Class: MCPClient::ServerStreamableHTTP

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

Overview

Implementation of MCP server that communicates via Streamable HTTP transport (MCP 2025-06-18) This transport uses HTTP POST for RPC calls with optional SSE responses, and GET for event streams Compliant with MCP specification version 2025-06-18

Key features:

  • Supports server-sent events (SSE) for real-time notifications
  • Handles ping/pong keepalive mechanism
  • Thread-safe connection management
  • Automatic reconnection with exponential backoff

Defined Under Namespace

Modules: JsonRpcTransport

Constant Summary collapse

DEFAULT_READ_TIMEOUT =

Default values for connection settings

30
DEFAULT_MAX_RETRIES = 
3
SSE_CONNECTION_TIMEOUT =

SSE connection settings

300
SSE_RECONNECT_DELAY =

5 minutes

1
SSE_MAX_RECONNECT_DELAY =

Initial reconnect delay in seconds

30
THREAD_JOIN_TIMEOUT =

Maximum reconnect delay in seconds

5
MIN_RESUMPTION_RECONNECT_DELAY =

Floor for the delay between resumption GETs. SEP-1699's polling pattern wants fast reconnects, so this is far smaller than the events-stream floor — it only prevents a peer-supplied "retry: 0" from turning the deadline window into a back-to-back request loop.

0.01
MAX_EVENT_ID_LENGTH =

Maximum length of a server-supplied SSE event id retained as the resumption cursor. The id is echoed in the Last-Event-ID header of subsequent requests, so an unbounded value means unbounded retained memory and oversized outbound headers.

1024
EVENT_ID_PATTERN =

Characters allowed in a retained event id: printable ASCII, since the value becomes an HTTP header value. Notably excludes CR/LF. The range starts at 0x20 because a space is legal inside a field value, and rejecting ids like "cursor 42" would silently strand resumption on a stale cursor.

/\A[\x20-\x7E]+\z/
MAX_CONCURRENT_RESPONSE_POSTS =

Ceiling on concurrent threads POSTing server-initiated responses (pongs, roots/sampling/elicitation replies, error responses). Each server request on the events stream costs one blocking HTTP POST in its own thread; without a bound, a peer flooding requests could accumulate threads and connections until the host is exhausted. Responses beyond the budget are dropped, with saturation logged at most once per SATURATION_LOG_INTERVAL seconds.

8
SATURATION_LOG_INTERVAL =

Minimum gap between "response budget saturated" warnings

5
MIN_EVENTS_RECONNECT_DELAY =

Floor for server-supplied retry directives on the long-lived events stream. The directive is peer-controlled: honoring "retry: 0" literally would let a hostile server that closes every stream drive a tight reconnect loop (sustained CPU/TLS/connection churn). Waiting longer than the directive stays SEP-1699 compliant — the retry field is a lower bound on the reconnect delay, not an exact schedule.

0.1
MAX_SSE_BUFFER_BYTES =

Maximum bytes an SSE parse buffer (events stream or resumption GET) may hold while waiting for an event terminator. The stream is peer-controlled: without a cap, a hostile server could withhold the blank-line delimiter forever and grow the buffer until the host runs out of memory. Generous enough for any legitimate JSON-RPC event.

32 * 1024 * 1024

Constants included from JsonRpcTransport

JsonRpcTransport::DECOMPRESS_CHUNK_BYTES, JsonRpcTransport::MAX_DECOMPRESSED_BODY_BYTES

Constants included from HttpTransportBase

HttpTransportBase::AUTH_PARAM, HttpTransportBase::AUTH_PARAMS_RUN

Constants included from JsonRpcCommon

JsonRpcCommon::NON_IDEMPOTENT_METHODS

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 HttpTransportBase

#resend_after_session_restart, #rpc_notify, #rpc_request, #send_cancellation_notification, #valid_server_url?, #valid_session_id?

