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.



24
25
26
27
28
29
30
31
32
33
34
35
36
37
 
# 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
end

Instance Attribute Details

#args ⇒ Object (readonly)

Returns the value of attribute args.



22
23
24
 
# File 'lib/mcp/client/stdio.rb', line 22

def args
  @args
end

#command ⇒ Object (readonly)

Returns the value of attribute command.



22
23
24
 
# File 'lib/mcp/client/stdio.rb', line 22

def command
  @command
end

#env ⇒ Object (readonly)

Returns the value of attribute env.



22
23
24
 
# File 'lib/mcp/client/stdio.rb', line 22

def env
  @env
end

#server_info ⇒ Object (readonly)

Returns the value of attribute server_info.



22
23
24
 
# File 'lib/mcp/client/stdio.rb', line 22

def server_info
  @server_info
end

Instance Method Details

#close ⇒ Object 



171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
 
# File 'lib/mcp/client/stdio.rb', line 171

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.



58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
 
# File 'lib/mcp/client/stdio.rb', line 58

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) 


131
132
133
 
# File 'lib/mcp/client/stdio.rb', line 131

def connected?
  @initialized
end

#send_request(request:) ⇒ Object 



135
136
137
138
139
140
141
 
# File 'lib/mcp/client/stdio.rb', line 135

def send_request(request:)
  start unless @started
  connect unless @initialized

  write_message(request)
  read_response(request)
end

#start ⇒ Object 



143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
 
# File 'lib/mcp/client/stdio.rb', line 143

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