Module: MCPClient::ServerStdio::JsonRpcTransport

Includes:
JsonRpcCommon
Included in:
MCPClient::ServerStdio
Defined in:
lib/mcp_client/server_stdio/json_rpc_transport.rb

Overview

JSON-RPC request/notification plumbing for stdio transport

Constant Summary

Constants included from JsonRpcCommon

JsonRpcCommon::CORE_RESULT_TYPES, JsonRpcCommon::EXTENSION_ID_PATTERN, JsonRpcCommon::INPUT_RETRY_DELAY, JsonRpcCommon::INPUT_RETRY_MAX_DELAY, JsonRpcCommon::LEGACY_RESULT_TYPES, JsonRpcCommon::LOG_LEVELS, JsonRpcCommon::MAX_INPUT_ROUND_TRIPS, JsonRpcCommon::MAX_PEER_LOG_TEXT_LENGTH, JsonRpcCommon::MRTR_METHODS, JsonRpcCommon::NAME_HEADER_SOURCES, JsonRpcCommon::NON_IDEMPOTENT_METHODS, JsonRpcCommon::REMOVED_MODERN_NOTIFICATIONS, JsonRpcCommon::RESULT_TYPE_EXTENSIONS, JsonRpcCommon::TASKS_EXTENSION, JsonRpcCommon::TASK_METHODS

Constants included from MCPClient::SessionPin

MCPClient::SessionPin::SESSION_PINS, MCPClient::SessionPin::WRITE_GUARDS

Constants included from RequestMetadata

RequestMetadata::CACHE_NEUTRAL_META_KEYS, RequestMetadata::META_CLIENT_CAPABILITIES, RequestMetadata::META_CLIENT_INFO, RequestMetadata::META_LOG_LEVEL, RequestMetadata::META_PROTOCOL_VERSION, RequestMetadata::META_SERVER_INFO, RequestMetadata::META_SUBSCRIPTION_ID, RequestMetadata::OPAQUE_PARAMS, RequestMetadata::PROTECTED_META_KEYS, RequestMetadata::UNRECORDED_PARAMS

Constants included from ResultCaching

ResultCaching::CACHE_INIT_LOCK, ResultCaching::LEGACY_ENTRY, ResultCaching::LIST_CHANGE_NOTIFICATIONS, ResultCaching::LIST_METHOD_KINDS, ResultCaching::LIST_VALUE_KINDS, ResultCaching::MAX_CACHED_READS, ResultCaching::MAX_READ_GENERATIONS, ResultCaching::PLACEHOLDER_KINDS

Constants included from InputRoundTrips

InputRoundTrips::INPUT_REQUEST_HANDLERS

Constants included from MCPClient::SubscriptionSupport

MCPClient::SubscriptionSupport::CONTROL_NOTIFICATIONS, MCPClient::SubscriptionSupport::DEFAULT_ACK_TIMEOUT

Constants included from JsonRpcCommon::ErrorBodies

JsonRpcCommon::ErrorBodies::MAX_ERROR_BODY_BYTES

Instance Attribute Summary

Attributes included from JsonRpcCommon

#request_meta, #send_client_info

Instance Method Summary collapse

Methods included from JsonRpcCommon

#accepted_result_types, #apply_discover_result, #begin_era_probe, #build_jsonrpc_notification, #build_jsonrpc_request, #build_named_request_params, #cancellable_request?, #client_capabilities, #client_info_payload, #declare_extension, #declare_sampling_tools, #declared_extensions, #describe_body_size, #describe_jsonrpc_message, #describe_parse_error, #discovery_cache_scope, #discovery_clock, #discovery_fresh?, #encode_header_value, #era_probe_in_flight?, #host_request_meta, #implemented_extension_result_types, #initialization_params, #mcp_name_header_value, #merge_meta_spellings, #modern?, #modern_request_headers, #notify_cache_invalidation, #ping, #process_jsonrpc_response, #protocol_era, #protocol_version, #record_discovery_freshness, #record_server_info, #refused_undeclared_sampling_tools?, #registered_callback?, #reject_task_result_discover!, #reject_task_result_on_unsupported_method!, #request_meta_claim, #required_request_meta, #reserved_meta_supplied?, #resolve_input_round_trips, restore_wire_keys, result_type, #sampling_tools_supported?, #sanitize_log_text, #select_protocol_version, #send_client_info?, #settle_era_probe, #spend_held_request_meta, #split_request_meta, #supported_versions, #suppressed_modern_notification?, #tasks_extension_declared?, #validate_log_level!, #validate_protocol_version!, #validate_result_type!, #with_request_meta, #with_retry

Methods included from MCPClient::SessionPin

#check_session_pin!, #guarded_writes, #pinned_to_session, #unpinned_session

Methods included from RequestMetadata

#adoptable_request_meta_hold, #claimable_request_meta_hold, #close_request_meta_hold, #current_params_fingerprint, #deep_sort_keys, #held_request_meta, #held_request_meta_key, #holding_request_meta, #note_request_params, #note_request_params_pending, #offer_request_meta_hold, #offered_request_meta_key, #open_request_meta_hold, #outside_request_meta_hold, #params_fingerprint_of, #recorded_request_params, #release_held_request_meta, #request_params_fingerprint, #request_params_key, #restore_request_params, #take_offered_request_meta_hold, #withdraw_request_meta_hold

Methods included from ResultCaching

#assume_zero_ttl?, #attach_list_value, #authorization_fingerprint, authorization_header_value, #authorization_header_value, #bind_authorization_context, #bump_cache_epoch, #bump_cache_generation, #cache_entries, #cache_entries_mutex, #cache_entry_for, #cache_entry_fresh?, #cache_entry_hinted?, #cache_entry_token, #cache_epoch, #cache_fresh?, #cache_generation, #cache_info, #cached_list_value, #clear_response_received_at, #clear_result_cache, #discard_paginated_list, #empty_list_copy?, #entry_for_current_params?, #entry_in_current_context?, #entry_matches_authorization?, faraday_headers, #faraday_headers, #fetching_list_page, #forget_served_entries, #forget_transport_thread_state, #fresh_list_value, #hinted_list_value, #invalid_cursor_error?, #invalidate_cache, #invalidate_cache_for_notification, #invalidate_read_cache, #list_cache_epoch, #list_kind_for, #mixed_pages_placeholder, #monotonic_now, #note_legacy_served, #note_response_received_at, #note_served_entry, #on_cache_invalidation, #private_entry_for_current_context, #prune_read_entries, #read_cache_key, #read_resource_with_cache, #record_cache_hint, #record_list_cache_hint, #record_paginated_cache_hint, #recorded_entries_key, #release_serving_request_meta, #remember_recorded_entry, #response_received_at, #response_received_key, #sent_authorization_known?, #served_entries_key, #stale_fallback_for, #stale_list_entry, #stale_list_value, #store_read_entry, #take_served_entry, #transport_thread_local_keys

Methods included from InputRoundTrips

#fulfil_input_request, #fulfil_input_requests, #undeclared_sampling_tool_use?

