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, MissingRequiredClientCapabilityError, RequestHandlerError, ResourceNotFoundError, URLElicitationRequiredError, UnsupportedProtocolVersionError, 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.



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
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
 
# File 'lib/mcp/server.rb', line 146

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.



144
145
146
 
# File 'lib/mcp/server.rb', line 144

def cache_scope
  @cache_scope
end

#capabilities ⇒ Object

Returns the value of attribute capabilities.



143
144
145
 
# File 'lib/mcp/server.rb', line 143

def capabilities
  @capabilities
end

#client_capabilities ⇒ Object (readonly)

Returns the value of attribute client_capabilities.



144
145
146
 
# File 'lib/mcp/server.rb', line 144

def client_capabilities
  @client_capabilities
end

#configuration ⇒ Object

Returns the value of attribute configuration.



143
144
145
 
# File 'lib/mcp/server.rb', line 143

def configuration
  @configuration
end

#description ⇒ Object

Returns the value of attribute description.



143
144
145
 
# File 'lib/mcp/server.rb', line 143

def description
  @description
end

#icons ⇒ Object

Returns the value of attribute icons.



143
144
145
 
# File 'lib/mcp/server.rb', line 143

def icons
  @icons
end

#instructions ⇒ Object

Returns the value of attribute instructions.



143
144
145
 
# File 'lib/mcp/server.rb', line 143

def instructions
  @instructions
end

#logging_message_notification ⇒ Object

Returns the value of attribute logging_message_notification.



143
144
145
 
# File 'lib/mcp/server.rb', line 143

def logging_message_notification
  @logging_message_notification
end

#name ⇒ Object

Returns the value of attribute name.



143
144
145
 
# File 'lib/mcp/server.rb', line 143

def name
  @name
end

#page_size ⇒ Object

Returns the value of attribute page_size.



144
145
146
 
# File 'lib/mcp/server.rb', line 144

def page_size
  @page_size
end

#prompts ⇒ Object

Returns the value of attribute prompts.



143
144
145
 
# File 'lib/mcp/server.rb', line 143

def prompts
  @prompts
end

#resource_templates ⇒ Object

Returns the value of attribute resource_templates.



143
144
145
 
# File 'lib/mcp/server.rb', line 143

def resource_templates
  @resource_templates
end

#resources ⇒ Object

Returns the value of attribute resources.



144
145
146
 
# File 'lib/mcp/server.rb', line 144

def resources
  @resources
end

#server_context ⇒ Object

Returns the value of attribute server_context.



143
144
145
 
# File 'lib/mcp/server.rb', line 143

def server_context
  @server_context
end

#title ⇒ Object

Returns the value of attribute title.



143
144
145
 
# File 'lib/mcp/server.rb', line 143

def title
  @title
end

#tools ⇒ Object

Returns the value of attribute tools.



143
144
145
 
# File 'lib/mcp/server.rb', line 143

def tools
  @tools
end

#transport ⇒ Object

Returns the value of attribute transport.



143
144
145
 
# File 'lib/mcp/server.rb', line 143

def transport
  @transport
end

#ttl_ms ⇒ Object

Returns the value of attribute ttl_ms.



144
145
146
 
# File 'lib/mcp/server.rb', line 144

def ttl_ms
  @ttl_ms
end

#version ⇒ Object

Returns the value of attribute version.



143
144
145
 
# File 'lib/mcp/server.rb', line 143

def version
  @version
end

#website_url ⇒ Object

Returns the value of attribute website_url.



143
144
145
 
# File 'lib/mcp/server.rb', line 143

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 



415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
 
# File 'lib/mcp/server.rb', line 415

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.



393
394
395
 
# File 'lib/mcp/server.rb', line 393

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

#define_custom_method(method_name:, &block) ⇒ Object 



290
291
292
293
294
295
296
 
# File 'lib/mcp/server.rb', line 290

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 



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

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 



262
263
264
265
266
267
268
269
270
271
272
 
# File 'lib/mcp/server.rb', line 262

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 



274
275
276
277
278
279
280
281
282
283
 
# File 'lib/mcp/server.rb', line 274

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 



245
246
247
248
249
250
251
252
253
 
# File 'lib/mcp/server.rb', line 245

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.



226
227
228
229
230
 
# File 'lib/mcp/server.rb', line 226

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.



239
240
241
242
243
 
# File 'lib/mcp/server.rb', line 239

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.



354
355
356
357
358
359
360
361
362
363
364
 
# File 'lib/mcp/server.rb', line 354

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 



335
336
337
338
339
340
341
 
# File 'lib/mcp/server.rb', line 335

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 



343
344
345
346
347
348
349
 
# File 'lib/mcp/server.rb', line 343

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 



327
328
329
330
331
332
333
 
# File 'lib/mcp/server.rb', line 327

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.



384
385
386
 
# File 'lib/mcp/server.rb', line 384

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.



402
403
404
 
# File 'lib/mcp/server.rb', line 402

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.



411
412
413
 
# File 'lib/mcp/server.rb', line 411

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).



374
375
376
 
# File 'lib/mcp/server.rb', line 374

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