Class: MCPClient::Client

Inherits:
Object
  • Object
show all
Includes:
CacheSlices, ListAggregation, NotificationRouting, SamplingValidation, TaskApi, TaskSupport
Defined in:
lib/mcp_client/client.rb,
lib/mcp_client/client/task_api.rb,
lib/mcp_client/client/task_shape.rb,
lib/mcp_client/client/cache_slices.rb,
lib/mcp_client/client/task_support.rb,
lib/mcp_client/client/task_updates.rb,
lib/mcp_client/client/task_workers.rb,
lib/mcp_client/client/task_registry.rb,
lib/mcp_client/client/task_lifetimes.rb,
lib/mcp_client/client/list_aggregation.rb,
lib/mcp_client/client/sampling_validation.rb,
lib/mcp_client/client/notification_routing.rb,
lib/mcp_client/client/task_wait_boundaries.rb

Overview

MCP Client for integrating with the Model Context Protocol This is the main entry point for using MCP tools

Defined Under Namespace

Modules: CacheSlices, ListAggregation, NotificationRouting, SamplingValidation, TaskApi, TaskLifetimes, TaskRegistry, TaskShape, TaskSupport, TaskUpdates, TaskWaitBoundaries, TaskWorkers

Constant Summary collapse

MAX_VIOLATION_TEXT =

Ceiling on the schema-violation text that reaches a log line or an exception (the validator already bounds its error count).

4000
SUPPORTED_ELICITATION_MODES =

Elicitation modes implemented by this client (MCP 2025-11-25). Requests with a mode outside this set are rejected with -32602.

%w[form url].freeze
STRUCTURED_CONTENT_MODES =

Supported modes for structuredContent validation (MCP 2025-11-25): :warn logs a warning on mismatch, :strict raises a ValidationError.

%i[warn strict].freeze
SENSITIVE_CONFIG_KEYS =

Server-config keys whose values carry credentials (HTTP headers, the subprocess environment, inline tokens). Their values are replaced before a config is written to the log.

%i[headers env token access_token api_key auth authorization
password secret client_secret oauth_provider].freeze
REDACTED =

Placeholder written in place of a redacted value.

'[REDACTED]'
CACHE_INVALIDATION_MARK =

Where NotificationRouting#register_notification_handlers leaves word, on the thread that is routing, that the caches for the notification in hand have already been dropped by the transport's invalidation hook.

:mcp_client_cache_invalidation
MAX_PEER_LOG_MESSAGE_LENGTH =

Maximum characters of a peer-supplied log message written to the host log. The remote server controls this content, so an unbounded message would let it inflate log storage at will.

4096
CACHED_LIST_KINDS =

The list kinds this client caches, each with the transport-level cache behind it.

%i[tools prompts resources].freeze

Constants included from TaskSupport

TaskSupport::DEFAULT_TASK_POLL_INTERVAL, TaskSupport::MAX_TASK_INPUT_ROUNDS, TaskSupport::MAX_TASK_POLL_INTERVAL, TaskSupport::MAX_TASK_REQUEST_TIMEOUT, TaskSupport::MIN_TASK_POLL_INTERVAL, TaskSupport::MIN_TASK_REQUEST_TIMEOUT

Constants included from TaskLifetimes

TaskLifetimes::MAX_TRACKED_TASK_LIFETIMES, TaskLifetimes::TRACKED_TASK_LIFETIMES_LOW_WATER

Constants included from TaskWorkers

TaskWorkers::MAX_PENDING_TASK_REQUESTS

Instance Attribute Summary collapse

Instance Method Summary collapse

Methods included from TaskApi

#call_tool_as_task, #cancel_task, #get_task, #get_task_result, #list_tasks

Methods included from TaskSupport

#tasks_extension?, #update_task, #wait_for_task

Constructor Details

#initialize(mcp_server_configs: [], logger: nil, elicitation_handler: nil, roots: nil, sampling_handler: nil, sampling_supports_tools: false, client_info: nil, validate_structured_content: :warn, request_meta: nil, extensions: nil) ⇒ Client

Initialize a new MCPClient::Client

Parameters:

  • mcp_server_configs (Array<Hash>) (defaults to: []) —

    configurations for MCP servers

  • logger (Logger, nil) (defaults to: nil) —

    optional logger, defaults to STDOUT

  • elicitation_handler (Proc, nil) (defaults to: nil) —

    optional handler for elicitation requests (MCP 2025-06-18)

  • roots (Array<MCPClient::Root, Hash>, nil) (defaults to: nil) —

    optional list of roots (MCP 2025-06-18). Deprecated since MCP 2026-07-28 (SEP-2577); earliest removal is the first revision released on or after 2027-07-28. Pass directories or files through tool parameters, resource URIs or server configuration instead.