Methods included from MCPClient::SubscriptionSupport

#await_acknowledgment_deadline, #close_subscription_gracefully, #confirm_resource_subscription, #deliver_subscription_notification, #discard_mapped_resource_subscription, #drop_unacknowledged_resource_subscriptions, #ensure_modern_listen!, #expire_unacknowledged_subscription, #handle_server_cancellation, #handle_subscription_acknowledgment, #handle_subscription_control, #handle_subscription_response, #listen, #live_resource_subscription, #notify_host, #open_listen, #open_resource_subscription, #rearm_acknowledgment_deadline, #recheck_mapped_resource_subscription, #register_subscription, #report_declined_subscription_types, #require_resource_watch, #resource_subscription_mutex, #resource_subscription_mutexes, #resource_subscriptions, #route_notification, #settled_resource_subscription, #subscribe_resource_via_listen, #subscription_ack_timeout, #subscription_by_id, #subscription_delivery_target, #subscription_for_notification, #subscriptions, #subscriptions_mutex, #unmap_resource_subscription, #unregister_subscription, #unregister_subscription_id, #unsubscribe_resource_via_listen

Methods included from DeprecationNotices

#warn_input_request_answer_deprecated, #warn_input_request_deprecated, #warn_logging_deprecated, #warn_request_log_level_deprecated, #warn_roots_deprecated, #warn_sampling_deprecated

Methods included from JsonRpcCommon::InputWaits

#on_input_required_wait, #reject_input_required_discover!, #resume_input_required

Methods included from JsonRpcCommon::ErrorBodies

#decoded_error_body, #gunzip_bounded, #jsonrpc_error_from_http_response, #jsonrpc_error_in_body, #oversized_error_body?

Instance Method Details

#build_registered_request(method, params, req_id) ⇒ Hash

Build a JSON-RPC request under an id #next_id has already registered as outstanding. Building can fail — the host's request_meta provider is evaluated here and may raise — and a request that was never built is never sent and never answered, so its marker has to go with it; otherwise every such failure leaks an entry into @awaiting.

Parameters:

  • method (String) —

    JSON-RPC method

  • params (Hash, nil) —

    parameters for the request

  • req_id (Integer) —

    the registered request id

Returns:

  • (Hash) —

    the JSON-RPC request



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# File 'lib/mcp_client/server_stdio/json_rpc_transport.rb', line 835

def build_registered_request(method, params, req_id)
  build_jsonrpc_request(method, params, req_id)
rescue StandardError
  @mutex.synchronize { @awaiting.delete(req_id) }
  raise
end

#call_tool_streaming(tool_name, parameters) ⇒ Enumerator

Stream tool call fallback for stdio transport (yields single result)

Parameters:

  • tool_name (String) —

    the name of the tool to call

  • parameters (Hash) —

    the parameters to pass to the tool

Returns:

  • (Enumerator) —

    a stream containing a single result



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# File 'lib/mcp_client/server_stdio/json_rpc_transport.rb', line 961

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

#crash_looping?(carrier) ⇒ Boolean

Whether the process these subscriptions were last given to died too soon after receiving them for another process to be worth spawning. The two moments are stamped on that process's own record, by its own lifecycle, so this answer cannot be spoiled by whatever another thread is doing to another process.

Parameters:

Returns:

  • (Boolean) 


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# File 'lib/mcp_client/server_stdio/json_rpc_transport.rb', line 396

def crash_looping?(carrier)
  carrier&.died_carrying_subscriptions?(
    MCPClient::ServerStdio::SUBSCRIPTION_RESTART_MIN_INTERVAL
  ) || false
end

#declared_protocol_version(req) ⇒ String?

The protocol version a built request declares in its _meta. Read back from the request rather than from the transport: building it evaluates the host's metadata provider, during which a concurrent request may settle the transport on a different version.

Parameters:

  • req (Hash) —

    a JSON-RPC request

Returns:

  • (String, nil) —

    the declared version, nil for a legacy request



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# File 'lib/mcp_client/server_stdio/json_rpc_transport.rb', line 1093

def declared_protocol_version(req)
  params = req['params']
  meta = params.is_a?(Hash) ? params['_meta'] : nil
  meta.is_a?(Hash) ? meta[JsonRpcCommon::META_PROTOCOL_VERSION] : nil
end

#defer_reestablished_attempt(subscription, id, error) ⇒ void

This method returns an undefined value.

Put a subscription whose hand-over could not be written back on the queue the next session drains, and say so.

It has just been taken off that queue by #reopen_subscriptions and out of the registry above, so leaving it alone would strand it: no session would re-send it and no cleanup would find it again. The queue is where a subscription waiting for a process belongs, and the process that could not be written to is on its way out — its reader reaches EOF and restarts, and the crash-loop bound then decides whether another one is worth spawning. The subscription is :reconnecting again already — that transition was decided with the verdict, under the one lock (MCPClient::Subscription#fail_attempt).

Parameters:

  • subscription (MCPClient::Subscription)
  • id (Integer, String, nil) —

    the listen id this attempt sent under; nil when it failed before taking one

  • error (StandardError) —

    why it failed



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# File 'lib/mcp_client/server_stdio/json_rpc_transport.rb', line 233

def defer_reestablished_attempt(subscription, id, error)
  enqueue_reconnecting_subscriptions([subscription])
  @logger.debug("#{id ? "subscriptions/listen #{id}" : 'a subscriptions/listen request'} failed while the " \
                "subscription was being handed to a new process (#{error.message}); it will be re-sent to " \
                'the next one')
end

#discover_result?(result) ⇒ Boolean

Returns whether it has the DiscoverResult shape.

Parameters:

  • result (Object) —

    a JSON-RPC result

Returns:

  • (Boolean) —

    whether it has the DiscoverResult shape



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# File 'lib/mcp_client/server_stdio/json_rpc_transport.rb', line 1029

def discover_result?(result)
  result.is_a?(Hash) && result['supportedVersions'].is_a?(Array)
end

#dropped_requests ⇒ Set<Integer>

Ids of requests a teardown found outstanding: recorded under @mutex by cleanup, consumed by their waiters (see #wait_response).

Returns:

  • (Set<Integer>) 


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# File 'lib/mcp_client/server_stdio/json_rpc_transport.rb', line 338

def dropped_requests
  @dropped_requests ||= Set.new
end

#enqueue_reconnecting_locked(subscriptions) ⇒ Array<MCPClient::Subscription>

Returns the whole queue afterwards.

Parameters:

Returns:



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# File 'lib/mcp_client/server_stdio/json_rpc_transport.rb', line 445

def enqueue_reconnecting_locked(subscriptions)
  queue = (@reconnecting_subscriptions ||= [])
  subscriptions.each do |subscription|
    queue << subscription unless queue.any? { |queued| queued.equal?(subscription) }
  end
  queue.dup
end

#enqueue_reconnecting_subscriptions(subscriptions) ⇒ Array<MCPClient::Subscription>

Put subscriptions on the queue the next session drains, at most once each.

Two paths write to this queue and they overlap: MCPClient::ServerStdio#cleanup moves the open subscriptions onto it, and #defer_reestablished_attempt puts back a hand-over whose write failed — and the second happens inside the window the first leaves between taking the registry snapshot and writing it here. An unguarded Array concated by one and <<ed by the other is undefined in MRI: the same window can lose the entry, and a subscription no session re-sends and no cleanup finds again is a stream the spec says MUST be re-established, stranded with the host never told. Scanning that Array with equal? while another thread grows it does not make the append safe either — it only decided, unreliably, whether to make a second one. So both paths come through here, under the one lock that also guards the take, and membership is by identity: a handle appears on the queue once, and one hand-over goes out for it.

Parameters:

Returns:



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# File 'lib/mcp_client/server_stdio/json_rpc_transport.rb', line 421

def enqueue_reconnecting_subscriptions(subscriptions)
  reconnecting_mutex.synchronize { enqueue_reconnecting_locked(subscriptions) }
end

#ensure_initialized ⇒ void

This method returns an undefined value.

