Class: MCP::Server

Inherits:
Object
  • Object
show all
Includes:
Instrumentation, Pagination
Defined in:
lib/mcp/server.rb,
lib/mcp/server/pagination.rb,
lib/mcp/server/transports.rb,
lib/mcp/server/capabilities.rb,
lib/mcp/server/transports/stdio_transport.rb,
lib/mcp/server/transports/streamable_http_transport.rb

Defined Under Namespace

Modules: Pagination, Transports Classes: Capabilities, MethodAlreadyDefinedError, RequestHandlerError, ResourceNotFoundError, URLElicitationRequiredError, ValidationError

Constant Summary collapse

DEFAULT_VERSION = 
"0.1.0"
UNSUPPORTED_PROPERTIES_UNTIL_2025_06_18 = 
[:description, :icons].freeze
UNSUPPORTED_PROPERTIES_UNTIL_2025_03_26 = 
[:title, :websiteUrl].freeze
DEFAULT_COMPLETION_RESULT = 
{ completion: { values: [], hasMore: false } }.freeze
MAX_COMPLETION_VALUES =

Servers return an array of completion values ranked by relevance, with maximum 100 items per response. https://modelcontextprotocol.io/specification/2025-11-25/server/utilities/completion#completion-results

100
CACHE_SCOPES =

Allowed values for the SEP-2549 cacheScope cache hint.

["public", "private"].freeze

Instance Attribute Summary collapse

Instance Method Summary collapse

Methods included from Instrumentation

#add_instrumentation_data, #instrument_call

Constructor Details

#initialize(description: nil, icons: [], name: "model_context_protocol", title: nil, version: DEFAULT_VERSION, website_url: nil, instructions: nil, tools: [], prompts: [], resources: [], resource_templates: [], server_context: nil, configuration: nil, capabilities: nil, page_size: nil, ttl_ms: nil, cache_scope: nil, transport: nil) ⇒ Server

Returns a new instance of Server.



111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
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
170
171
172
173
174
175
176
177
178
179
180
181
182
 
# File 'lib/mcp/server.rb', line 111

def initialize(
  description: nil,
  icons: [],
  name: "model_context_protocol",
  title: nil,
  version: DEFAULT_VERSION,
  website_url: nil,
  instructions: nil,
  tools: [],
  prompts: [],
  resources: [],
  resource_templates: [],
  server_context: nil,
  configuration: nil,
  capabilities: nil,
  page_size: nil,
  ttl_ms: nil,
  cache_scope: nil,
  transport: nil
)
  @description = description
  @icons = icons
  @name = name
  @title = title
  @version = version
  @website_url = website_url
  @instructions = instructions
  @tool_names = tools.map(&:name_value)
  @tools = tools.to_h { |t| [t.name_value, t] }
  @prompts = prompts.to_h { |p| [p.name_value, p] }
  @resources = resources
  @resource_templates = resource_templates
  @resource_index = index_resources_by_uri(resources)
  @server_context = server_context
  self.page_size = page_size
  self.ttl_ms = ttl_ms
  self.cache_scope = cache_scope
  @configuration = MCP.configuration.merge(configuration)
  @client = nil

  validate!

  # Accept either a plain Hash or an `MCP::Server::Capabilities` builder.
  @capabilities = if capabilities.is_a?(Capabilities)
    capabilities.to_h
  else
    capabilities || default_capabilities
  end
  @client_capabilities = nil
  @logging_message_notification = nil

  @handlers = {
    Methods::RESOURCES_LIST => method(:list_resources),
    Methods::RESOURCES_READ => method(:read_resource),
    Methods::RESOURCES_TEMPLATES_LIST => method(:list_resource_templates),
    Methods::RESOURCES_SUBSCRIBE => ->(_) { {} },
    Methods::RESOURCES_UNSUBSCRIBE => ->(_) { {} },
    Methods::TOOLS_LIST => method(:list_tools),
    Methods::TOOLS_CALL => method(:call_tool),
    Methods::PROMPTS_LIST => method(:list_prompts),
    Methods::PROMPTS_GET => method(:get_prompt),
    Methods::INITIALIZE => method(:init),
    Methods::SERVER_DISCOVER => method(:discover),
    Methods::PING => ->(_) { {} },
    Methods::NOTIFICATIONS_INITIALIZED => ->(_) {},
    Methods::NOTIFICATIONS_PROGRESS => ->(_) {},
    Methods::NOTIFICATIONS_ROOTS_LIST_CHANGED => ->(_) {},
    Methods::COMPLETION_COMPLETE => ->(_) { DEFAULT_COMPLETION_RESULT },
    Methods::LOGGING_SET_LEVEL => method(:configure_logging_level),
  }
  @transport = transport
end

Instance Attribute Details

#cache_scope ⇒ Object

Returns the value of attribute cache_scope.