  • sampling_handler (Proc, nil) (defaults to: nil) —

    optional handler for sampling requests (MCP 2025-11-25). Deprecated since MCP 2026-07-28 (SEP-2577); earliest removal is the first revision released on or after 2027-07-28. Integrate directly with the LLM provider API instead.

  • sampling_supports_tools (Boolean) (defaults to: false) —

    whether the sampling handler supports tool use (MCP 2025-11-25 / SEP-1577); declares the sampling.tools capability and forwards tools/toolChoice params to the handler instead of rejecting tool-enabled requests

  • client_info (Hash, nil) (defaults to: nil) —

    host-provided Implementation info sent as clientInfo (name and version required; title, description, websiteUrl, icons optional)

  • validate_structured_content (Symbol) (defaults to: :warn) —

    how to treat a tools/call result whose structuredContent does not match the tool's declared outputSchema (MCP 2025-11-25: "Clients SHOULD validate structured results against this schema"): :warn (default) logs a warning, :strict raises MCPClient::Errors::ValidationError

  • request_meta (Hash, #call, nil) (defaults to: nil) —

    metadata merged into every request's _meta (a Hash, or a callable returning one, evaluated per request) — e.g. OpenTelemetry trace context (traceparent, tracestate, baggage, MCP 2026-07-28) or vendor-prefixed keys. Reserved protocol keys cannot be set this way.

  • extensions (Array<String>, Hash{String => Hash}, nil) (defaults to: nil) —

    MCP 2026-07-28 extensions to declare in every request's clientCapabilities (identifier, or identifier => settings), e.g. ['io.modelcontextprotocol/tasks'] to let servers answer tools/call with a task



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# File 'lib/mcp_client/client.rb', line 116

def initialize(mcp_server_configs: [], logger: nil, elicitation_handler: nil, roots: nil, sampling_handler: nil,
               sampling_supports_tools: false, client_info: nil, validate_structured_content: :warn,
               request_meta: nil, extensions: nil)
  unless STRUCTURED_CONTENT_MODES.include?(validate_structured_content)
    raise ArgumentError, "validate_structured_content must be one of #{STRUCTURED_CONTENT_MODES.inspect}, " \
                         "got #{validate_structured_content.inspect}"
  end

  @validate_structured_content = validate_structured_content
  @extensions = normalize_extensions(extensions)
  # Preserve a caller-supplied logger's formatter (only tag progname), and
  # install the default formatter solely on a logger we create ourselves.
  # Overwriting the formatter of an application's logger would silently
  # reformat every log line it emits elsewhere.
  if logger
    @logger = logger
    @logger.progname = self.class.name
  else
    @logger = Logger.new($stdout, level: Logger::WARN)
    @logger.progname = self.class.name
    @logger.formatter = proc { |severity, _datetime, progname, msg| "#{severity} [#{progname}] #{msg}\n" }
  end
  @servers = mcp_server_configs.map do |config|
    @logger.debug("Creating server with config: #{redact_config(config).inspect}")
    MCPClient::ServerFactory.create(config, logger: @logger)
  end
  @tool_cache = {}
  # Bumped whenever the tool cache is emptied, so a list_tools that was
  # already in flight can tell that its definitions were superseded while
  # it ran (MCP 2026-07-28: a HeaderMismatch refresh announces itself as a
  # tools/list_changed).
  @tool_cache_generation = 0
  @cache_mutex = Mutex.new
  # The effective-parameter fingerprint each server's slice of a list
  # cache was filled under (MCP 2026-07-28 caching: a result is served
  # only to a request that would carry the same parameters).
  @cache_params = Hash.new { |h, k| h[k] = {}.compare_by_identity }
  # Which servers have filled their slice of a list cache, so a snapshot
  # is known to be complete however few items it holds: a server that
  # legitimately lists nothing must be served from the cache too, not
  # asked again on every call.
  @cache_filled = {}
  # One lock for the list caches and their parameter tags: a freshness
  # check and the copy it approves are one snapshot, and the notification
  # thread's clears wait for it.
  @cache_mutex = Mutex.new
  # Bumped by every write under @cache_mutex, so a freshness verdict
  # reached outside the lock can be revalidated before a copy is served.
  @cache_version = 0
  # Active progressToken -> callback registrations (MCP progress utility)
  @progress_callbacks = {}
  @progress_mutex = Mutex.new
  @prompt_cache = {}
  @resource_cache = {}
  # JSON-RPC notification listeners
  @notification_listeners = []
  # Elicitation handler (MCP 2025-06-18)
  @elicitation_handler = elicitation_handler
  # Sampling handler (MCP 2025-11-25; deprecated in 2026-07-28, SEP-2577)
  @sampling_handler = sampling_handler
  MCPClient::Deprecations.warn(:sampling, @logger) if sampling_handler
  # Whether the sampling handler supports tool use (SEP-1577)
  @sampling_supports_tools = sampling_supports_tools
  # Roots (MCP 2025-06-18; deprecated in 2026-07-28, SEP-2577)
  @roots = normalize_roots(roots)
  MCPClient::Deprecations.warn(:roots, @logger) unless @roots.empty?
  # Register default and user-defined notification handlers on each server
  @servers.each do |server|
    configure_server_identity(server, client_info, request_meta)
    register_notification_handlers(server)
    # Register feature callbacks only for features the host actually
    # supports: transports derive their declared client capabilities from
    # the callbacks registered before connecting, and MCP forbids using
    # capabilities that were not negotiated.
    # The transports call the callback with (request_id, params) only, so
    # the asking server is closed over here: the URL-mode host contract
    # depends on its protocol era (MCP 2026-07-28 removed elicitationId).
    if @elicitation_handler && server.respond_to?(:on_elicitation_request)
      server.on_elicitation_request do |request_id, request_params|
        handle_elicitation_request(request_id, request_params, server)
      end
    end
    # The client always implements the roots feature (roots/list and
    # list_changed notifications), independent of the current roots list.
    server.on_roots_list_request(&method(:handle_roots_list_request)) if server.respond_to?(:on_roots_list_request)
    next unless @sampling_handler && server.respond_to?(:on_sampling_request)