Methods included from JsonRpcCommon

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

Methods inherited from ServerBase

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

Constructor Details

#initialize(base_url:, **options) ⇒ ServerStreamableHTTP

Returns a new instance of ServerStreamableHTTP.

Parameters:

  • base_url (String) —

    The base URL of the MCP server

  • options (Hash) —

    Server configuration options (same as ServerHTTP)

Raises:

  • (ArgumentError) 


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# File 'lib/mcp_client/server_streamable_http.rb', line 100

def initialize(base_url:, **options)
  opts = default_options.merge(options)
  super(name: opts[:name])
  initialize_logger(opts[:logger])

  @max_retries = opts[:retries]
  @retry_backoff = opts[:retry_backoff]

  # Validate and normalize base_url
  raise ArgumentError, "Invalid or insecure server URL: #{base_url}" unless valid_server_url?(base_url)

  # Normalize base_url and handle cases where full endpoint is provided in base_url
  uri = URI.parse(base_url.chomp('/'))

  # Helper to build base URL without default ports
  build_base_url = lambda do |parsed_uri|
    port_part = if parsed_uri.port &&
                   !((parsed_uri.scheme == 'http' && parsed_uri.port == 80) ||
                     (parsed_uri.scheme == 'https' && parsed_uri.port == 443))
                  ":#{parsed_uri.port}"
                else
                  ''
                end
    "#{parsed_uri.scheme}://#{parsed_uri.host}#{port_part}"
  end

  @base_url = build_base_url.call(uri)
  @endpoint = if uri.path && !uri.path.empty? && uri.path != '/' && opts[:endpoint] == '/rpc'
                # If base_url contains a path and we're using default endpoint,
                # treat the path as the endpoint and use the base URL without path
                uri.path
              else
                # Standard case: base_url is just scheme://host:port, endpoint is separate
                opts[:endpoint]
              end

  # Set up headers for Streamable HTTP requests
  @headers = opts[:headers].merge({
                                    'Content-Type' => 'application/json',
                                    'Accept' => 'text/event-stream, application/json',
                                    'Accept-Encoding' => 'gzip',
                                    'User-Agent' => "ruby-mcp-client/#{MCPClient::VERSION}",
                                    'Cache-Control' => 'no-cache'
                                  })

  @read_timeout = opts[:read_timeout]
  @faraday_config = opts[:faraday_config]
  @max_decompressed_body_bytes = validate_decompression_limit(opts[:max_decompressed_body_bytes])
  @tools = nil
  @tools_data = nil
  @prompts = nil
  @prompts_data = nil
  @resources = nil
  @resources_data = nil
  @request_id = 0
  @mutex = Monitor.new
  @connection_established = false
  @initialized = false
  @http_conn = nil
  @session_id = nil
  @last_event_id = nil
  @sse_retry_ms = nil
  @pending_stream_responses = {}
  @response_post_count = 0
  # Saturation bookkeeping for the response-POST budget
  @dropped_response_posts = 0
  @last_saturation_log_at = nil
  @oauth_provider = opts[:oauth_provider]

  # SSE events connection state
  @events_connection = nil
  @events_thread = nil
  @buffer = +'' # Buffer for partial SSE event data
  # How much of @buffer has already been searched for an event terminator
  @buffer_scanned = 0
  @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
end

Instance Attribute Details

#base_url ⇒ String (readonly)

Returns The base URL of the MCP server.

Returns:

  • (String) —

    The base URL of the MCP server



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# File 'lib/mcp_client/server_streamable_http.rb', line 88

def base_url
  @base_url
end

#capabilities ⇒ Hash? (readonly)

Server capabilities from initialize response

Returns:

  • (Hash, nil) —

    Server capabilities



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# File 'lib/mcp_client/server_streamable_http.rb', line 96

def capabilities
  @capabilities
end

#endpoint ⇒ String (readonly)

Returns The JSON-RPC endpoint path.

