Introduce stateless mode to allow for HA

- Create failing tests for `stateless` mode
- Implement `stateless` mode by turning of SSE in Streamable HTTP,
  making the interaction a standard HTTP Req/Resp

Implement `stateless` mode

Author: bigH

lib/mcp/server/transports/streamable_http_transport.rb

@@ -8,11 +8,13 @@ module MCP
   class Server
     module Transports
       class StreamableHTTPTransport < Transport
-        def initialize(server)
-          super
+        def initialize(server, stateless: false)
+          super(server)
           # { session_id => { stream: stream_object }
           @sessions = {}
           @mutex = Mutex.new
+
+          @stateless = stateless
         end
 
         def handle_request(request)
@@ -24,7 +26,7 @@ def handle_request(request)
           when "DELETE"
             handle_delete(request)
           else
-            [405, { "Content-Type" => "application/json" }, [{ error: "Method not allowed" }.to_json]]
+            method_not_allowed_response
           end
         end
 
@@ -35,6 +37,9 @@ def close
         end
 
         def send_notification(method, params = nil, session_id: nil)
+          # Stateless mode doesn't support notifications
+          raise "Stateless mode does not support notifications" if @stateless
+
           notification = {
             jsonrpc: "2.0",
             method:,
@@ -117,6 +122,10 @@ def handle_post(request)
         end
 
         def handle_get(request)
+          if @stateless
+            return method_not_allowed_response
+          end
+
           session_id = extract_session_id(request)
 
           return missing_session_id_response unless session_id
@@ -126,6 +135,13 @@ def handle_get(request)
         end
 
         def handle_delete(request)
+          success_response = [200, { "Content-Type" => "application/json" }, [{ success: true }.to_json]]
+
+          if @stateless
+            # Stateless mode doesn't support sessions, so we can just return a success response
+            return success_response
+          end
+
           session_id = request.env["HTTP_MCP_SESSION_ID"]
 
           return [
@@ -135,7 +151,7 @@ def handle_delete(request)
           ] unless session_id
 
           cleanup_session(session_id)
-          [200, { "Content-Type" => "application/json" }, [{ success: true }.to_json]]
+          success_response
         end
 
         def cleanup_session(session_id)
@@ -167,31 +183,40 @@ def parse_request_body(body_string)
         end
 
         def handle_initialization(body_string, body)
-          session_id = SecureRandom.uuid
+          session_id = nil
 
-          @mutex.synchronize do
-            @sessions[session_id] = {
-              stream: nil,
-            }
+          unless @stateless
+            session_id = SecureRandom.uuid
+
+            @mutex.synchronize do
+              @sessions[session_id] = {
+                stream: nil,
+              }
+            end
           end
 
           response = @server.handle_json(body_string)
 
           headers = {
             "Content-Type" => "application/json",
-            "Mcp-Session-Id" => session_id,
           }
 
+          headers["Mcp-Session-Id"] = session_id if session_id
+
           [200, headers, [response]]
         end
 
         def handle_regular_request(body_string, session_id)
-          # If session ID is provided, but not in the sessions hash, return an error
-          if session_id && !@sessions.key?(session_id)
-            return [400, { "Content-Type" => "application/json" }, [{ error: "Invalid session ID" }.to_json]]
+          unless @stateless
+            # If session ID is provided, but not in the sessions hash, return an error
+            if session_id && !@sessions.key?(session_id)
+              return [400, { "Content-Type" => "application/json" }, [{ error: "Invalid session ID" }.to_json]]
+            end
           end
 
           response = @server.handle_json(body_string)
+
+          # Stream can be nil since stateless mode doesn't retain streams
           stream = get_session_stream(session_id) if session_id
 
           if stream
@@ -222,6 +247,10 @@ def session_exists?(session_id)
           @mutex.synchronize { @sessions.key?(session_id) }
         end
 