    server.on_sampling_request(&method(:handle_sampling_request))
    # Declare the sampling.tools sub-capability (SEP-1577) only when the
    # host opted in; the transport derives its initialize declaration
    # from this before connecting.
    server.declare_sampling_tools if @sampling_supports_tools && server.respond_to?(:declare_sampling_tools)
  end
end

Instance Attribute Details

#logger ⇒ Logger (readonly)

Returns logger for client operations.

Returns:

  • (Logger) —

    logger for client operations



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# File 'lib/mcp_client/client.rb', line 55

def logger
  @logger
end

#prompt_cache ⇒ Hash<String, MCPClient::Prompt> (readonly)

Returns cache of prompts by composite key (server_id:name).

Returns:

  • (Hash<String, MCPClient::Prompt>) —

    cache of prompts by composite key (server_id:name)



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# File 'lib/mcp_client/client.rb', line 49

def prompt_cache
  @prompt_cache
end

#resource_cache ⇒ Hash<String, MCPClient::Resource> (readonly)

Returns cache of resources by composite key (server_id:uri).

Returns:



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# File 'lib/mcp_client/client.rb', line 52

def resource_cache
  @resource_cache
end

#roots ⇒ Array<MCPClient::Root>

Deprecated.

Roots is deprecated since MCP 2026-07-28 (SEP-2577); earliest removal is the first revision released on or after 2027-07-28. Reading the list is not itself a first use of the feature — the notice follows configuring a root or serving one — but the list is a deprecated feature's state, and a host reading it is holding one. Pass directories or files through tool parameters, resource URIs or server configuration instead.

Returns list of MCP roots (MCP 2025-06-18).

Returns:



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# File 'lib/mcp_client/client.rb', line 64

def roots
  @roots
end

#servers ⇒ Array<MCPClient::ServerBase> (readonly)

Returns list of servers.

Returns:



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# File 'lib/mcp_client/client.rb', line 43

def servers
  @servers
end

#tool_cache ⇒ Hash<String, MCPClient::Tool> (readonly)

Returns cache of tools by composite key (server_id:name).