Returns:

  • (String) —

    The JSON-RPC endpoint path



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# File 'lib/mcp_client/server_streamable_http.rb', line 88

attr_reader :base_url, :endpoint, :tools

#server_info ⇒ Hash? (readonly)

Server information from initialize response

Returns:

  • (Hash, nil) —

    Server information



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# File 'lib/mcp_client/server_streamable_http.rb', line 92

def server_info
  @server_info
end

#tools ⇒ Object (readonly)

Returns the value of attribute tools.



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# File 'lib/mcp_client/server_streamable_http.rb', line 88

attr_reader :base_url, :endpoint, :tools

Instance Method Details

#apply_request_headers(req, request) ⇒ Object

Override apply_request_headers to add session and SSE headers for MCP protocol



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# File 'lib/mcp_client/server_streamable_http.rb', line 455

def apply_request_headers(req, request)
  super

  # Add session and protocol version headers for non-initialize requests
  return unless request['method'] != 'initialize'

  if @session_id
    req.headers['Mcp-Session-Id'] = @session_id
    @logger.debug("Adding session header: Mcp-Session-Id: #{@session_id}")
  end

  return unless @protocol_version

  req.headers['Mcp-Protocol-Version'] = @protocol_version
  @logger.debug("Adding protocol version header: Mcp-Protocol-Version: #{@protocol_version}")

  # NOTE: Last-Event-ID is deliberately NOT sent on POSTs — per SEP-1699,
  # resumption is always via HTTP GET with Last-Event-ID.
end

#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 (with string keys for backward compatibility)

Raises:



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# File 'lib/mcp_client/server_streamable_http.rb', line 253

def call_tool(tool_name, parameters)
  rpc_request('tools/call', build_named_request_params(tool_name, parameters))
rescue MCPClient::Errors::ConnectionError, MCPClient::Errors::TransportError
  # Re-raise connection/transport errors directly to match test expectations
  raise
rescue StandardError => e
  # For all other errors, wrap in ToolCallError
  raise MCPClient::Errors::ToolCallError, "Error calling tool '#{tool_name}': #{e.message}"
end

#call_tool_streaming(tool_name, parameters) ⇒ Enumerator

Stream tool call (default implementation returns single-value stream)

Parameters:

  • tool_name (String) —

    the name of the tool to call

  • parameters (Hash) —

    the parameters to pass to the tool

Returns:

  • (Enumerator) —

    stream of results



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# File 'lib/mcp_client/server_streamable_http.rb', line 267

def call_tool_streaming(tool_name, parameters)
  Enumerator.new do |yielder|
    yielder << call_tool(tool_name, parameters)
  end
end

#cleanup ⇒ Object

Clean up the server connection Properly closes HTTP connections, stops threads, and clears cached state



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# File 'lib/mcp_client/server_streamable_http.rb', line 507

def cleanup
  @mutex.synchronize do
    return unless @connection_established || @initialized

    @logger.info('Cleaning up Streamable HTTP connection')

    # Mark connection as closed to stop reconnection attempts
    @connection_established = false
    @initialized = false

    # Attempt to terminate session before cleanup
    begin
      terminate_session if @session_id
    rescue StandardError => e
      @logger.warn("Failed to terminate session: #{e.message}")
    end

    # Stop events thread gracefully
    if @events_thread&.alive?
      @logger.debug('Stopping events thread...')
      @events_thread.kill
      @events_thread.join(THREAD_JOIN_TIMEOUT)
    end
    @events_thread = nil

    # Clear connections and state
    @http_conn = nil
    @events_connection = nil
    @session_id = nil
    @last_event_id = nil
    @sse_retry_ms = nil
    @pending_stream_responses.each_value(&:close)
    @pending_stream_responses.clear

    # Clear cached data
    @tools = nil
    @tools_data = nil
    @prompts = nil
    @prompts_data = nil
    @resources = nil
    @resources_data = nil
    @buffer = +''
    @buffer_scanned = 0

