Building an MCP Client

The MCP::Client class provides an interface for interacting with MCP servers.

This class supports:

  • Lifecycle negotiation and connection via MCP::Client#connect, adopting the modern lifecycle when the server serves it; see Lifecycle
  • Server discovery via the server/discover method (MCP::Client#discover); see Explicit Discovery
  • Liveness check via the ping method (MCP::Client#ping)
  • Tool listing via the tools/list method (MCP::Client#tools)
  • Tool invocation via the tools/call method (MCP::Client#call_tool)
  • Resource listing via the resources/list method (MCP::Client#resources)
  • Resource template listing via the resources/templates/list method (MCP::Client#resource_templates)
  • Resource reading via the resources/read method (MCP::Client#read_resource)
  • Prompt listing via the prompts/list method (MCP::Client#prompts)
  • Prompt retrieval via the prompts/get method (MCP::Client#get_prompt)
  • Completion requests via the completion/complete method (MCP::Client#complete); see Completions
  • Automatic driving of multi round-trip input_required results once on_elicitation, on_sampling, or on_roots handlers are registered; see Multi-Round-Trip Results
  • Cancellation of in-flight requests via the cancellation: keyword; see Cancellation
  • Cursor-based page iteration on the list_* methods and whole-collection fetching with the max_pages guard; see Pagination
  • Automatic JSON-RPC 2.0 message formatting
  • UUID request ID generation

Clients are initialized with a transport layer instance that handles the low-level communication mechanics. Authorization is handled by the transport layer; see Authorization.

Tool Objects

The client provides a wrapper class for tools returned by the server:

  • MCP::Client::Tool - Represents a single tool with its metadata

This class provides easy access to tool properties like name, description, input schema, and output schema.