Returns:

  • (Hash<String, MCPClient::Tool>) —

    cache of tools by composite key (server_id:name)



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# File 'lib/mcp_client/client.rb', line 46

def tool_cache
  @tool_cache
end

Instance Method Details

#call_tool(tool_name, parameters, server: nil, progress: nil) ⇒ Object

Calls a specific tool by name with the given parameters

Parameters:

  • tool_name (String) —

    the name of the tool to call

  • parameters (Hash) —

    the parameters to pass to the tool

  • server (String, Symbol, Integer, MCPClient::ServerBase, nil) (defaults to: nil) —

    optional server to use

Returns:

  • (Object) —

    the result of the tool invocation

Raises:

  • (MCPClient::Errors::ValidationError) —

    when the parameters miss a required property, or the tool's inputSchema declares a JSON Schema dialect this client does not support (MCP 2026-07-28)



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# File 'lib/mcp_client/client.rb', line 343

def call_tool(tool_name, parameters, server: nil, progress: nil)
  tool = resolve_tool(tool_name, server: server)

  # Validate parameters against tool schema
  validate_params!(tool, parameters)
  reject_task_required!(tool, tool_name)

  # Use the tool's associated server
  server = tool.server
  raise MCPClient::Errors::ServerNotFound, "No server found for tool '#{tool_name}'" unless server

  # MCP progress utility: attach an auto-generated progressToken to the
  # request _meta and route matching notifications/progress to the
  # caller's callback while the request is active.
  parameters, token = setup_progress_tracking(parameters, progress)
  # The session the call is made in: a task it comes back as belongs to
  # that session, not to one that replaced it while the answer was read.
  # The call goes into that very session and no other — a transport that
  # reconnects inside the request would otherwise run the tool in the
  # replacement session while the task is stamped with the sampled one,
  # and the wait would then refuse a task whose (possibly non-idempotent)
  # tool has already run, inviting a duplicate retry.
  task_epoch = tasks_extension? ? invocation_session_epoch(server) : nil

  # The call and the re-resolve that follows it share one slot for the
  # definition the transport's request goes out under, so a call that a
  # notification listener nests inside this one cannot leave its own
  # there.
  with_called_tool_definition(server) do
    result = begin
      pinned_to_session(server, task_epoch) { server.call_tool(tool_name, parameters) }
    rescue MCPClient::Errors::ConnectionError => e
      # Add server identity information to the error for better context
      server_id = server.name ? "#{server.class}[#{server.name}]" : server.class.name
      raise MCPClient::Errors::ToolCallError,
            "Error calling tool '#{tool_name}': #{e.message} (Server: #{server_id})"
    ensure
      # Tokens are only valid for the lifetime of the request: dropping the
      # registration filters out stale post-completion notifications.
      unregister_progress_callback(token) if token
    end

    # MCP 2026-07-28 HeaderMismatch recovery re-derives a call's
    # Mcp-Param-* headers from a refreshed tools/list, so the attempt that
    # was answered may have gone out under a definition this client never
    # resolved. Validate against that one -- never against the transport's
    # current list, which a tools/list_changed racing the call may already
    # have replaced with a definition the server never used. It is read
    # here, before a task's result is waited for: a refresh that lands
    # during a wait that may take minutes belongs to another invocation and
    # says nothing about the definition this one was answered under.
    called = called_tool_definition(server, tool_name)

    # MCP 2026-07-28 tasks extension: the server may have turned the call
    # into a task; drive it to its final result so the contract of this
    # method does not change.
    result = complete_task_result(tool_name, server, result, task_epoch)

    validate_called_result!(called || tool, result)
  end
end

#call_tool_streaming(tool_name, parameters, server: nil) ⇒ Enumerator

Stream call of a specific tool by name with the given parameters. Returns an Enumerator yielding streaming updates if supported.

Parameters:

  • tool_name (String) —

    the name of the tool to call

  • parameters (Hash) —

    the parameters to pass to the tool

  • server (String, Symbol, Integer, MCPClient::ServerBase, nil) (defaults to: nil) —

    optional server to use

Returns:

  • (Enumerator) —

    streaming enumerator or single-value enumerator

Raises:



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# File 'lib/mcp_client/client.rb', line 562

def call_tool_streaming(tool_name, parameters, server: nil)
  tool = resolve_tool(tool_name, server: server)

  # Validate parameters against tool schema
  validate_params!(tool, parameters)
  reject_task_required!(tool, tool_name)

  # Use the tool's associated server
  server = tool.server
  raise MCPClient::Errors::ServerNotFound, "No server found for tool '#{tool_name}'" unless server