    @logger.info('Cleanup completed')
  end
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_streamable_http.rb', line 279

def complete(ref:, argument:, context: nil)
  ensure_connected
  require_capability!('completions', method: 'completion/complete')
  params = { ref: ref, argument: argument }
  params[:context] = context if context
  result = rpc_request('completion/complete', params)
  result['completion'] || { 'values' => [] }
rescue MCPClient::Errors::ConnectionError, MCPClient::Errors::TransportError,
       MCPClient::Errors::CapabilityError
  raise
rescue StandardError => e
  raise MCPClient::Errors::ServerError, "Error requesting completion: #{e.message}"
end

#connect ⇒ Boolean

Connect to the MCP server over Streamable HTTP

Returns:

  • (Boolean) —

    true if connection was successful

Raises:



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# File 'lib/mcp_client/server_streamable_http.rb', line 183

def connect
  return true if @mutex.synchronize { @connection_established }

  begin
    @mutex.synchronize do
      @connection_established = false
      @initialized = false
    end

    # Test connectivity with a simple HTTP request
    test_connection

    # Perform MCP initialization handshake
    perform_initialize

    # Start long-lived GET connection for server events
    start_events_connection

    @mutex.synchronize do
      @connection_established = true
      @initialized = true
    end

    true
  rescue MCPClient::Errors::ConnectionError => e
    cleanup
    raise e
  rescue StandardError => e
    cleanup
    raise MCPClient::Errors::ConnectionError, "Failed to connect to MCP server at #{@base_url}: #{e.message}"
  end
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 (with string keys for backward compatibility)

Raises:



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# File 'lib/mcp_client/server_streamable_http.rb', line 341

def get_prompt(prompt_name, parameters)
  rpc_request('prompts/get', build_named_request_params(prompt_name, parameters))
rescue MCPClient::Errors::ConnectionError, MCPClient::Errors::TransportError
  # Re-raise connection/transport errors directly
  raise
rescue StandardError => e
  # For all other errors, wrap in PromptGetError
  raise MCPClient::Errors::PromptGetError, "Error getting prompt '#{prompt_name}': #{e.message}"
end

#handle_successful_response(response, request) ⇒ Object

Override handle_successful_response to capture session ID



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# File 'lib/mcp_client/server_streamable_http.rb', line 476

def handle_successful_response(response, request)
  super

  # Capture session ID from initialize response with validation
  return unless request['method'] == 'initialize' && response.success?

  session_id = response.headers['mcp-session-id'] || response.headers['Mcp-Session-Id']
  if session_id
    if valid_session_id?(session_id)
      @session_id = session_id
      @logger.debug("Captured session ID: #{@session_id}")
    else
      @logger.warn("Invalid session ID format received: #{session_id.inspect}")
    end
  else
    @logger.warn('No session ID found in initialize response headers')
  end
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_streamable_http.rb', line 312

def list_prompts
  @mutex.synchronize do
    return @prompts if @prompts
  end

  begin
    ensure_connected

    prompts_data = request_prompts_list
    @mutex.synchronize do
      @prompts = prompts_data.map do |prompt_data|
        MCPClient::Prompt.from_json(prompt_data, server: self)
      end
    end

    @mutex.synchronize { @prompts }
  rescue MCPClient::Errors::ConnectionError, MCPClient::Errors::TransportError, MCPClient::Errors::ServerError
    # Re-raise these errors directly
    raise
  rescue StandardError => e
    raise MCPClient::Errors::PromptGetError, "Error listing prompts: #{e.message}"
  end
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_streamable_http.rb', line 406

def list_resource_templates(cursor: nil)
  params = {}
  params['cursor'] = cursor if cursor
  result = rpc_request('resources/templates/list', params)

  templates = (result['resourceTemplates'] || []).map do |template_data|
    MCPClient::ResourceTemplate.from_json(template_data, server: self)
  end