Ensure the server process is started and initialized (handshake)

Raises:



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# File 'lib/mcp_client/server_stdio/json_rpc_transport.rb', line 14

def ensure_initialized
  # Read the retirement flag FIRST. A restart clears @initialized and
  # then the flag; a thread reading them the other way round could see
  # a stale handshake next to a cleared flag and skip the lock, then
  # register a request against a transport that is being replaced.
  return if !transport_retired? && @initialized

  @init_lock.synchronize do
    # The subprocess behind a completed handshake exited under it:
    # release its pipes and reader threads before connect overwrites
    # the handles, then negotiate again against the fresh process.
    release_retired_transport if transport_retired?
    return if @initialized

    begin
      # Ordinary requests are refused while the replacement is being
      # negotiated (see #send_request): the restart clears the
      # retirement before it negotiates, so the generation alone would
      # judge a half-restarted transport current.
      @transport_lock.synchronize { @negotiating = true }
      # A process the host connected explicitly is negotiated, not
      # replaced: spawning again would orphan it.
      spawned = !live_process?
      connect if spawned
      # The record of the process that is now the session. Everything the
      # crash-loop bound needs is written on it, by its own lifecycle: see
      # {MCPClient::ServerStdio::ChildSession}. A process spawned here gets
      # a fresh record; one the host connected explicitly keeps the record
      # it already has, or gets its first.
      session = spawned || @session.nil? ? (@session = MCPClient::ServerStdio::ChildSession.new) : @session
      start_reader unless @reader_thread&.alive?
      start_stderr_reader unless @stderr_thread&.alive?
      negotiate_protocol
    rescue StandardError
      # A failed negotiation must not leave the subprocess, its pipes
      # and its reader threads behind. @initialized stays false, so the
      # next request runs connect again and overwrites @stdin/@stdout/
      # @wait_thread — putting the first process permanently out of
      # cleanup's reach.
      release_transport
      raise
    ensure
      @transport_lock.synchronize { @negotiating = false }
    end

    @initialized = true
    # The record is passed rather than read back: a cleanup on another
    # thread can retire @session while this one is still handing the
    # subscriptions to the process it established, and the crash-loop
    # bound must be stamped on the process that actually received them.
    reopen_subscriptions(session)
  end
end

#ensure_session_ready ⇒ void

This method returns an undefined value.



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# File 'lib/mcp_client/server_stdio/json_rpc_transport.rb', line 69

def ensure_session_ready
  ensure_initialized
end

#fail_open_attempt(subscription, id, error, io: nil) ⇒ void

This method returns an undefined value.

Undo the listen attempt that just failed — but only when the subscription is this attempt's to undo.

A write can block long enough for the child to exit and for the restart that follows to take the subscription over. Two things can have happened by then, and neither is this attempt's to tear down:

  • the restart already re-opened it under a newer id. The registry is keyed by listen id, so unregistering "the subscription" would delete the new registration and finishing it would close a stream the fresh process is serving. Naming the id the write went out with keeps this attempt to its own. It only says so in the log — unless that newer stream has itself already failed, in which case the caller must be told rather than handed a closed handle with no explanation.
  • the subscription is one a session is being handed (MCPClient::Subscription#reestablishing?): it is a stream the spec requires to be re-sent, not one this caller asked for, so a write that failed because the process was gone (stdin closed under it, an EPIPE to a child that exited on sight, or a nested restart holding the init lock) leaves it for the next session instead of ending it. The question is asked of the subscription rather than of its state: taking the new listen id has already moved it from :reconnecting to :pending by the time the write raises, so the state says "being opened" for the very hand-over that is failing.

Which of those it is, and the transition that follows, are decided in one step on the subscription (MCPClient::Subscription#fail_attempt). Asked first and acted on afterwards, the answer went stale in between: a restart re-opened the subscription under a newer id and had it acknowledged after the ownership check had passed, and this attempt then finished the healthy replacement. Only the registration comes first — it is scoped to this attempt's id, so a newer one is never touched by it.

An attempt that failed before the subscription took its id at all (the request could not be built) arrives with no id: nothing was registered for it, and no newer attempt can have superseded it, so the failure is the caller's — or the next session's — to hear about. A failure that is the caller's own abandons the request the write may have put on the pipe, and abandoning a request on stdio is a cancellation naming its id (basic/transports/stdio "Cancellation"): left unnamed, the server was serving listen(n) for a subscription that had ended with the error, and nothing this client held could cancel it any more. It is sent best effort, to the pipe the request went to. A superseded attempt's request went to a process the restart has replaced, and a deferred hand-over's to one on its way out — neither is cancelled on a pipe that is gone.

Parameters:

  • subscription (MCPClient::Subscription)
  • id (Integer, String, nil) —

    the listen id this attempt sent under; nil when it failed before taking one

  • error (StandardError) —

    why it failed

  • io (IO, nil) (defaults to: nil) —

    the pipe the request was written to

Raises:

  • (StandardError) —

    the failure, when it was still this attempt's



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# File 'lib/mcp_client/server_stdio/json_rpc_transport.rb', line 197

def fail_open_attempt(subscription, id, error, io: nil)
  unregister_subscription_id(subscription, id) if id
  failure = subscription_failure(error)
  case subscription.fail_attempt(id, failure)
  when :superseded then fail_superseded_attempt(subscription, id, error)
  when :deferred then defer_reestablished_attempt(subscription, id, error)
  else
    cancel_outstanding_listens(subscription, io: io) if io && id
    raise failure
  end
end

#fail_reconnecting_subscriptions(error) ⇒ void

This method returns an undefined value.

End the subscriptions waiting for a process that is not coming back.

Parameters:

  • error (StandardError) —

    why it is not



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# File 'lib/mcp_client/server_stdio/json_rpc_transport.rb', line 539

def fail_reconnecting_subscriptions(error)
  fail_subscriptions(take_reconnecting_subscriptions, error)
end

#fail_subscriptions(pending, error) ⇒ void

This method returns an undefined value.

Parameters:



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# File 'lib/mcp_client/server_stdio/json_rpc_transport.rb', line 546

def fail_subscriptions(pending, error)
  failure = subscription_failure(error)
  pending.each { |subscription| subscription.finish(gracefully: false, error: failure) }
end

#fail_superseded_attempt(subscription, id, error) ⇒ void

This method returns an undefined value.

A failure the subscription has already moved on from: harmless while the stream that replaced it stands, and the caller's answer when it does not.

Parameters:

  • subscription (MCPClient::Subscription)
  • id (Integer, String) —

    the listen id this attempt sent under

  • error (StandardError) —

    why it failed

Raises:



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# File 'lib/mcp_client/server_stdio/json_rpc_transport.rb', line 248

def fail_superseded_attempt(subscription, id, error)
  replacement_error = subscription.closed? ? subscription.error : nil
  if replacement_error
    @logger.warn("subscriptions/listen #{id} failed (#{error.message}) and the stream that replaced it " \
                 "failed too: #{sanitize_log_text(replacement_error.message)}")
    raise replacement_error
  end

  @logger.debug("subscriptions/listen #{id} failed after the subscription was re-opened " \
                "(#{error.message}); the newer stream stands")
end

#fall_forward_to_modern(error) ⇒ Hash

Resume the modern path after a fallback handshake was refused by a modern server: the rejection settles the era, so server/discover is re-issued with a version the server named and initialize is never sent again.