  begin
    task_epoch = tasks_extension? ? invocation_session_epoch(server) : nil
    # Use the streaming API if it's available, opened in the session the
    # epoch was sampled for: a chunk that comes back as a task is stamped
    # with that session, so the call must not have been written into the
    # one that replaced it (see #call_tool). The stream every built-in
    # transport hands back is lazy, so the call itself goes out under the
    # pin the enumeration takes (see #streamed_call_chunks) — this one
    # covers a transport that sends while building it.
    stream = pinned_to_session(server, task_epoch) { server.call_tool_streaming(tool_name, parameters) }
    # Every stream goes through the wrapper, whether or not tasks are in
    # play: "Clients SHOULD validate structured results against this
    # schema" is about a result, not about the method that fetched it, and
    # so is the dialect a result's schema declares.
    streamed_call_chunks(stream, tool, tool_name, server, epoch: task_epoch)
  rescue MCPClient::Errors::ConnectionError => e
    # Add server identity information to the error for better context
    server_id = server.name ? "#{server.class}[#{server.name}]" : server.class.name
    msg = "Error calling streaming tool '#{tool_name}': #{e.message} (Server: #{server_id})"
    raise MCPClient::Errors::ToolCallError, msg
  end
end

#call_tools(calls) ⇒ Array<Object>

Call multiple tools in batch

Parameters:

  • calls (Array<Hash>) —

    array of call hashes with keys:

    • name: tool name (required)
    • parameters: tool parameters (optional, default empty hash)
    • server: server name for routing (optional)

Returns:

  • (Array<Object>) —

    array of results for each tool invocation



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# File 'lib/mcp_client/client.rb', line 547

def call_tools(calls)
  calls.map do |call|
    name = call[:name] || call['name']
    params = call[:parameters] || call['parameters'] || {}
    server = call[:server] || call['server']
    call_tool(name, params, server: server)
  end
end

#cleanup ⇒ Object

Clean up all server connections



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# File 'lib/mcp_client/client.rb', line 433

def cleanup
  servers.each(&:cleanup)
  # The transports forgot their results; the slices built from them go too.
  clear_cache
  clear_task_states
end

#clear_cache ⇒ void

This method returns an undefined value.

Clear the cached lists so that the next list_tools, list_prompts or list_resources fetches fresh data.



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# File 'lib/mcp_client/client.rb', line 447

def clear_cache
  clear_tool_cache
  @cache_mutex.synchronize do
    @cache_version += 1
    @prompt_cache.clear
    @resource_cache.clear
    # A slice's tag goes with the slice: a leftover tag must not vouch
    # for a server whose slice a later, partial refill never rebuilt.
    @cache_params.clear
    @cache_filled.clear
  end
  # The promise is fresh data, and a transport holding a list the server
  # bounded with a positive `ttlMs` (MCP 2026-07-28
  # server/utilities/caching) would answer the next listing from it
  # without sending anything at all. Dropped outside this client's lock:
  # each transport takes its own.
  servers.each do |server|
    CACHED_LIST_KINDS.each { |kind| refresh_server_cache(server, kind) }
    forget_schema_checks
  end
end

#complete(ref:, argument:, context: nil, server: nil) ⇒ Hash

Request completion suggestions from a 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), e.g., { 'arguments' => { 'arg1' => 'value1' } } for previously-resolved arguments

  • server (Integer, String, Symbol, MCPClient::ServerBase, nil) (defaults to: nil) —

    server selector

Returns:

  • (Hash) —

    completion result with 'values', optional 'total', and 'hasMore' fields

Raises:



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# File 'lib/mcp_client/client.rb', line 650

def complete(ref:, argument:, context: nil, server: nil)
  srv = select_server(server)
  srv.complete(ref: ref, argument: argument, context: context)
end

#find_server(name) ⇒ MCPClient::ServerBase?

Find a server by name

Parameters:

  • name (String) —

    the name of the server to find

Returns:



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# File 'lib/mcp_client/client.rb', line 522

def find_server(name)
  @servers.find { |s| s.name == name }
end

#find_tool(pattern) ⇒ MCPClient::Tool?

Find the first tool whose name matches the given pattern

Parameters:

  • pattern (String, Regexp) —

    pattern to match tool names

Returns:



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# File 'lib/mcp_client/client.rb', line 537

def find_tool(pattern)
  find_tools(pattern).first
end

#find_tools(pattern) ⇒ Array<MCPClient::Tool>

Find all tools whose name matches the given pattern (String or Regexp)

Parameters:

  • pattern (String, Regexp) —

    pattern to match tool names

Returns:



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# File 'lib/mcp_client/client.rb', line 529

def find_tools(pattern)
  rx = pattern.is_a?(Regexp) ? pattern : /#{Regexp.escape(pattern)}/
  list_tools.select { |t| t.name.match(rx) }
end

#get_prompt(prompt_name, parameters, server: nil) ⇒ Object

Gets a specific prompt by name with the given parameters

Parameters:

  • prompt_name (String) —

    the name of the prompt to get

  • parameters (Hash) —

    the parameters to pass to the prompt

  • server (String, Symbol, Integer, MCPClient::ServerBase, nil) (defaults to: nil) —

    optional server to use

Returns:

  • (Object) —

    the final prompt

Raises:



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# File 'lib/mcp_client/client.rb', line 232

def get_prompt(prompt_name, parameters, server: nil)
  prompts = list_prompts

  if server
    # Use the specified server
    srv = select_server(server)
    # Find the prompt on this specific server
    prompt = prompts.find { |t| t.name == prompt_name && t.server == srv }
    unless prompt
      raise MCPClient::Errors::PromptNotFound,
            "Prompt '#{prompt_name}' not found on server '#{srv.name || srv.class.name}'"
    end
  else
    # Find the prompt across all servers
    matching_prompts = prompts.select { |t| t.name == prompt_name }

    if matching_prompts.empty?
      raise MCPClient::Errors::PromptNotFound, "Prompt '#{prompt_name}' not found"
    elsif matching_prompts.size > 1
      # If multiple matches, disambiguate with server names
      server_names = matching_prompts.map { |t| t.server&.name || 'unnamed' }
      raise MCPClient::Errors::AmbiguousPromptName,
            "Multiple prompts named '#{prompt_name}' found across servers (#{server_names.join(', ')}). " \
            "Please specify a server using the 'server' parameter."
    end

    prompt = matching_prompts.first
  end

  # Use the prompt's associated server
  server = prompt.server
  raise MCPClient::Errors::ServerNotFound, "No server found for prompt '#{prompt_name}'" unless server

  begin
    server.get_prompt(prompt_name, parameters)
  rescue MCPClient::Errors::ConnectionError => e
    # Add server identity information to the error for better context
    server_id = server.name ? "#{server.class}[#{server.name}]" : server.class.name
    raise MCPClient::Errors::PromptGetError,
          "Error getting prompt '#{prompt_name}': #{e.message} (Server: #{server_id})"
  end
end

#list_prompts(cache: true) ⇒ Array<MCPClient::Prompt>

Lists all available prompts from all connected MCP servers

Parameters:

  • cache (Boolean) (defaults to: true) —

    whether to use cached prompts or fetch fresh

Returns:

Raises:



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# File 'lib/mcp_client/client.rb', line 216

def list_prompts(cache: true)
  holding_request_meta('prompts/list') do
    if cache && (snapshot = cached_snapshot(:prompts, @prompt_cache))
      release_held_request_meta
      return snapshot
    end

    collect_prompts_from_servers(cache)
  end
end

#list_resources(cache: true, cursor: nil) ⇒ Hash

Lists all available resources from all connected MCP servers

Parameters:

  • cache (Boolean) (defaults to: true) —

    whether to use cached resources or fetch fresh

  • cursor (String, nil) (defaults to: nil) —

    optional cursor for pagination (only works with single server)

Returns:

  • (Hash) —

    result containing 'resources' array and optional 'nextCursor'

Raises:



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# File 'lib/mcp_client/client.rb', line 281

def list_resources(cache: true, cursor: nil)
  holding_request_meta('resources/list') do
    # If cursor is provided, we can only query one server (the one that provided the cursor)
    # This is a limitation of aggregating multiple servers
    if cursor
      # For now, just use the first server when cursor is provided
      return servers.first.list_resources(cursor: cursor) if servers.any?

      return { 'resources' => [], 'nextCursor' => nil }
    end

    # Use cache if available and no cursor
    if cache && (snapshot = cached_snapshot(:resources, @resource_cache))
      release_held_request_meta
      return { 'resources' => snapshot, 'nextCursor' => nil }
    end

    collect_resources_from_servers(cache)
  end
end

#list_tools(cache: true) ⇒ Array<MCPClient::Tool>

Lists all available tools from all connected MCP servers

Parameters:

  • cache (Boolean) (defaults to: true) —

    whether to use cached tools or fetch fresh

Returns:

Raises:



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# File 'lib/mcp_client/client.rb', line 324

def list_tools(cache: true)
  holding_request_meta('tools/list') do
    if cache && (snapshot = cached_snapshot(:tools, @tool_cache))
      release_held_request_meta
      return snapshot
    end

    collect_tools_from_servers(cache)
  end
end

#listen(notifications:, server: nil, ack_timeout: nil) {|method, params| ... } ⇒ MCPClient::Subscription

Open a long-lived notification stream on a server (MCP 2026-07-28 subscriptions/listen). The subscription's notifications also flow through the client's regular notification handling (cache invalidation, on_notification listeners).