+        def method_not_allowed_response
+          [405, { "Content-Type" => "application/json" }, [{ error: "Method not allowed" }.to_json]]
+        end
+
         def missing_session_id_response
           [400, { "Content-Type" => "application/json" }, [{ error: "Missing session ID" }.to_json]]
         end

test/mcp/server/transports/streamable_http_transport_test.rb

@@ -575,6 +575,108 @@ class StreamableHTTPTransportTest < ActiveSupport::TestCase
           assert_equal "Method not allowed", body["error"]
         end
 
+        test "stateless mode allows requests without session IDs, responding with no session ID" do
+          stateless_transport = StreamableHTTPTransport.new(@server, stateless: true)
+
+          init_request = create_rack_request(
+            "POST",
+            "/",
+            { "CONTENT_TYPE" => "application/json" },
+            { jsonrpc: "2.0", method: "initialize", id: "init" }.to_json,
+          )
+          init_response = stateless_transport.handle_request(init_request)
+          assert_nil init_response[1]["Mcp-Session-Id"]
+        end
+
+        test "stateless mode responds without any session ID when session ID is present" do
+          stateless_transport = StreamableHTTPTransport.new(@server, stateless: true)
+
+          request = create_rack_request(
+            "POST",
+            "/",
+            {
+              "CONTENT_TYPE" => "application/json",
+              "HTTP_MCP_SESSION_ID" => "unseen_session_id",
+            },
+            { jsonrpc: "2.0", method: "ping", id: "123" }.to_json,
+          )
+
+          response = stateless_transport.handle_request(request)
+          assert_equal 200, response[0]
+          assert_equal(
+            {
+              "Content-Type" => "application/json",
+            },
+            response[1],
+          )
+
+          body = JSON.parse(response[2][0])
+          assert_equal "2.0", body["jsonrpc"]
+          assert_equal "123", body["id"]
+        end
+
+        test "stateless mode responds with 405 when SSE is requested" do
+          stateless_transport = StreamableHTTPTransport.new(@server, stateless: true)
+
+          get_request = create_rack_request(
+            "GET",
+            "/",
+            {
+              "CONTENT_TYPE" => "application/json,text/event-stream",
+            },
+          )
+          response = stateless_transport.handle_request(get_request)
+          assert_equal 405, response[0]
+          assert_equal({ "Content-Type" => "application/json" }, response[1])
+
+          body = JSON.parse(response[2][0])
+          assert_equal "Method not allowed", body["error"]
+        end
+
+        test "stateless mode silently responds with success to session DELETE when session ID is not present" do
+          stateless_transport = StreamableHTTPTransport.new(@server, stateless: true)
+
+          delete_request = create_rack_request(
+            "DELETE",
+            "/",
+            {},
+          )
+          response = stateless_transport.handle_request(delete_request)
+          assert_equal 200, response[0]
+          assert_equal({ "Content-Type" => "application/json" }, response[1])
+
+          body = JSON.parse(response[2][0])
+          assert body["success"]
+        end
+
+        test "stateless mode silently responds with success to session DELETE when session ID is provided" do
+          stateless_transport = StreamableHTTPTransport.new(@server, stateless: true)
+
+          delete_request = create_rack_request(
+            "DELETE",
+            "/",
+            { "HTTP_MCP_SESSION_ID" => "session_id" },
+          )
+          response = stateless_transport.handle_request(delete_request)
+          assert_equal 200, response[0]
+          assert_equal({ "Content-Type" => "application/json" }, response[1])
+
+          body = JSON.parse(response[2][0])
+          assert body["success"]
+        end
+
+        test "stateless mode does not support notifications" do
+          stateless_transport = StreamableHTTPTransport.new(@server, stateless: true)
+
+          assert_raises(RuntimeError, "Stateless mode does not support notifications") do
+            stateless_transport.send_notification(
+              "test_notification",
+              { message: "Hello" },
+              session_id: "some_session_id",
+            )
+          end
+        end
+
         test "handle post request with a standard error" do
           request = create_rack_request(
             "POST",