Add basic http client support

.gitignore

@@ -8,3 +8,6 @@
 /spec/reports/
 /tmp/
 Gemfile.lock
+
+# Mac stuff
+.DS_Store

Gemfile

@@ -22,3 +22,8 @@ gem "activesupport"
 gem "debug"
 gem "rake", "~> 13.0"
 gem "sorbet-static-and-runtime"
+
+group :test do
+  gem "webmock"
+  gem "faraday", ">= 2.0"
+end

README.md

@@ -10,6 +10,21 @@ Add this line to your application's Gemfile:
 gem 'mcp'
 ```
 
+### Optional Dependencies
+
+The MCP gem has different dependency requirements depending on your use case:
+
+**For Server-only usage:**
+```ruby
+gem 'mcp'
+```
+
+**For client HTTP transport usage:**
+```ruby
+gem 'mcp'
+gem 'faraday', '>= 2.0'
+```
+
 And then execute:
 
 ```console
@@ -216,6 +231,67 @@ $ ruby examples/stdio_server.rb
 {"jsonrpc":"2.0","id":"2","method":"tools/list"}
 ```
 
+## MCP Client
+
+The `MCP::Client` module provides client implementations for interacting with MCP servers. Currently, it supports HTTP transport for making JSON-RPC requests to MCP servers.
+
+**Note:** The client HTTP transport requires the `faraday` gem. Add `gem 'faraday', '>= 2.0'` to your Gemfile if you plan to use the client HTTP transport functionality.
+
+### HTTP Client
+
+The `MCP::Client::Http` class provides a simple HTTP client for interacting with MCP servers:
+
+```ruby
+client = MCP::Client::Http.new(url: "https://api.example.com/mcp")
+
+# List available tools
+tools = client.tools
+tools.each do |tool|
+  puts "Tool: #{tool.name}"
+  puts "Description: #{tool.description}"
+  puts "Input Schema: #{tool.input_schema}"
+end
+
+# Call a specific tool
+response = client.call_tool(
+  tool: tools.first,
+  input: { message: "Hello, world!" }
+)
+```
+
+The HTTP client supports:
+- Tool listing via the `tools/list` method
+- Tool invocation via the `tools/call` method
+- Automatic JSON-RPC 2.0 message formatting
+- UUID v7 request ID generation
+- Setting headers for things like authorization
+
+### HTTP Authorization
+
+By default, the HTTP client has no authentication, but it supports custom headers for authentication. For example, to use Bearer token authentication:
+
+```ruby
+client = MCP::Client::Http.new(
+  url: "https://api.example.com/mcp",
+  headers: {
+    "Authorization" => "Bearer my_token"
+  }
+)
+
+client.tools # will make the call using Bearer auth
+```
+
+You can add any custom headers needed for your authentication scheme. The client will include these headers in all requests.
+
+### Tool Objects
+
+The client provides wrapper objects for tools returned by the server:
+
+- `MCP::Client::Tool` - Represents a single tool with its metadata
+- `MCP::Client::Tools` - Collection of tools with enumerable functionality
+
+These objects provide easy access to tool properties like name, description, and input schema.
+
 ## Configuration
 
 The gem can be configured using the `MCP.configure` block:

lib/mcp/client.rb

@@ -0,0 +1,22 @@
+# frozen_string_literal: true
+
+# require "json_rpc_handler"
+# require_relative "shared/instrumentation"
+# require_relative "shared/methods"
+
+module MCP
+  module Client
+    # Can be made an abstract class if we need shared behavior
+
+    class RequestHandlerError < StandardError
+      attr_reader :error_type, :original_error, :request
+
+      def initialize(message, request, error_type: :internal_error, original_error: nil)
+        super(message)
+        @request = request
+        @error_type = error_type
+        @original_error = original_error
+      end
+    end
+  end
+end

lib/mcp/client/http.rb

@@ -0,0 +1,115 @@
+# frozen_string_literal: true
+
+module MCP
+  module Client
+    class Http
+      DEFAULT_VERSION = "0.1.0"
+
+      attr_reader :url, :version
+
+      def initialize(url:, version: DEFAULT_VERSION, headers: {})
+        @url = url
+        @version = version
+        @headers = headers
+      end
+
+      def tools
+        response = make_request(method: "tools/list").body
+
+        ::MCP::Client::Tools.new(response)
+      end
+
+      def call_tool(tool:, input:)