Parameters:

  • notifications (Hash) —

    the SubscriptionFilter: tools_list_changed, prompts_list_changed, resources_list_changed (booleans), resource_subscriptions, task_ids (arrays of strings)

  • server (Integer, String, Symbol, MCPClient::ServerBase, nil) (defaults to: nil) —

    server selector

  • ack_timeout (Numeric, false, nil) (defaults to: nil) —

    seconds to wait for the server's acknowledgment before giving the listen up and cancelling it; nil takes the transport's own read timeout, false waits for ever

Yields:

  • (method, params) —

    notifications delivered on the subscription

Returns:

Raises:



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# File 'lib/mcp_client/client.rb', line 669

def listen(notifications:, server: nil, ack_timeout: nil, &listener)
  srv = select_server(server)
  filter = MCPClient::Subscription.normalize_filter(notifications)
  if filter.key?('taskIds')
    unless tasks_extension?
      raise MCPClient::Errors::CapabilityError,
            'Task notifications (taskIds) require the tasks extension: pass ' \
            "extensions: ['#{MCPClient::JsonRpcCommon::TASKS_EXTENSION}'] to MCPClient::Client.new"
    end
    # The server must have negotiated the extension too (it answers a
    # taskIds filter from a non-declaring client with -32021).
    ensure_task_capability!(srv, 'listen')
  end

  srv.listen(notifications: notifications, ack_timeout: ack_timeout, &listener)
end

#log_level=(level) ⇒ Array<Hash>

Deprecated.

Logging is deprecated since MCP 2026-07-28 (SEP-2577); earliest removal is the first revision released on or after 2027-07-28. Have the server log to stderr (stdio) or use OpenTelemetry instead.

Set the logging level on all connected servers (MCP 2025-06-18) To set on a specific server, use: client.find_server('name').log_level = 'debug'

Parameters:

  • level (String) —

    the log level ('debug', 'info', 'notice', 'warning', 'error', 'critical', 'alert', 'emergency')

Returns:

  • (Array<Hash>) —

    results from servers

Raises:



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# File 'lib/mcp_client/client.rb', line 696

def log_level=(level)
  MCPClient::Deprecations.warn(:logging, @logger)
  @servers.filter_map do |srv|
    # MCP lifecycle: only use capabilities that were successfully
    # negotiated — skip servers whose NEGOTIATED set lacks logging.
    # Unconnected servers proceed: the transport-level gate re-checks
    # after its handshake establishes the capability set. A 2026-07-28
    # server needs no capability at all: the level is a per-request
    # field of every request's _meta, not a logging/setLevel call.
    unless !capabilities_known?(srv) || srv.capability?('logging') || modern_server?(srv)
      @logger.debug("Skipping logging/setLevel for #{srv.name || srv.class.name}: " \
                    'logging capability not negotiated')
      next
    end

    srv.log_level = level
  end
end

#on_input_required_wait(&block) ⇒ void

This method returns an undefined value.

Register the host's control over a multi round-trip request's out-of-band wait on every server (MCP 2026-07-28 client/elicitation "URL Mode": manual retry/cancel controls). See JsonRpcCommon#on_input_required_wait for the contract.

Parameters:

  • block (Proc) —

    callback that receives an InputRequiredWait



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# File 'lib/mcp_client/client.rb', line 482

def on_input_required_wait(&block)
  @input_required_wait_handler = block
  @servers.each do |server|
    server.on_input_required_wait(&block) if server.respond_to?(:on_input_required_wait)
  end
end

#on_notification {|server, method, params| ... } ⇒ void

This method returns an undefined value.

Register a callback for JSON-RPC notifications from servers

Yields:

  • (server, method, params) 


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# File 'lib/mcp_client/client.rb', line 472

def on_notification(&block)
  @notification_listeners << block
end

#ping(server_index: nil) ⇒ Object

Ping the MCP server to check connectivity (zero-parameter heartbeat call)

Parameters:

  • server_index (Integer, nil) (defaults to: nil) —

    optional index of a specific server to ping, nil for first available

Returns:

  • (Object) —

    result from the ping request

Raises:



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# File 'lib/mcp_client/client.rb', line 600

def ping(server_index: nil)
  if server_index.nil?
    # Ping first available server
    raise MCPClient::Errors::ServerNotFound, 'No server available for ping' if @servers.empty?

