client: add session initialization

- Send initialize request with protocolVersion and clientInfo
  - Store session_info from server response
  - Send notifications/initialized after successful init
  - Methods now automatically initialize session on first use
- Add session management with Mcp-Session-Id header
  - Track session_id from server responses
  - Automatically include Mcp-Session-Id header in subsequent requests
  - Expose session_id as readable `Client` attribute

Author: topherbullock

lib/mcp/client.rb

@@ -27,6 +27,44 @@ def initialize(transport:, client_info: nil, protocol_version: "2025-06-18")
     # So keeping it public
     attr_reader :transport, :client_info, :protocol_version, :session_info
 
+    # Initializes the MCP session with the server.
+    # This method performs the MCP handshake: sends an initialize request,
+    # receives session information, and sends the initialized notification.
+    #
+    # @return [Hash] The session information including server_info
+    #
+    # @example
+    #   client.init
+    #   # => { server_info: { name: "my-server", version: "1.0.0" }, ... }
+    def init
+      return @session_info if @initialized
+
+      response = transport.send_request(request: {
+        jsonrpc: JsonRpcHandler::Version::V2_0,
+        id: request_id,
+        method: "initialize",
+        params: {
+          protocolVersion: protocol_version,
+          capabilities: {},
+          clientInfo: {
+            name: client_info[:name] || client_info["name"],
+            version: client_info[:version] || client_info["version"],
+          },
+        },
+      })
+
+      @session_info = response.dig("result")
+      @initialized = true
+
+      # Send the initialized notification
+      transport.send_notification(notification: {
+        jsonrpc: JsonRpcHandler::Version::V2_0,
+        method: "notifications/initialized",
+      })
+
+      @session_info
+    end
+
     # Returns the list of tools available from the server.
     # Each call will make a new request – the result is not cached.
     #
@@ -38,6 +76,8 @@ def initialize(transport:, client_info: nil, protocol_version: "2025-06-18")
     #     puts tool.name
     #   end
     def tools
+      init unless @initialized
+
       response = transport.send_request(request: {
         jsonrpc: JsonRpcHandler::Version::V2_0,
         id: request_id,
@@ -58,6 +98,8 @@ def tools
     #
     # @return [Array<Hash>] An array of available resources.
     def resources
+      init unless @initialized
+
       response = transport.send_request(request: {
         jsonrpc: JsonRpcHandler::Version::V2_0,
         id: request_id,
@@ -82,6 +124,8 @@ def resources
     #   The exact requirements for `arguments` are determined by the transport layer in use.
     #   Consult the documentation for your transport (e.g., MCP::Client::HTTP) for details.
     def call_tool(tool:, arguments: nil)
+      init unless @initialized
+
       transport.send_request(request: {
         jsonrpc: JsonRpcHandler::Version::V2_0,
         id: request_id,
@@ -95,6 +139,8 @@ def call_tool(tool:, arguments: nil)
     # @param uri [String] The URI of the resource to read.
     # @return [Array<Hash>] An array of resource contents (text or blob).
     def read_resource(uri:)
+      init unless @initialized
+
       response = transport.send_request(request: {
         jsonrpc: JsonRpcHandler::Version::V2_0,
         id: request_id,

lib/mcp/client/http.rb

@@ -3,18 +3,29 @@
 module MCP
   class Client
     class HTTP
-      attr_reader :url
+      attr_reader :url, :session_id
 
       def initialize(url:, headers: {})
         @url = url
         @headers = headers
+        @session_id = nil
       end
 
       def send_request(request:)
         method = request[:method] || request["method"]
         params = request[:params] || request["params"]
 
-        client.post("", request).body
+        # Update session header if we have one
+        update_session_header!
+        
+        response = client.post("", request)
+        
+        # Store session ID from response headers if present
+        if response.headers["Mcp-Session-Id"]
+          @session_id = response.headers["Mcp-Session-Id"]
+        end
+
+        response.body
       rescue Faraday::BadRequestError => e
         raise RequestHandlerError.new(
           "The #{method} request is invalid",
@@ -76,6 +87,17 @@ def client
         end
       end
 
+      # Updates the session header on the Faraday client
+      def update_session_header!
+        return unless @client
+
+        if @session_id
+          @client.headers["Mcp-Session-Id"] = @session_id
+        else
+          @client.headers.delete("Mcp-Session-Id")
+        end
+      end
+
       def require_faraday!
         require "faraday"
       rescue LoadError