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# File 'lib/mcp_client/server_stdio.rb', line 637

def send_sampling_response(request_id, result)
  # Check if result contains an error
  if result.is_a?(Hash) && result['error']
    send_error_response(request_id, result['error']['code'] || -1, result['error']['message'] || 'Sampling error')
    return
  end

  response = {
    'jsonrpc' => '2.0',
    'id' => request_id,
    'result' => result
  }
  send_message(response)
end

#signal_server_process(signal) ⇒ void

This method returns an undefined value.

Send a signal to the server process, tolerating a process that has already exited or cannot be signalled.

Parameters:

  • signal (String) —

    signal name, e.g. 'TERM' or 'KILL'



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# File 'lib/mcp_client/server_stdio.rb', line 769

def signal_server_process(signal)
  Process.kill(signal, @wait_thread.pid)
rescue Errno::ESRCH, Errno::EPERM => e
  @logger.debug("Could not send SIG#{signal} to server process: #{e.class}")
end

#start_reader ⇒ Thread

Spawn a reader thread to collect JSON-RPC responses

Returns:

  • (Thread) —

    the reader thread



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# File 'lib/mcp_client/server_stdio.rb', line 117

def start_reader
  @reader_thread = Thread.new do
    @stdout.each_line do |line|
      handle_line(line)
    end
  rescue StandardError
    # Reader thread aborted unexpectedly
  end
end

#start_stderr_reader ⇒ Thread

Spawn a thread to continuously drain the subprocess stderr.

The child's stderr pipe has a fixed OS buffer (typically 64KB). If it is never read, a server that logs verbosely to stderr eventually blocks on write once the buffer fills, which stalls the whole subprocess (it stops producing stdout / reading stdin) and deadlocks the client. Draining stderr keeps the pipe empty; lines are surfaced at debug level.

Reads happen in bounded chunks (not IO#each_line) so that a server which writes to stderr without newline delimiters cannot make the client buffer a single "line" without limit: any pending fragment larger than STDERR_MAX_LINE_SIZE is flushed rather than retained.

Returns:

  • (Thread) —

    the stderr reader thread



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# File 'lib/mcp_client/server_stdio.rb', line 140

def start_stderr_reader
  @stderr_thread = Thread.new do
    buffer = +''
    loop do
      buffer << @stderr.readpartial(STDERR_READ_CHUNK_SIZE)
      flush_stderr_lines(buffer)
      flush_stderr_overflow(buffer)
    end
  rescue IOError
    # EOFError (a subclass of IOError) on EOF, or IOError on close;
    # emit any trailing partial line before exiting
    @logger.debug("[stderr] #{buffer.chomp}") if buffer && !buffer.empty?
  rescue StandardError
    # reader aborted unexpectedly; nothing actionable
  end
end

#subscribe_resource(uri) ⇒ Boolean

Subscribe to resource updates

Parameters:

  • uri (String) —

    the URI of the resource to subscribe to

Returns:

  • (Boolean) —

    true if subscription successful

Raises:



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# File 'lib/mcp_client/server_stdio.rb', line 337

def subscribe_resource(uri)
  ensure_initialized
  require_capability!('resources', 'subscribe', method: 'resources/subscribe')
  req_id = next_id
  req = {
    'jsonrpc' => '2.0',
    'id' => req_id,
    'method' => 'resources/subscribe',
    'params' => { 'uri' => uri }
  }
  send_request(req)
  res = wait_response(req_id)
  if (err = res['error'])
    raise MCPClient::Errors::ServerError, err['message']
  end

  true
rescue MCPClient::Errors::CapabilityError
  raise
rescue StandardError => e
  raise MCPClient::Errors::ResourceReadError, "Error subscribing to resource '#{uri}': #{e.message}"
end

#terminate_server_process ⇒ void

This method returns an undefined value.

Terminate the spawned server process per the MCP 2025-11-25 stdio shutdown sequence (basic/lifecycle.mdx): stdin has already been closed, so wait for the process to exit on its own; if it does not exit within the grace period send SIGTERM, wait again, and finally send SIGKILL.



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# File 'lib/mcp_client/server_stdio.rb', line 754

def terminate_server_process
  return unless @wait_thread
  return if @wait_thread.join(SHUTDOWN_GRACE_PERIOD)

  signal_server_process('TERM')
  return if @wait_thread.join(SHUTDOWN_GRACE_PERIOD)

  signal_server_process('KILL')
  @wait_thread.join(SHUTDOWN_GRACE_PERIOD)
end

#unsubscribe_resource(uri) ⇒ Boolean

Unsubscribe from resource updates

Parameters:

  • uri (String) —

    the URI of the resource to unsubscribe from

Returns:

  • (Boolean) —

    true if unsubscription successful

Raises:



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# File 'lib/mcp_client/server_stdio.rb', line 365

def unsubscribe_resource(uri)
  ensure_initialized
  require_capability!('resources', 'subscribe', method: 'resources/unsubscribe')
  req_id = next_id
  req = {
    'jsonrpc' => '2.0',
    'id' => req_id,
    'method' => 'resources/unsubscribe',
    'params' => { 'uri' => uri }
  }
  send_request(req)
  res = wait_response(req_id)
  if (err = res['error'])
    raise MCPClient::Errors::ServerError, err['message']
  end

  true
rescue MCPClient::Errors::CapabilityError
  raise
rescue StandardError => e
  raise MCPClient::Errors::ResourceReadError, "Error unsubscribing from resource '#{uri}': #{e.message}"
end