Parameters:

Returns:

  • (Hash) —

    the DiscoverResult



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# File 'lib/mcp_client/server_stdio/json_rpc_transport.rb', line 805

def fall_forward_to_modern(error)
  version = select_protocol_version(error.supported)
  @logger.info('The server refused the initialize handshake and supports ' \
               "#{error.supported.join(', ')}; it is a modern server — retrying server/discover with #{version}")
  @protocol_version = version
  perform_discover
end

#fall_forward_to_modern?(error) ⇒ Boolean

Whether a rejected initialize handshake should send this connection back to the modern path. Only a well-formed rejection counts — a bare -32022 from a legacy endpoint identifies nothing — and only one that names a version this client speaks, since the retry has to declare one. A host that configured protocol: :legacy asked for the 2025-11-25 handshake and gets the error instead.

Parameters:

Returns:

  • (Boolean) 


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# File 'lib/mcp_client/server_stdio/json_rpc_transport.rb', line 793

def fall_forward_to_modern?(error)
  return false if @protocol_mode == :legacy

  error.modern_protocol_error? && !select_protocol_version(error.supported).nil?
end

#hand_over_to_established_process ⇒ void

This method returns an undefined value.

Hand the queue to a process that is already established.

#ensure_initialized re-sends the queue itself, but only when it negotiated the process: a host request that observed the exit while this teardown was still parking its subscriptions established the replacement, found the queue empty and returned, and the restart above then took the initialized fast path — leaving the subscriptions parked on a queue nothing was going to read. Whichever of the two finishes last drains what it finds here, so the re-send MCP 2026-07-28 basic/patterns/subscriptions requires after a stdio reconnect happens however the two threads interleave.

Under the initialization lock, like every other hand-over: the process must not be replaced underneath the writes, and a queue taken while a negotiation is in flight would be re-sent to the process that negotiation is replacing.



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# File 'lib/mcp_client/server_stdio/json_rpc_transport.rb', line 528

def hand_over_to_established_process
  @init_lock.synchronize do
    next unless @initialized

    reopen_subscriptions
  end
end

#identifies_modern_server?(msg) ⇒ Boolean

Whether a response, as it comes off the wire, could only have been written by a modern server: a result carrying a 2026-07-28 marker, or one of the spec-defined modern errors in its mandated shape.

Parameters:

  • msg (Hash) —

    a JSON-RPC response

Returns:

  • (Boolean) 


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# File 'lib/mcp_client/server_stdio/json_rpc_transport.rb', line 731

def identifies_modern_server?(msg)
  return true if modern_discover_answer?(msg)
  return false unless msg.key?('error')

  MCPClient::Errors::ServerError.from_jsonrpc(msg['error']).modern_protocol_error?
end

#interpret_discover_answer(res) ⇒ Hash

Turn the probe's answer into a DiscoverResult, or into the failure that says what kind of server sent it.

The stdio fallback rule is keyed to the probe being answered with an error, or not answered at all — never to a result. resultType does not exist before 2026-07-28, so a result carrying it came from a modern server even when the rest of it is unusable: treating that as a legacy answer would pin a dual-era server to the 2025-11-25 handshake for the life of the process, and would make a modern-only server fail to connect after it had already answered the probe. Such an answer therefore fails the negotiation instead of falling back. A result with no 2026-07-28 marker at all is still a legacy answer: a permissive server answering an unknown method with some object.

Parameters:

  • res (Hash) —

    the JSON-RPC response to the probe

Returns:

  • (Hash) —

    the applied DiscoverResult

Raises:



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# File 'lib/mcp_client/server_stdio/json_rpc_transport.rb', line 700

def interpret_discover_answer(res)
  modern_answer = modern_discover_answer?(res)
  # Named so the identity of an answer that fails to validate below is
  # not recorded: apply_discover_result records it once the whole
  # result has validated.
  result = process_jsonrpc_response(res, method: 'server/discover')
  reject_input_required_discover!(result)
  reject_task_result_discover!(result)
  unless discover_result?(result)
    raise invalid_discover_answer(modern_answer, 'answered without a DiscoverResult')
  end

  apply_discover_result(result)
rescue MCPClient::Errors::InvalidResultError, MCPClient::Errors::InputRequiredError => e
  raise invalid_discover_answer(modern_answer, "answered without a DiscoverResult (#{e.message})")
end

#invalid_discover_answer(modern_answer, message) ⇒ StandardError

Returns the failure the probe should propagate.

Parameters:

  • modern_answer (Boolean) —

    whether the answer identified a modern server

  • message (String) —

    what was wrong with it

Returns:

  • (StandardError) —

    the failure the probe should propagate



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# File 'lib/mcp_client/server_stdio/json_rpc_transport.rb', line 741

def invalid_discover_answer(modern_answer, message)
  return MCPClient::Errors::ServerError.new("server/discover was #{message}") unless modern_answer

  MCPClient::Errors::ConnectionError.new("Server is modern but incompatible: server/discover was #{message}")
end

#legacy_after_probe(error) ⇒ void

This method returns an undefined value.

Not a recognized modern error: a legacy server (or one that never answered).

Parameters:

  • error (StandardError) —

    the probe failure

Raises:



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# File 'lib/mcp_client/server_stdio/json_rpc_transport.rb', line 636

def legacy_after_probe(error)
  e = error
  @protocol_version = nil
  if @protocol_mode == :modern
    raise MCPClient::Errors::ConnectionError,
          "Server did not answer server/discover as a modern MCP server (#{e.message}); it is most likely " \
          'a legacy server expecting the initialize handshake. Use protocol: :auto or :legacy to allow that.'
  end

  @logger.debug("server/discover probe failed (#{e.class}); treating the server as legacy")
end

#live_process? ⇒ Boolean

Returns whether a subprocess is connected and still running.

Returns:

  • (Boolean) —

    whether a subprocess is connected and still running



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# File 'lib/mcp_client/server_stdio/json_rpc_transport.rb', line 329

def live_process?
  return false unless @stdin.respond_to?(:closed?) && !@stdin.closed?

  @wait_thread.respond_to?(:alive?) && @wait_thread.alive?
end

#modern_discover_answer?(res) ⇒ Boolean

Returns whether its result could only have come from a 2026-07-28 server.

Parameters:

  • res (Hash) —

    the JSON-RPC response to the probe

Returns:

  • (Boolean) —

    whether its result could only have come from a 2026-07-28 server



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# File 'lib/mcp_client/server_stdio/json_rpc_transport.rb', line 719

def modern_discover_answer?(res)
  result = res.is_a?(Hash) ? res['result'] : nil
  return false unless result.is_a?(Hash)

  result.key?('resultType') || result.key?(:resultType) || discover_result?(result)
end

#negotiate_protocol ⇒ void

This method returns an undefined value.

Establish the server's protocol era (MCP 2026-07-28 basic/transports/stdio "Backward Compatibility"): probe with server/discover unless configured legacy-only, and fall back to the initialize handshake when the probe shows a legacy server.

Raises:



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# File 'lib/mcp_client/server_stdio/json_rpc_transport.rb', line 557

def negotiate_protocol
  return perform_initialize if @protocol_mode == :legacy
  return if probe_modern_server

  perform_initialize
rescue StandardError
  # Nothing was negotiated. The probe only PROPOSES its version, and a
  # failure that never reached one of the classified outcomes — the
  # host's request_meta provider raising, say, so no probe was even
  # sent — would otherwise leave that proposal behind as a settled
  # modern era. The next attempt would then take a legacy server's
  # startup request for prohibited modern traffic and drop it, and a
  # server waiting for that response answers nothing: the recovery
  # deadlocks until it times out.
  @protocol_version = nil
  settle_era_probe
  raise
end

#next_id ⇒ Integer

Generate a new unique request ID and mark it as awaiting a response. Registering the id before the request is sent lets the reader thread distinguish expected responses from late/unsolicited ones.

