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/pending_response.rb,
lib/mcp/server/input_required_result.rb,
lib/mcp/server/request_state_security.rb,
lib/mcp/server/transports/stdio_transport.rb,
lib/mcp/server/transports/streamable_http_transport.rb

Defined Under Namespace

Modules: Pagination, Transports Classes: Capabilities, InputRequiredResult, MethodAlreadyDefinedError, MissingRequiredClientCapabilityError, PendingResponse, RequestHandlerError, RequestStateSecurity, RequestTimeoutError, 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
CACHEABLE_RESULT_METHODS =

Methods whose results are cacheable per SEP-2549. On the modern wire (2026-07-28) the ttlMs/cacheScope hints are REQUIRED on these results, so unset hints get the spec defaults there; on stable protocol versions emission stays opt-in via apply_cache_metadata.

[
  Methods::TOOLS_LIST,
  Methods::PROMPTS_LIST,
  Methods::RESOURCES_LIST,
  Methods::RESOURCES_TEMPLATES_LIST,
  Methods::RESOURCES_READ,
].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, request_state_security: nil, input_required_legacy_shim: true, transport: nil) ⇒ Server

Returns a new instance of Server.



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# File 'lib/mcp/server.rb', line 187

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,
  request_state_security: nil,
  input_required_legacy_shim: true,
  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)
  @resources_list_handler = nil
  @server_context = server_context
  self.page_size = page_size
  self.ttl_ms = ttl_ms
  self.cache_scope = cache_scope
  @request_state_security = request_state_security

  # Dual-era authoring (SEP-2322): on the legacy wire, an `input_required` result is fulfilled
  # through real server-to-client requests and the handler re-runs, so handlers written
  # in the 2026 style serve both eras. `false` restores the strict rejection of `input_required`
  # on legacy requests. Matches the TypeScript SDK's default-on legacy shim.
  @input_required_legacy_shim = input_required_legacy_shim
  @configuration = MCP.configuration.merge(configuration)
  @client = nil
  @client_protocol_version = 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.



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# File 'lib/mcp/server.rb', line 185

def cache_scope
  @cache_scope
end

#capabilities ⇒ Object

Returns the value of attribute capabilities.



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# File 'lib/mcp/server.rb', line 184

def capabilities
  @capabilities
end

#client_capabilities ⇒ Object (readonly)

Returns the value of attribute client_capabilities.



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# File 'lib/mcp/server.rb', line 185

def client_capabilities
  @client_capabilities
end

#configuration ⇒ Object

Returns the value of attribute configuration.



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# File 'lib/mcp/server.rb', line 184

def configuration
  @configuration
end

#description ⇒ Object

Returns the value of attribute description.



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# File 'lib/mcp/server.rb', line 184

def description
  @description
end

#icons ⇒ Object

Returns the value of attribute icons.



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# File 'lib/mcp/server.rb', line 184

def icons
  @icons
end

#instructions ⇒ Object

Returns the value of attribute instructions.



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# File 'lib/mcp/server.rb', line 184

def instructions
  @instructions
end

#logging_message_notification ⇒ Object

Returns the value of attribute logging_message_notification.



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# File 'lib/mcp/server.rb', line 184

def logging_message_notification
  @logging_message_notification
end

#name ⇒ Object

Returns the value of attribute name.



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# File 'lib/mcp/server.rb', line 184

def name
  @name
end

#page_size ⇒ Object

Returns the value of attribute page_size.



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# File 'lib/mcp/server.rb', line 185

def page_size
  @page_size
end

#prompts ⇒ Object

Returns the value of attribute prompts.



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# File 'lib/mcp/server.rb', line 184

def prompts
  @prompts
end

#request_state_security ⇒ Object (readonly)

Returns the value of attribute request_state_security.



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# File 'lib/mcp/server.rb', line 185

def request_state_security
  @request_state_security
end

#resource_templates ⇒ Object

Returns the value of attribute resource_templates.



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# File 'lib/mcp/server.rb', line 184

def resource_templates
  @resource_templates
end

#resources ⇒ Object

Returns the value of attribute resources.



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# File 'lib/mcp/server.rb', line 185

def resources
  @resources
end

#server_context ⇒ Object

Returns the value of attribute server_context.



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# File 'lib/mcp/server.rb', line 184

def server_context
  @server_context
end

#title ⇒ Object

Returns the value of attribute title.



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# File 'lib/mcp/server.rb', line 184

def title
  @title
end

#tools ⇒ Object

Returns the value of attribute tools.



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# File 'lib/mcp/server.rb', line 184

def tools
  @tools
end

#transport ⇒ Object

Returns the value of attribute transport.



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# File 'lib/mcp/server.rb', line 184

def transport
  @transport
end

#ttl_ms ⇒ Object

Returns the value of attribute ttl_ms.



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# File 'lib/mcp/server.rb', line 185

def ttl_ms
  @ttl_ms
end

#version ⇒ Object

Returns the value of attribute version.



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# File 'lib/mcp/server.rb', line 184

def version
  @version
end

#website_url ⇒ Object

Returns the value of attribute website_url.



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# File 'lib/mcp/server.rb', line 184

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 



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# File 'lib/mcp/server.rb', line 489

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.



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# File 'lib/mcp/server.rb', line 461

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

#define_custom_method(method_name:, &block) ⇒ Object 



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# File 'lib/mcp/server.rb', line 342

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 



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# File 'lib/mcp/server.rb', line 307

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 



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# File 'lib/mcp/server.rb', line 314

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 



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# File 'lib/mcp/server.rb', line 326

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 



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# File 'lib/mcp/server.rb', line 297

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.



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# File 'lib/mcp/server.rb', line 278

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.



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# File 'lib/mcp/server.rb', line 291

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.



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# File 'lib/mcp/server.rb', line 406

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 



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# File 'lib/mcp/server.rb', line 387

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 



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

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 



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# File 'lib/mcp/server.rb', line 379

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_list_handler {|params, server_context:| ... } ⇒ Object

Sets a custom handler for resources/list requests, letting the visible list depend on request context such as the authenticated principal or granted scope. The block returns the resource collection to serve; the framework paginates it and stamps SEP-2549 cache hints exactly as it does for the constructor-provided resources, so the block returns only the array, not the paginated result. A block that declares a server_context: keyword receives an MCP::ServerContext. When no handler is set, the constructor-provided resources array is served unchanged.

The block is invoked once per page, so it must return a stable ordering across the pages of one logical query; the cursor is a positional offset into the returned collection.

Yields:

  • (params, server_context:) —

    The request params, and an MCP::ServerContext when declared.

Yield Returns:



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# File 'lib/mcp/server.rb', line 442

def resources_list_handler(&block)
  @resources_list_handler = block
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.



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# File 'lib/mcp/server.rb', line 452

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 response is an empty result, except that a _meta hash the block returns is passed through - the spec defines no other member for this result, so any other field the block returns is dropped. Nest a subscription identifier or other advisory data under _meta.

Yields:

  • (params) —

    The request params containing :uri.

Yield Returns:

  • (Hash, nil) —

    Optionally { _meta: { ... } }; any other shape yields an empty result.



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# File 'lib/mcp/server.rb', line 473

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 response is an empty result, except that a _meta hash the block returns is passed through - the spec defines no other member for this result, so any other field the block returns is dropped. Nest a subscription identifier or other advisory data under _meta.

Yields:

  • (params) —

    The request params containing :uri.

Yield Returns:

  • (Hash, nil) —

    Optionally { _meta: { ... } }; any other shape yields an empty result.



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# File 'lib/mcp/server.rb', line 485

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).



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# File 'lib/mcp/server.rb', line 426

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