109
110
111
 
# File 'lib/mcp/server.rb', line 109

def cache_scope
  @cache_scope
end

#capabilities ⇒ Object

Returns the value of attribute capabilities.



108
109
110
 
# File 'lib/mcp/server.rb', line 108

def capabilities
  @capabilities
end

#client_capabilities ⇒ Object (readonly)

Returns the value of attribute client_capabilities.



109
110
111
 
# File 'lib/mcp/server.rb', line 109

def client_capabilities
  @client_capabilities
end

#configuration ⇒ Object

Returns the value of attribute configuration.



108
109
110
 
# File 'lib/mcp/server.rb', line 108

def configuration
  @configuration
end

#description ⇒ Object

Returns the value of attribute description.



108
109
110
 
# File 'lib/mcp/server.rb', line 108

def description
  @description
end

#icons ⇒ Object

Returns the value of attribute icons.



108
109
110
 
# File 'lib/mcp/server.rb', line 108

def icons
  @icons
end

#instructions ⇒ Object

Returns the value of attribute instructions.



108
109
110
 
# File 'lib/mcp/server.rb', line 108

def instructions
  @instructions
end

#logging_message_notification ⇒ Object

Returns the value of attribute logging_message_notification.



108
109
110
 
# File 'lib/mcp/server.rb', line 108

def logging_message_notification
  @logging_message_notification
end

#name ⇒ Object

Returns the value of attribute name.



108
109
110
 
# File 'lib/mcp/server.rb', line 108

def name
  @name
end

#page_size ⇒ Object

Returns the value of attribute page_size.



109
110
111
 
# File 'lib/mcp/server.rb', line 109

def page_size
  @page_size
end

#prompts ⇒ Object

Returns the value of attribute prompts.



108
109
110
 
# File 'lib/mcp/server.rb', line 108

def prompts
  @prompts
end

#resource_templates ⇒ Object

Returns the value of attribute resource_templates.



108
109
110
 
# File 'lib/mcp/server.rb', line 108

def resource_templates
  @resource_templates
end

#resources ⇒ Object

Returns the value of attribute resources.



109
110
111
 
# File 'lib/mcp/server.rb', line 109

def resources
  @resources
end

#server_context ⇒ Object

Returns the value of attribute server_context.



108
109
110
 
# File 'lib/mcp/server.rb', line 108

def server_context
  @server_context
end

#title ⇒ Object

Returns the value of attribute title.



108
109
110
 
# File 'lib/mcp/server.rb', line 108

def title
  @title
end

#tools ⇒ Object

Returns the value of attribute tools.



108
109
110
 
# File 'lib/mcp/server.rb', line 108

def tools
  @tools
end

#transport ⇒ Object

Returns the value of attribute transport.



108
109
110
 
# File 'lib/mcp/server.rb', line 108

def transport
  @transport
end

#ttl_ms ⇒ Object

Returns the value of attribute ttl_ms.



109
110
111
 
# File 'lib/mcp/server.rb', line 109

def ttl_ms
  @ttl_ms
end

#version ⇒ Object

Returns the value of attribute version.



108
109
110
 
# File 'lib/mcp/server.rb', line 108

def version
  @version
end

#website_url ⇒ Object

Returns the value of attribute website_url.



108
109
110
 
# File 'lib/mcp/server.rb', line 108

def website_url
  @website_url
end

Instance Method Details

#build_sampling_params(capabilities, messages:, max_tokens:, system_prompt: nil, model_preferences: nil, include_context: nil, temperature: nil, stop_sequences: nil, metadata: nil, tools: nil, tool_choice: nil) ⇒ Object 



380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
 
# File 'lib/mcp/server.rb', line 380

def build_sampling_params(
  capabilities,
  messages:,
  max_tokens:,
  system_prompt: nil,
  model_preferences: nil,
  include_context: nil,
  temperature: nil,
  stop_sequences: nil,
  metadata: nil,
  tools: nil,
  tool_choice: nil
)
  unless capabilities&.dig(:sampling)
    raise "Client does not support sampling."
  end

  if tools && !capabilities.dig(:sampling, :tools)
    raise "Client does not support sampling with tools."
  end

  if tool_choice && !capabilities.dig(:sampling, :tools)
    raise "Client does not support sampling with tool_choice."
  end

  {
    messages: messages,
    maxTokens: max_tokens,
    systemPrompt: system_prompt,
    modelPreferences: model_preferences,
    includeContext: include_context,
    temperature: temperature,
    stopSequences: stop_sequences,
    metadata: metadata,
    tools: tools,
    toolChoice: tool_choice,
  }.compact
end

#completion_handler {|params| ... } ⇒ Object

Sets a custom handler for completion/complete requests. The block receives the parsed request params and should return completion values.