Returns:

  • (Integer) —

    a unique request ID



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# File 'lib/mcp_client/server_stdio/json_rpc_transport.rb', line 817

def next_id
  @mutex.synchronize do
    id = @next_id
    @next_id += 1
    @awaiting[id] = true
    id
  end
end

#open_subscription(subscription) ⇒ void

This method returns an undefined value.

Send the subscriptions/listen request for a subscription (a fresh id each time it is opened or re-opened).

Parameters:



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# File 'lib/mcp_client/server_stdio/json_rpc_transport.rb', line 77

def open_subscription(subscription)
  # The pipe this attempt writes to, taken before anything is recorded
  # about it and written to whatever happens next.
  #
  # Reading the transport's *current* stdin at the write instead let a
  # listen that was still pending when the process exited be written to
  # the process that replaced it: the teardown had already forgotten that
  # id (nothing written to a dead process is outstanding, and none of its
  # ids may be cancelled on its successor), so the replacement was
  # serving a second stream this client could no longer name — and the
  # restart's own listen was the only one `close` cancelled. Pinning the
  # pipe makes the bookkeeping follow the process actually written to: a
  # write that lands late goes to the pipe it was recorded against, and
  # once the teardown has closed that pipe it fails into the error paths
  # below instead.
  # Read with the generation it belongs to, so the two agree: the pipe
  # is what this attempt writes to, and the generation is what a
  # teardown compares its claim against when it decides whose
  # subscriptions to park.
  stdin, generation = @transport_lock.synchronize { [@stdin, @transport_generation] }
  # Whether the subscription has taken this attempt's id yet. A failure
  # before that — the request could not be built — is nobody's to have
  # superseded, and was filed as exactly that while the id it never
  # took was compared with the one it had (see {#fail_open_attempt}).
  taken = false
  id = next_id
  # No caller waits on this id: the response, if any, is the server's
  # graceful closure and is routed to the subscription itself.
  @mutex.synchronize { @awaiting.delete(id) }
  request = build_jsonrpc_request('subscriptions/listen', { 'notifications' => subscription.requested }, id)
  # A {Subscription#close} racing with a re-open must not leave the
  # server holding a subscription this client can no longer cancel:
  # taking the id and registering it happen under the subscription's own
  # lock (so a close that wins stops the re-open outright), and a close
  # that cancelled this id while the request was still going out is
  # named again below, once the server has seen the listen.
  taken = true
  return unless subscription.with_open_id(id, generation) { register_subscription(subscription) }

  # Recorded before the write, and whatever the write does: from here on
  # the server may be serving this listen, and {#cancel_subscription}
  # has to be able to name it even after a later request has taken the
  # subscription's own id (see {Subscription#record_outstanding_listen}).
  # It is not cancellable until the write has finished, though — a close
  # that named it while the pipe still held nothing would put
  # `cancelled(n)` on the wire ahead of `listen(n)`. So this attempt
  # marks it written and then cancels it itself, if that close has
  # happened by then.
  #
  # Both of those happen however the write ends. A write that raised may
  # still have put the request on the pipe, and the id is recorded for
  # exactly that reason; whether that request is then cancelled is
  # decided with the failure's verdict (see {#fail_open_attempt}). The
  # pipe is named on both, so the cancellation goes to the process the
  # request went to and to no other.
  subscription.record_outstanding_listen(id, stdin)
  begin
    send_request(request, io: stdin)
  ensure
    subscription.mark_listen_written(id)
  end
  cancel_outstanding_listens(subscription, io: stdin) if subscription.closed_by_client?
rescue StandardError => e
  fail_open_attempt(subscription, taken ? id : nil, e, io: stdin)
end

#perform_discover ⇒ Hash

Send server/discover and apply the DiscoverResult. Bounded by discover_timeout rather than the general read timeout so a silent legacy server delays the fallback only briefly.

Returns:

  • (Hash) —

    the DiscoverResult



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# File 'lib/mcp_client/server_stdio/json_rpc_transport.rb', line 670

def perform_discover
  req_id = next_id
  req = build_registered_request('server/discover', {}, req_id)
  send_request(req)
  begin
    res = wait_response(req_id, timeout: @discover_timeout)
  rescue MCPClient::Errors::RequestTimeoutError
    send_cancellation_notification(req_id)
    raise
  end
  interpret_discover_answer(res)
end

#perform_initialize ⇒ void

This method returns an undefined value.

Handshake: send initialize request and initialized notification

Raises:



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# File 'lib/mcp_client/server_stdio/json_rpc_transport.rb', line 750

def perform_initialize
  # Initialize request
  init_id = next_id
  init_req = build_registered_request('initialize', initialization_params, init_id)
  send_request(init_req)
  res = wait_response(init_id)
  begin
    result = process_jsonrpc_response(res) || {}
  rescue MCPClient::Errors::UnsupportedProtocolVersionError => e
    # A modern-only server SHOULD name the versions it supports when
    # rejecting initialize (basic/versioning). When one of them is
    # mutual the era is settled after all — the fallback ran only
    # because the probe was too slow — so go back to server/discover
    # instead of ending the session. A legacy-only configuration has
    # opted out of the modern era and gets the error.
    return fall_forward_to_modern(e) if fall_forward_to_modern?(e)

    raise MCPClient::Errors::ConnectionError,
          "Initialize failed: #{e.message} (server supports: #{e.supported.join(', ')})"
  rescue MCPClient::Errors::ServerError => e
    raise MCPClient::Errors::ConnectionError, "Initialize failed: #{e.message}"
  end

  # Store negotiated protocol version, server info and capabilities.
  # Disconnects if the server negotiated a version we cannot speak.
  @protocol_version = validate_protocol_version!(result)
  @server_info = result['serverInfo']
  @capabilities = result['capabilities']
  @instructions = result['instructions']

  # Send initialized notification
  notif = build_jsonrpc_notification('notifications/initialized', {})
  @stdin.puts(notif.to_json)
end

#probe_modern_server ⇒ Boolean

Send the server/discover probe with this client's preferred modern version. Three outcomes, per the stdio backward-compatibility rules: a DiscoverResult (modern: select a version from supportedVersions), a recognized modern error such as UnsupportedProtocolVersionError (modern: retry with an advertised version, never fall back), or any other error / a timeout (legacy: fall back to initialize). The fallback is deliberately not keyed to one error code — legacy servers answer pre-initialize requests with implementation-defined errors.

Returns:

  • (Boolean) —

    true when the server is modern and a version was selected

Raises:



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# File 'lib/mcp_client/server_stdio/json_rpc_transport.rb', line 587

def probe_modern_server
  # The probe DECLARES this version; it does not establish it. Until the
  # answer arrives the era stays unknown, so an incoming server request
  # is still handled — a legacy server MAY ping during initialization and
  # the receiver MUST respond promptly, and a server waiting for that
  # response answers nothing until it arrives.
  @protocol_version = MCPClient::LATEST_PROTOCOL_VERSION
  begin_era_probe
  modern_confirmed = false
  begin
    perform_discover
  rescue MCPClient::Errors::UnsupportedProtocolVersionError => e
    raise unless e.modern_protocol_error?