+        response = make_request(
+          method: "tools/call",
+          params: { name: tool.name, arguments: input },
+        ).body
+
+        response.dig("result", "content", 0, "text")
+      end
+
+      private
+
+      attr_reader :headers
+
+      def client
+        require_faraday!
+        @client ||= Faraday.new(url) do |faraday|
+          faraday.request(:json)
+          faraday.response(:json)
+          faraday.response(:raise_error)
+
+          headers.each do |key, value|
+            faraday.headers[key] = value
+          end
+        end
+      end
+
+      def require_faraday!
+        require "faraday"
+      rescue LoadError
+        raise LoadError, "The 'faraday' gem is required to use the MCP client HTTP transport. " \
+          "Add it to your Gemfile: gem 'faraday', '>= 2.0'"
+      end
+
+      def make_request(method:, params: nil)
+        client.post(
+          "",
+          {
+            jsonrpc: "2.0",
+            id: request_id,
+            method:,
+            params:,
+            mcp: { jsonrpc: "2.0", id: request_id, method:, params: }.compact,
+          }.compact,
+        )
+      rescue Faraday::BadRequestError => e
+        raise RequestHandlerError.new(
+          "The #{method} request is invalid",
+          { method:, params: },
+          error_type: :bad_request,
+          original_error: e,
+        )
+      rescue Faraday::UnauthorizedError => e
+        raise RequestHandlerError.new(
+          "You are unauthorized to make #{method} requests",
+          { method:, params: },
+          error_type: :unauthorized,
+          original_error: e,
+        )
+      rescue Faraday::ForbiddenError => e
+        raise RequestHandlerError.new(
+          "You are forbidden to make #{method} requests",
+          { method:, params: },
+          error_type: :forbidden,
+          original_error: e,
+        )
+      rescue Faraday::ResourceNotFound => e
+        raise RequestHandlerError.new(
+          "The #{method} request is not found",
+          { method:, params: },
+          error_type: :not_found,
+          original_error: e,
+        )
+      rescue Faraday::UnprocessableEntityError => e
+        raise RequestHandlerError.new(
+          "The #{method} request is unprocessable",
+          { method:, params: },
+          error_type: :unprocessable_entity,
+          original_error: e,
+        )
+      rescue Faraday::Error => e # Catch-all
+        raise RequestHandlerError.new(
+          "Internal error handling #{method} request",
+          { method:, params: },
+          error_type: :internal_error,
+          original_error: e,
+        )
+      end
+
+      def request_id
+        SecureRandom.uuid_v7
+      end
+    end
+  end
+end

lib/mcp/client/tool.rb

@@ -0,0 +1,26 @@
+# typed: false
+# frozen_string_literal: true
+
+module MCP
+  module Client
+    class Tool
+      attr_reader :payload
+
+      def initialize(payload)
+        @payload = payload
+      end
+
+      def name
+        payload["name"]
+      end
+
+      def description
+        payload["description"]
+      end
+
+      def input_schema
+        payload["inputSchema"]
+      end
+    end
+  end
+end

lib/mcp/client/tools.rb

@@ -0,0 +1,30 @@
+# typed: false
+# frozen_string_literal: true
+
+module MCP
+  module Client
+    class Tools
+      include Enumerable
+
+      attr_reader :response
+
+      def initialize(response)
+        @response = response
+      end
+
+      def each(&block)
+        tools.each(&block)
+      end
+
+      def all
+        tools
+      end
+
+      private
+
+      def tools
+        @tools ||= @response.dig("result", "tools")&.map { |tool| Tool.new(tool) } || []
+      end
+    end
+  end
+end

mcp.gemspec

@@ -29,4 +29,6 @@ Gem::Specification.new do |spec|
 
   spec.add_dependency("json_rpc_handler", "~> 0.1")
   spec.add_dependency("json-schema", ">= 4.1")
+
+  # Faraday is required for the client HTTP transport layer
 end