Yields:

  • (params) —

    The request params containing :ref, :argument, and optionally :context.

Yield Returns:

  • (Hash) —

    A hash with :completion key containing :values, optional :total, and :hasMore.



358
359
360
 
# File 'lib/mcp/server.rb', line 358

def completion_handler(&block)
  @handlers[Methods::COMPLETION_COMPLETE] = block
end

#define_custom_method(method_name:, &block) ⇒ Object 



255
256
257
258
259
260
261
 
# File 'lib/mcp/server.rb', line 255

def define_custom_method(method_name:, &block)
  if @handlers.key?(method_name)
    raise MethodAlreadyDefinedError, method_name
  end

  @handlers[method_name] = block
end

#define_prompt(name: nil, title: nil, description: nil, arguments: [], &block) ⇒ Object 



220
221
222
223
224
225
 
# File 'lib/mcp/server.rb', line 220

def define_prompt(name: nil, title: nil, description: nil, arguments: [], &block)
  prompt = Prompt.define(name: name, title: title, description: description, arguments: arguments, &block)
  @prompts[prompt.name_value] = prompt

  validate!
end

#define_resource(uri: nil, name: nil, title: nil, description: nil, icons: [], mime_type: nil, annotations: nil, size: nil, meta: nil, &block) ⇒ Object 



227
228
229
230
231
232
233
234
235
236
237
 
# File 'lib/mcp/server.rb', line 227

def define_resource(uri: nil, name: nil, title: nil, description: nil, icons: [], mime_type: nil, annotations: nil, size: nil, meta: nil, &block)
  resource = Resource.define(
    uri: uri, name: name, title: title, description: description, icons: icons,
    mime_type: mime_type, annotations: annotations, size: size, meta: meta,
    &block
  )
  @resources << resource
  @resource_index[resource.uri] = resource

  validate!
end

#define_resource_template(uri_template: nil, name: nil, title: nil, description: nil, icons: [], mime_type: nil, annotations: nil, meta: nil, &block) ⇒ Object 



239
240
241
242
243
244
245
246
247
248
 
# File 'lib/mcp/server.rb', line 239

def define_resource_template(uri_template: nil, name: nil, title: nil, description: nil, icons: [], mime_type: nil, annotations: nil, meta: nil, &block)
  resource_template = ResourceTemplate.define(
    uri_template: uri_template, name: name, title: title, description: description, icons: icons,
    mime_type: mime_type, annotations: annotations, meta: meta,
    &block
  )
  @resource_templates << resource_template

  validate!
end

#define_tool(name: nil, title: nil, description: nil, input_schema: nil, output_schema: nil, annotations: nil, meta: nil, &block) ⇒ Object 



210
211
212
213
214
215
216
217
218
 
# File 'lib/mcp/server.rb', line 210

def define_tool(name: nil, title: nil, description: nil, input_schema: nil, output_schema: nil, annotations: nil, meta: nil, &block)
  tool = Tool.define(name: name, title: title, description: description, input_schema: input_schema, output_schema: output_schema, annotations: annotations, meta: meta, &block)
  tool_name = tool.name_value

  @tool_names << tool_name
  @tools[tool_name] = tool

  validate!
end

#handle(request, session: nil) ⇒ Hash?

Processes a parsed JSON-RPC request and returns the response as a Hash.

Parameters:

  • request (Hash) —

    A parsed JSON-RPC request.

  • session (ServerSession, nil) (defaults to: nil) —

    Per-connection session. Passed by ServerSession#handle for session-scoped notification delivery. When nil, progress and logging notifications from tool handlers are silently skipped.

Returns:

  • (Hash, nil) —

    The JSON-RPC response, or nil for notifications.



191
192
193
194
195
 
# File 'lib/mcp/server.rb', line 191

def handle(request, session: nil)
  JsonRpcHandler.handle(request) do |method, request_id|
    handle_request(request, method, session: session, related_request_id: request_id)
  end
end

#handle_json(request, session: nil) ⇒ String?

Processes a JSON-RPC request string and returns the response as a JSON string.

Parameters:

  • request (String) —

    A JSON-RPC request as a JSON string.