    # A well-formed rejection settles the era: whatever the retried
    # probe does next, this server is modern and never gets initialize.
    modern_confirmed = true
    settle_era_probe
    retry_discover_with_advertised_version(e)
  end
  true
rescue MCPClient::Errors::ConnectionError
  # A DiscoverResult (or advertised list) with no mutual version: the
  # server is modern but incompatible. Nothing was negotiated.
  @protocol_version = nil
  raise
rescue MCPClient::Errors::ServerError, MCPClient::Errors::TransportError => e
  # A recognized modern error (-32020/-32021, or -32022 with no usable
  # version) identifies a modern server: surface it, never fall back.
  # Anything else — including a 2xx-style result that is not a
  # DiscoverResult — is a legacy server, unless the era was already
  # settled by a well-formed rejection.
  if modern_confirmed || e.modern_protocol_error_for_probe?
    @protocol_version = nil
    raise MCPClient::Errors::ConnectionError, "Server is modern but incompatible: #{e.message}"
  end

  legacy_after_probe(e)
  false
ensure
  # However the probe ended, it is no longer proposing anything.
  settle_era_probe
end

#queue_subscriptions_of_ended_process(subscriptions) ⇒ void

This method returns an undefined value.

Hand the subscriptions of a process that is being torn down to the next one, forgetting the listen ids that process was holding: nothing written to it is outstanding any more, and none of those ids may be cancelled on the process that replaces it (see MCPClient::Subscription#record_outstanding_listen).

Both steps happen under the lock a hand-over takes them off the queue under, so a session that is already re-sending them cannot have the ids it has just written forgotten by this teardown: it cannot reach its own writes until this has finished.

Parameters:



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# File 'lib/mcp_client/server_stdio/json_rpc_transport.rb', line 437

def queue_subscriptions_of_ended_process(subscriptions)
  reconnecting_mutex.synchronize do
    enqueue_reconnecting_locked(subscriptions).each(&:discard_outstanding_listens)
  end
end

#reconnecting_mutex ⇒ Mutex

Returns guards the queue of subscriptions waiting for a process (created by MCPClient::ServerStdio#initialize, so no two threads ever race to make it).

Returns:



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# File 'lib/mcp_client/server_stdio/json_rpc_transport.rb', line 472

def reconnecting_mutex
  @reconnecting_mutex ||= Mutex.new
end

#reconnecting_subscriptions ⇒ Array<MCPClient::Subscription>

Returns the subscriptions on the queue that a process could still be re-sent to.

Returns:



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# File 'lib/mcp_client/server_stdio/json_rpc_transport.rb', line 465

def reconnecting_subscriptions
  reconnecting_mutex.synchronize { (@reconnecting_subscriptions || []).select(&:reconnectable?) }
end

#release_retired_transport ⇒ void

This method returns an undefined value.

Discard a transport whose subprocess exited after a successful handshake, so the next negotiation starts from a clean slate rather than on top of the dead process's handles.



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# File 'lib/mcp_client/server_stdio/json_rpc_transport.rb', line 351

def release_retired_transport
  @logger.info('The MCP server subprocess exited; restarting it for this request')
  @initialized = false
  @transport_retired = false
  release_transport
end

#release_transport ⇒ void

This method returns an undefined value.

Tear down a transport that is not going to be used again — a handshake that never completed, or a subprocess that exited under a completed one. Failures are swallowed: the transport being unusable is often the reason it is being released, and the original error is the one worth raising.



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# File 'lib/mcp_client/server_stdio/json_rpc_transport.rb', line 364

def release_transport
  cleanup
rescue StandardError => e
  @logger.debug("Releasing the stdio transport did not complete cleanly: #{e.message}")
end

#reopen_refusal(carrier) ⇒ StandardError?

Why this session must not be given the open subscriptions, if it must not (see #reopen_subscriptions).

Parameters:

Returns:

  • (StandardError, nil) 


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# File 'lib/mcp_client/server_stdio/json_rpc_transport.rb', line 375

def reopen_refusal(carrier)
  unless modern?
    return MCPClient::Errors::CapabilityError.new(
      'the re-established server process negotiated ' \
      "#{protocol_version || 'no version'}, which cannot carry a subscriptions/listen stream"
    )
  end
  return nil unless crash_looping?(carrier)

  @logger.error('MCP server process exited again right after it was given its subscriptions; closing them')
  MCPClient::Errors::TransportError.new('MCP server process exited again right after a restart')
end

#reopen_subscriptions(session = @session) ⇒ void

This method returns an undefined value.

After the process was re-established, re-send subscriptions/listen for every subscription the host still holds open ("the server holds no subscription state across reconnections").

This is the one place a session is handed the subscriptions, so it is the one place that decides whether to hand them over at all:

  • a re-established session that turns out to be legacy cannot carry them. On protocol: :auto the restarted process may negotiate an older revision than the one that died, and MCPClient::ServerStdio#cleanup has already moved the open subscriptions out of the registry — returning would leave them :reconnecting for ever with the host never told.
  • neither can a session that would only continue a crash loop: if the last process these subscriptions were given died less than SUBSCRIPTION_RESTART_MIN_INTERVAL after receiving them, handing them over again would spawn the same corpse for ever. Deciding here rather than at the restart is what makes the bound hold: a process is re-established by whichever thread gets there first — the reader's restart or a host request — and only the re-send is common to both.

Either way the subscriptions end with the error, so the host learns from closed?/error rather than waiting on a stream that is not coming back.

Parameters:



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# File 'lib/mcp_client/server_stdio/json_rpc_transport.rb', line 287

def reopen_subscriptions(session = @session)
  pending = take_reconnecting_subscriptions
  # A session handed nothing asks nothing, and must not spend the record
  # either: the subscriptions are still open on the session this one
  # replaced (a nested restart re-establishes the process between a
  # hand-over and the next exit), and the process that carried them is
  # still what the next hand-over has to be judged against.
  return if pending.empty?

  # The record of the process that last carried subscriptions answers
  # exactly one question — whether handing them over again would only
  # respawn the same corpse — and this is the moment it is asked. Asking
  # spends it, whatever the answer: the loop it recorded is either
  # broken here (these subscriptions are closed and never handed on) or
  # replaced below by the record of the process that takes them. Left
  # standing it outlived the loop it described, and the next hand-over —
  # of a subscription opened directly on the replacement, which then ran
  # healthily for hours — was refused for a crash it had no part in.
  carrier = @subscription_carrier
  @subscription_carrier = nil

  refusal = reopen_refusal(carrier)
  return fail_subscriptions(pending, refusal) if refusal

  # Stamped on the process before the writes go out: one that exits
  # while they are still going to it survived receiving them by no time
  # at all, which is what the next hand-over needs to know.
  session&.carrying_subscriptions
  @subscription_carrier = session
  pending.each do |subscription|
    open_subscription(subscription)
    # A re-sent listen is a new request the replacement has to
    # acknowledge, so it carries the deadline the first one did: a
    # process that takes it and then says nothing is otherwise bounded
    # by nothing at all on stdio.
    rearm_acknowledgment_deadline(subscription)
  rescue StandardError => e
    @logger.warn("Could not re-establish subscription: #{e.message}")
  end
end

#restart_for_open_subscriptions ⇒ void

This method returns an undefined value.

Restart the process the reader just watched exit, for the subscriptions the host still holds.

A subscription is a standing request the host does not repeat: while it only waits for notifications there is no RPC for #ensure_initialized to re-establish the process on, so leaving the restart to "the next request" leaves every subscription :reconnecting for ever, with the host neither notified nor served. Restarting is also what MCP 2026-07-28 stdio "Unexpected Termination" asks of a client, and re-sending the subscriptions afterwards is what this transport already promises. With no subscription open there is nothing standing, and the process stays lazily re-established on the next request.