    @servers.first.ping
  else
    # Ping specified server
    if server_index >= @servers.length
      raise MCPClient::Errors::ServerNotFound,
            "Server at index #{server_index} not found"
    end

    @servers[server_index].ping
  end
end

#read_resource(uri, server: nil) ⇒ Object

Reads a specific resource by URI

Parameters:

  • uri (String) —

    the URI of the resource to read

  • server (String, Symbol, Integer, MCPClient::ServerBase, nil) (defaults to: nil) —

    optional server to use

Returns:

  • (Object) —

    the resource contents



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# File 'lib/mcp_client/client.rb', line 306

def read_resource(uri, server: nil)
  result = list_resources
  resources = result['resources'] || []

  resource = if server
               find_resource_on_server(uri, resources, server)
             else
               find_resource_across_servers(uri, resources)
             end

  execute_resource_read(resource, uri)
end

#resume_input_required(error, timeout: nil) ⇒ Object

Resume a multi round-trip request from the continuation an Errors::InputRequiredError carries, on the transport that raised it. The result is the transport's, as #call_tool would have returned it before validation.

Parameters:

Returns:

  • (Object) —

    the final (complete) result

Raises:

  • (ArgumentError) —

    if the error carries no continuation or names no transport



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

def resume_input_required(error, timeout: nil)
  transport = error.respond_to?(:transport) ? error.transport : nil
  raise ArgumentError, 'the error names no transport to resume on' unless transport

  transport.resume_input_required(error, timeout: timeout)
end

#send_notification(method, params: {}, server: nil) ⇒ void

This method returns an undefined value.

Send a raw JSON-RPC notification to a server (no response expected)

Parameters:

  • method (String) —

    JSON-RPC method name

  • params (Hash) (defaults to: {}) —

    parameters for the notification

  • server (Integer, String, Symbol, MCPClient::ServerBase, nil) (defaults to: nil) —

    server selector



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# File 'lib/mcp_client/client.rb', line 636

def send_notification(method, params: {}, server: nil)
  srv = select_server(server)
  srv.rpc_notify(method, params)
end

#send_rpc(method, params: {}, server: nil, timeout: nil) ⇒ Object

Send a raw JSON-RPC request to a server

Parameters:

  • method (String) —

    JSON-RPC method name

  • params (Hash) (defaults to: {}) —

    parameters for the request

  • server (Integer, String, Symbol, MCPClient::ServerBase, nil) (defaults to: nil) —

    server selector

Returns:

  • (Object) —

    result from the JSON-RPC response



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# File 'lib/mcp_client/client.rb', line 622

def send_rpc(method, params: {}, server: nil, timeout: nil)
  srv = select_server(server)
  # Only pass the per-request timeout when set, so transports (and test
  # doubles) with the two-argument signature keep working.
  return srv.rpc_request(method, params) unless timeout

  srv.rpc_request(method, params, timeout: timeout)
end

#to_anthropic_tools(tool_names: nil) ⇒ Array<Hash>

Convert MCP tools to Anthropic Claude tool specifications

Parameters:

  • tool_names (Array<String>, nil) (defaults to: nil) —

    optional list of tool names to include, nil means all tools

Returns:

  • (Array<Hash>) —

    Anthropic Claude tool specifications



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# File 'lib/mcp_client/client.rb', line 417

def to_anthropic_tools(tool_names: nil)
  tools = list_tools
  tools = tools.select { |t| tool_names.include?(t.name) } if tool_names
  tools.map(&:to_anthropic_tool)
end

#to_google_tools(tool_names: nil) ⇒ Array<Hash>

Convert MCP tools to Google Vertex AI tool specifications

Parameters:

  • tool_names (Array<String>, nil) (defaults to: nil) —

    optional list of tool names to include, nil means all tools

Returns:

  • (Array<Hash>) —

    Google Vertex AI tool specifications with cleaned schemas



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# File 'lib/mcp_client/client.rb', line 426

def to_google_tools(tool_names: nil)
  tools = list_tools
  tools = tools.select { |t| tool_names.include?(t.name) } if tool_names
  tools.map(&:to_google_tool)
end

#to_openai_tools(tool_names: nil) ⇒ Array<Hash>

Convert MCP tools to OpenAI function specifications

Parameters:

  • tool_names (Array<String>, nil) (defaults to: nil) —

    optional list of tool names to include, nil means all tools

Returns:

  • (Array<Hash>) —

    OpenAI function specifications



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# File 'lib/mcp_client/client.rb', line 408

def to_openai_tools(tool_names: nil)
  tools = list_tools
  tools = tools.select { |t| tool_names.include?(t.name) } if tool_names
  tools.map(&:to_openai_tool)
end