  { 'resourceTemplates' => templates, 'nextCursor' => result['nextCursor'] }
rescue MCPClient::Errors::ConnectionError, MCPClient::Errors::TransportError, MCPClient::Errors::ServerError
  raise
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_streamable_http.rb', line 355

def list_resources(cursor: nil)
  @mutex.synchronize do
    return @resources_result if @resources_result && !cursor
  end

  begin
    ensure_connected

    params = {}
    params['cursor'] = cursor if cursor
    result = rpc_request('resources/list', params)

    resources = (result['resources'] || []).map do |resource_data|
      MCPClient::Resource.from_json(resource_data, server: self)
    end

    resources_result = { 'resources' => resources, 'nextCursor' => result['nextCursor'] }

    @mutex.synchronize do
      @resources_result = resources_result unless cursor
    end

    resources_result
  rescue MCPClient::Errors::ConnectionError, MCPClient::Errors::TransportError, MCPClient::Errors::ServerError
    # Re-raise these errors directly
    raise
  rescue StandardError => e
    raise MCPClient::Errors::ResourceReadError, "Error listing resources: #{e.message}"
  end
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_streamable_http.rb', line 221

def list_tools
  @mutex.synchronize do
    return @tools if @tools
  end

  begin
    ensure_connected

    tools_data = request_tools_list
    @mutex.synchronize do
      @tools = tools_data.map do |tool_data|
        MCPClient::Tool.from_json(tool_data, server: self)
      end
    end

    @mutex.synchronize { @tools }
  rescue MCPClient::Errors::ConnectionError, MCPClient::Errors::TransportError, MCPClient::Errors::ServerError
    # Re-raise these errors directly
    raise
  rescue StandardError => e
    raise MCPClient::Errors::ToolCallError, "Error listing tools: #{e.message}"
  end
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_streamable_http.rb', line 298

def log_level=(level)
  ensure_connected
  require_capability!('logging', method: 'logging/setLevel')
  rpc_request('logging/setLevel', { level: level })
rescue MCPClient::Errors::ConnectionError, MCPClient::Errors::TransportError,
       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_streamable_http.rb', line 558

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_streamable_http.rb', line 565

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_streamable_http.rb', line 572

def on_sampling_request(&block)
  @sampling_request_callback = block
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_streamable_http.rb', line 390

def read_resource(uri)
  result = rpc_request('resources/read', { uri: uri })
  contents = result['contents'] || []
  contents.map { |content| MCPClient::ResourceContent.from_json(content) }
rescue MCPClient::Errors::ConnectionError, MCPClient::Errors::TransportError
  # Re-raise connection/transport errors directly
  raise
rescue StandardError => e
  # For all other errors, wrap in ResourceReadError
  raise MCPClient::Errors::ResourceReadError, "Error reading resource '#{uri}': #{e.message}"
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_streamable_http.rb', line 426

def subscribe_resource(uri)
  ensure_connected
  require_capability!('resources', 'subscribe', method: 'resources/subscribe')
  rpc_request('resources/subscribe', { uri: uri })
  true
rescue MCPClient::Errors::ConnectionError, MCPClient::Errors::TransportError, MCPClient::Errors::ServerError,
       MCPClient::Errors::CapabilityError
  raise
rescue StandardError => e
  raise MCPClient::Errors::ResourceReadError, "Error subscribing to resource '#{uri}': #{e.message}"
end

#terminate_session ⇒ Boolean

Terminate the current session (if any)

Returns:

  • (Boolean) —

    true if termination was successful or no session exists



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# File 'lib/mcp_client/server_streamable_http.rb', line 497

def terminate_session
  @mutex.synchronize do
    return true unless @session_id

    super
  end
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_streamable_http.rb', line 442

def unsubscribe_resource(uri)
  ensure_connected
  require_capability!('resources', 'subscribe', method: 'resources/unsubscribe')
  rpc_request('resources/unsubscribe', { uri: uri })
  true
rescue MCPClient::Errors::ConnectionError, MCPClient::Errors::TransportError, MCPClient::Errors::ServerError,
       MCPClient::Errors::CapabilityError
  raise
rescue StandardError => e
  raise MCPClient::Errors::ResourceReadError, "Error unsubscribing from resource '#{uri}': #{e.message}"
end