A server that keeps exiting must not be respawned in a loop; that bound is enforced where the subscriptions are handed over rather than here (see #reopen_subscriptions), because the process is re-established by whichever thread gets there first — this restart or a host request that raced it — and only the hand-over is common to both. A restart that fails outright ends them here instead. Either way the host learns from closed?/error rather than waiting on a stream that is never coming back.



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# File 'lib/mcp_client/server_stdio/json_rpc_transport.rb', line 499

def restart_for_open_subscriptions
  pending = reconnecting_subscriptions
  return if pending.empty?

  @logger.info("Re-establishing the server process for #{pending.size} open subscription(s)")
  ensure_initialized
  hand_over_to_established_process
rescue StandardError => e
  @logger.warn("Could not re-establish the server process: #{e.message}")
  fail_reconnecting_subscriptions(e)
end

#retry_discover_with_advertised_version(error) ⇒ void

This method returns an undefined value.

After UnsupportedProtocolVersionError, pick a mutually supported version from the error's advertised list and re-issue the probe.

Parameters:



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# File 'lib/mcp_client/server_stdio/json_rpc_transport.rb', line 652

def retry_discover_with_advertised_version(error)
  version = select_protocol_version(error.supported)
  unless version
    raise MCPClient::Errors::ConnectionError,
          "Server rejected protocol version #{@protocol_version} and supports only " \
          "#{error.supported.join(', ')}, none of which this client speaks " \
          "(modern versions supported: #{MCPClient::MODERN_PROTOCOL_VERSIONS.join(', ')})"
  end

  @logger.info("Server does not support #{@protocol_version}; retrying server/discover with #{version}")
  @protocol_version = version
  perform_discover
end

#rpc_notify(method, params = {}) ⇒ void

This method returns an undefined value.

Send a JSON-RPC notification (no response expected)

Parameters:

  • method (String) —

    JSON-RPC method

  • params (Hash) (defaults to: {}) —

    parameters for the notification



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# File 'lib/mcp_client/server_stdio/json_rpc_transport.rb', line 1116

def rpc_notify(method, params = {})
  ensure_initialized
  if suppressed_modern_notification?(method)
    @logger.debug("Not sending #{method}: removed in MCP #{protocol_version}")
    return
  end

  notif = build_jsonrpc_notification(method, params)
  begin
    @stdin.puts(notif.to_json)
  rescue StandardError => e
    # The same failure a request write reports, reported the same way:
    # a notification writes on its own, outside the request path's
    # check-and-write, and a dead pipe there reached the host as a raw
    # IOError.
    raise MCPClient::Errors::TransportError, "Failed to send JSONRPC notification: #{e.message}"
  end
end

#rpc_request(method, params = {}, timeout: nil) ⇒ Object

Generic JSON-RPC request: send method with params and wait for result

Parameters:

  • method (String) —

    JSON-RPC method

  • params (Hash) (defaults to: {}) —

    parameters for the request

Returns:

  • (Object) —

    result from JSON-RPC response

Raises:



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# File 'lib/mcp_client/server_stdio/json_rpc_transport.rb', line 974

def rpc_request(method, params = {}, timeout: nil)
  freshly_probed = !@initialized || transport_retired?
  ensure_initialized
  if method == 'ping' && modern?
    # `ping` was removed in MCP 2026-07-28; the mandatory server/discover
    # request is the modern heartbeat. The probe that just established
    # the connection IS such a round trip, so answer from it rather than
    # paying for a second one.
    return @last_discover_result if freshly_probed && @last_discover_result

    method = 'server/discover'
  end

  # The multi round-trip resolver sits outside the per-attempt
  # recovery, so a retry that carries inputResponses/requestState keeps
  # them through transport retries, version renegotiation and the like.
  result = resolve_input_round_trips(method, params, timeout) do |attempt_params|
    # Every round is its own request, and the subprocess may have exited
    # between two of them — the host handler that gathers the input
    # waits for a person. MCP 2026-07-28 basic/transports/stdio
    # ("Unexpected Termination"): the client SHOULD restart a server
    # that terminated unexpectedly. The continuation is then re-issued
    # against the fresh process with the answers and the state it was
    # gathered for, rather than written to a dead pipe.
    ensure_initialized
    with_retry(method) do
      sent_version = nil
      begin
        send_request_and_wait(method, attempt_params, timeout) { |version| sent_version = version }
      rescue MCPClient::Errors::UnsupportedProtocolVersionError => e
        # MCP 2026-07-28 basic/versioning: "The client SHOULD select a
        # mutually supported version from the supported list and retry
        # the request". The server rejected the request before
        # processing it, so re-sending cannot duplicate a side effect.
        # Compared against the version THIS request declared, read back
        # from the request itself: a concurrent request may have moved
        # the transport on while this one was being built.
        version = select_protocol_version(e.supported)
        raise unless modern? && version && version != sent_version

        @logger.info("Server does not support protocol version #{sent_version}; " \
                     "retrying #{method} with #{version}")
        @protocol_version = version
        send_request_and_wait(method, attempt_params, timeout)
      end
    end
  end
  # Every server/discover answer is validated and applied: a later
  # heartbeat may advertise new versions or capabilities.
  result = apply_discover_result(result) if method == 'server/discover'
  result
end

#send_cancellation_notification(request_id) ⇒ void

This method returns an undefined value.

Best-effort notifications/cancelled for a request the client stopped waiting on. Failures are swallowed: the transport may be the reason the request timed out in the first place.

Parameters:

  • request_id (Integer) —

    id of the abandoned request



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# File 'lib/mcp_client/server_stdio/json_rpc_transport.rb', line 1104

def send_cancellation_notification(request_id)
  notif = build_jsonrpc_notification('notifications/cancelled',
                                     { 'requestId' => request_id, 'reason' => 'Request timed out' })
  @stdin.puts(notif.to_json)
rescue StandardError => e
  @logger.debug("Failed to send cancellation notification: #{e.message}")
end

#send_if_current(req, generation) ⇒ Hash?

Write a registered request, unless the transport it was registered on has been replaced since. Replacement is judged by the transport generation, which every teardown and every spawn bumps: a restart in between has dropped the id from @awaiting, and the process the request was built for is gone.

Parameters:

  • req (Hash) —

    the JSON-RPC request

  • generation (Integer) —

    the transport generation the id was registered on

Returns:

  • (Hash, nil) —

    the request once written, or nil when the transport was replaced under it before it was written (it was not sent)

Raises:



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# File 'lib/mcp_client/server_stdio/json_rpc_transport.rb', line 852

def send_if_current(req, generation)
  return req unless send_request(req, generation) == :replaced

  @logger.debug("The transport was replaced before #{req['method']} was sent; re-issuing it")
  # Nothing was written, so nobody will ever wait on this id: drop it
  # from the teardown's record as well as from the awaiting table. The
  # waiter is what normally consumes that record, and an id no request
  # carries has no waiter — it would accumulate for the process's life.
  @mutex.synchronize do
    @awaiting.delete(req['id'])
    dropped_requests.delete(req['id'])
  end
  nil
end

#send_on_current_transport(method, params) {|req| ... } ⇒ Array(Integer, Hash)

Register, build and write a request on the transport that is current when it is written. Between registering the id and writing, the host's request_meta provider runs, and in that window the subprocess may exit and another thread restart it: the restart drops every outstanding id, so a request written afterwards would go out unregistered and its answer be discarded as unsolicited. Nothing has been sent when that is detected, so the request is simply rebuilt — after waiting for the restart to complete — and sent registered.