  • session (ServerSession, nil) (defaults to: nil) —

    Per-connection session. Passed by ServerSession#handle_json for session-scoped notification delivery. When nil, progress and logging notifications from tool handlers are silently skipped.

Returns:

  • (String, nil) —

    The JSON-RPC response as JSON, or nil for notifications.



204
205
206
207
208
 
# File 'lib/mcp/server.rb', line 204

def handle_json(request, session: nil)
  JsonRpcHandler.handle_json(request) do |method, request_id|
    handle_request(request, method, session: session, related_request_id: request_id)
  end
end

#notify_log_message(data:, level:, logger: nil) ⇒ Object

Deprecated.

MCP Logging (logging/setLevel and notifications/message) is deprecated as of MCP protocol version 2026-07-28 (SEP-2577). Use stderr or OpenTelemetry instead.



319
320
321
322
323
324
325
326
327
328
329
 
# File 'lib/mcp/server.rb', line 319

def notify_log_message(data:, level:, logger: nil)
  return unless @transport
  return unless logging_message_notification&.should_notify?(level)

  params = { "data" => data, "level" => level }
  params["logger"] = logger if logger

  @transport.send_notification(Methods::NOTIFICATIONS_MESSAGE, params)
rescue => e
  report_exception(e, { notification: "log_message" })
end

#notify_prompts_list_changed ⇒ Object 



300
301
302
303
304
305
306
 
# File 'lib/mcp/server.rb', line 300

def notify_prompts_list_changed
  return unless @transport

  @transport.send_notification(Methods::NOTIFICATIONS_PROMPTS_LIST_CHANGED)
rescue => e
  report_exception(e, { notification: "prompts_list_changed" })
end

#notify_resources_list_changed ⇒ Object 



308
309
310
311
312
313
314
 
# File 'lib/mcp/server.rb', line 308

def notify_resources_list_changed
  return unless @transport

  @transport.send_notification(Methods::NOTIFICATIONS_RESOURCES_LIST_CHANGED)
rescue => e
  report_exception(e, { notification: "resources_list_changed" })
end

#notify_tools_list_changed ⇒ Object 



292
293
294
295
296
297
298
 
# File 'lib/mcp/server.rb', line 292

def notify_tools_list_changed
  return unless @transport

  @transport.send_notification(Methods::NOTIFICATIONS_TOOLS_LIST_CHANGED)
rescue => e
  report_exception(e, { notification: "tools_list_changed" })
end

#resources_read_handler {|params| ... } ⇒ Object

Sets a custom handler for resources/read requests. The block receives the parsed request params and should return resource contents. The return value is set as the contents field of the response.

Yields:

  • (params) —

    The request params containing :uri.

Yield Returns:

  • (Array<Hash>, Hash) —

    Resource contents.



349
350
351
 
# File 'lib/mcp/server.rb', line 349

def resources_read_handler(&block)
  @handlers[Methods::RESOURCES_READ] = block
end

#resources_subscribe_handler {|params| ... } ⇒ Object

Sets a custom handler for resources/subscribe requests. The block receives the parsed request params. The return value is ignored; the response is always an empty result {} per the MCP specification.

Yields:

  • (params) —

    The request params containing :uri.



367
368
369
 
# File 'lib/mcp/server.rb', line 367

def resources_subscribe_handler(&block)
  @handlers[Methods::RESOURCES_SUBSCRIBE] = block
end

#resources_unsubscribe_handler {|params| ... } ⇒ Object

Sets a custom handler for resources/unsubscribe requests. The block receives the parsed request params. The return value is ignored; the response is always an empty result {} per the MCP specification.

Yields:

  • (params) —

    The request params containing :uri.



376
377
378
 
# File 'lib/mcp/server.rb', line 376

def resources_unsubscribe_handler(&block)
  @handlers[Methods::RESOURCES_UNSUBSCRIBE] = block
end

#roots_list_changed_handler {|params| ... } ⇒ Object

Deprecated.

MCP Roots (roots/list and notifications/roots/list_changed) is deprecated as of MCP protocol version 2026-07-28 (SEP-2577). Use tool parameters, resource URIs, server configuration, or environment variables instead.

Sets a handler for notifications/roots/list_changed notifications. Called when a client notifies the server that its filesystem roots have changed.

Yields:

  • (params) —

    The notification params (typically nil).



339
340
341
 
# File 'lib/mcp/server.rb', line 339

def roots_list_changed_handler(&block)
  @handlers[Methods::NOTIFICATIONS_ROOTS_LIST_CHANGED] = block
end