Completions

MCP spec includes Completions, which enable servers to provide autocompletion suggestions for prompt arguments and resource URIs.

To enable completions, declare the completions capability and register a handler:

server = MCP::Server.new(
  name: "my_server",
  prompts: [CodeReviewPrompt],
  resource_templates: [FileTemplate],
  capabilities: { completions: {} },
)

server.completion_handler do |params|
  ref = params[:ref]
  argument = params[:argument]
  value = argument[:value]

  case ref[:type]
  when "ref/prompt"
    values = case argument[:name]
    when "language"
      ["python", "pytorch", "pyside"].select { |v| v.start_with?(value) }
    else
      []
    end
    { completion: { values: values, hasMore: false } }
  when "ref/resource"
    { completion: { values: [], hasMore: false } }
  end
end

The handler receives a params hash with:

• ref - The reference ({ type: "ref/prompt", name: "..." } or { type: "ref/resource", uri: "..." })
• argument - The argument being completed ({ name: "...", value: "..." })
• context (optional) - Previously resolved arguments ({ arguments: { ... } })

The handler must return a hash with a completion key containing values (array of strings), and optionally total and hasMore. The SDK automatically enforces the 100-item limit per the MCP specification.

The server validates that the referenced prompt, resource, or resource template is registered before calling the handler. Requests for unknown references return an error.