Parameters:

  • method (String) —

    JSON-RPC method

  • params (Hash) —

    parameters for the request

Yield Parameters:

  • req (Hash) —

    the request as built, before it is written

Returns:

  • (Array(Integer, Hash)) —

    the registered id and the request



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# File 'lib/mcp_client/server_stdio/json_rpc_transport.rb', line 1071

def send_on_current_transport(method, params)
  loop do
    # A subprocess that exited under the handshake is restarted here
    # (MCP 2026-07-28 stdio "Unexpected Termination": the client
    # SHOULD restart it) rather than written to.
    ensure_initialized if transport_retired?
    generation = @transport_generation
    req_id = next_id
    req = build_registered_request(method, params, req_id)
    yield req if block_given?
    return [req_id, req] if send_if_current(req, generation)

    ensure_initialized
  end
end

#send_request(req, generation = nil, io: @stdin) ⇒ Symbol

Send a JSON-RPC request. With a generation, the request is written only if that is still the transport's generation, and the check and the write are one step under the transport lock: a restart replaces the handles and bumps the generation under the same lock, so a request judged current cannot be written to the replacement process — where it would be executed unregistered and its answer discarded.

Parameters:

  • req (Hash) —

    the JSON-RPC request

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

    the transport generation the request was registered on

  • io (IO, nil) (defaults to: @stdin) —

    the pipe to write to; defaults to the live process's stdin, but a caller whose bookkeeping is tied to one particular process pins that process's pipe instead (see #open_subscription)

Returns:

  • (Symbol) —

    :sent, or :replaced when the transport was replaced and nothing was written

Raises:



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# File 'lib/mcp_client/server_stdio/json_rpc_transport.rb', line 880

def send_request(req, generation = nil, io: @stdin)
  @logger.debug("Sending JSONRPC request: #{describe_jsonrpc_message(req)}")
  # A request pinned to a session that has since ended is not written at
  # all: its payload names something else in the replacement session.
  # The pin is read outside the transport lock (the two locks never
  # nest); a restart completing between this check and the write moves
  # the transport generation, which the locked check below catches.
  @mutex.synchronize { check_session_pin! }
  @transport_lock.synchronize do
    # A replacement whose negotiation has not completed is not current
    # either, whatever its generation says: an ordinary request written
    # to it would reach the process before its handshake.
    return :replaced if generation && (generation != @transport_generation || @negotiating)
    raise IOError, 'the server process is gone' unless io

    # The write goes to the pipe this request was recorded against,
    # never to whichever pipe the transport holds by now.
    io.puts(req.to_json)
  end
  :sent
rescue MCPClient::Errors::SessionChangedError, MCPClient::Errors::TaskReplacedError
  # A refusal, not a failure: the pin (or the caller's own pre-write
  # guard, see {MCPClient::SessionPin#guarded_writes}) turned the write
  # down. Nothing was written, nothing will answer this id, and the
  # refusal keeps its type — a definite "this request is not to be
  # sent" must not reach the caller as an ambiguous transport failure.
  @mutex.synchronize { @awaiting.delete(req['id']) } if req.is_a?(Hash) && req['id']
  raise
rescue StandardError => e
  # A request that failed to send will never receive a response, so drop
  # its awaiting marker; otherwise a broken transport (e.g. the server
  # exited) would leak an entry per retry/attempt into @awaiting.
  @mutex.synchronize { @awaiting.delete(req['id']) } if req.is_a?(Hash) && req['id']
  raise MCPClient::Errors::TransportError, "Failed to send JSONRPC request: #{e.message}"
end

#send_request_and_wait(method, params, timeout) {|version| ... } ⇒ Object

One request/response exchange with its own JSON-RPC id.

Parameters:

  • method (String) —

    JSON-RPC method

  • params (Hash) —

    parameters for the request

  • timeout (Numeric, nil) —

    per-request timeout override

Yield Parameters:

  • version (String, nil) —

    the protocol version the request declares

Returns:

  • (Object) —

    result from the JSON-RPC response



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# File 'lib/mcp_client/server_stdio/json_rpc_transport.rb', line 1039

def send_request_and_wait(method, params, timeout)
  # As late as a request pinned to a session can be held back: every
  # reconnect on the way here (ensure_initialized, a retry after the
  # child exited) has happened by now.
  check_session_pin!
  req_id, = send_on_current_transport(method, params) do |built|
    clear_response_received_at if respond_to?(:clear_response_received_at, true)
    yield declared_protocol_version(built) if block_given?
  end
  begin
    res = wait_response(req_id, timeout: timeout)
  rescue MCPClient::Errors::RequestTimeoutError
    # MCP lifecycle: on timeout the sender SHOULD issue a cancellation
    # notification for the abandoned request and stop waiting.
    send_cancellation_notification(req_id) if cancellable_request?(method, params)
    raise
  end
  process_jsonrpc_response(res, method: method)
end

#subscription_failure(error) ⇒ MCPClient::Errors::MCPError

Returns the error a subscription ends with.

Parameters:

  • error (StandardError) —

    a failure

Returns:



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# File 'lib/mcp_client/server_stdio/json_rpc_transport.rb', line 211

def subscription_failure(error)
  error.is_a?(MCPClient::Errors::MCPError) ? error : MCPClient::Errors::TransportError.new(error.message)
end

#take_reconnecting_subscriptions ⇒ Array<MCPClient::Subscription>

Take the subscriptions waiting for a process, leaving the queue empty.

Returns:



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# File 'lib/mcp_client/server_stdio/json_rpc_transport.rb', line 455

def take_reconnecting_subscriptions
  reconnecting_mutex.synchronize do
    pending = (@reconnecting_subscriptions || []).select(&:reconnectable?)
    @reconnecting_subscriptions = []
    pending
  end
end

#transport_retired? ⇒ Boolean

Returns whether the subprocess behind the handshake exited.

Returns:

  • (Boolean) —

    whether the subprocess behind the handshake exited



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# File 'lib/mcp_client/server_stdio/json_rpc_transport.rb', line 343

def transport_retired?
  @transport_retired
end

#wait_response(id, timeout: nil) ⇒ Hash

Wait for a response with the given request ID

Parameters:

  • id (Integer) —

    the request ID

Returns:

  • (Hash) —

    the JSON-RPC response message

Raises:



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# File 'lib/mcp_client/server_stdio/json_rpc_transport.rb', line 920

def wait_response(id, timeout: nil)
  deadline = Time.now + (timeout || @read_timeout)
  @mutex.synchronize do
    until @pending.key?(id)
      # The subprocess exited: no answer is coming, however long the
      # timeout. (An answer that arrived before it exited is above.)
      # A restart another caller completed meanwhile has cleared the
      # retirement again, but it recorded this request as dropped —
      # the durable sign that the transport it went out on is gone.
      break if @transport_retired || dropped_requests.include?(id)

      remaining = deadline - Time.now
      break if remaining <= 0

      @cond.wait(@mutex, remaining)
    end
    # Remove the response and the awaiting marker on both success and
    # timeout so neither @pending nor @awaiting accumulates entries.
    msg = @pending.delete(id)
    transport_gone = @transport_retired || !dropped_requests.delete?(id).nil?
    arrival = (@response_arrivals ||= {}).delete(id)
    @awaiting.delete(id)
    if msg
      # The response's receipt time is the reader's, not this wake-up.
      note_response_received_at(arrival || monotonic_now) if respond_to?(:note_response_received_at, true)
      return msg
    end

    if transport_gone
      raise MCPClient::Errors::TransportError,
            "The MCP server subprocess exited before answering JSONRPC request id=#{id}"
    end

    raise MCPClient::Errors::RequestTimeoutError, "Timeout waiting for JSONRPC response id=#{id}"
  end
end