MCP Protocol 2026-07-28 Stateless Architecture Upgrade Deep Dive: Anthropic Model Context Protocol's Biggest Architectural Revision

// MCP 2026-07-28 Stateless Protocol Implementation
package main

import (
	"bytes"
	"context"
	"encoding/json"
	"fmt"
	"io"
	"log"
	"net/http"
	"sync"
	"time"
)

// MCPRequest Stateless MCP request
type MCPRequest struct {
	Method string                 `json:"method"`
	Params map[string]interface{} `json:"params"`
	Meta   map[string]interface{} `json:"_meta,omitempty"`
	ID     string                 `json:"id"`
}

// MCPResponse MCP response
type MCPResponse struct {
	ID     string                 `json:"id"`
	Result map[string]interface{} `json:"result,omitempty"`
	Error  *MCPError              `json:"error,omitempty"`
}

// MCPError MCP error
type MCPError struct {
	Code    int    `json:"code"`
	Message string `json:"message"`
}

// MCPTool MCP tool definition
type MCPTool struct {
	Name        string                 `json:"name"`
	Description string                 `json:"description"`
	InputSchema map[string]interface{} `json:"inputSchema"`
}

// MCPResource MCP resource definition
type MCPResource struct {
	URI         string `json:"uri"`
	Name        string `json:"name"`
	Description string `json:"description"`
	MimeType    string `json:"mimeType"`
}

// StatelessMCPServer Stateless MCP server
type StatelessMCPServer struct {
	mu        sync.RWMutex
	tools     map[string]MCPTool
	resources map[string]MCPResource
}

func NewStatelessMCPServer() *StatelessMCPServer {
	return &StatelessMCPServer{
		tools:     make(map[string]MCPTool),
		resources: make(map[string]MCPResource),
	}
}

func (s *StatelessMCPServer) RegisterTool(tool MCPTool) {
	s.mu.Lock()
	defer s.mu.Unlock()
	s.tools[tool.Name] = tool
}

func (s *StatelessMCPServer) RegisterResource(resource MCPResource) {
	s.mu.Lock()
	defer s.mu.Unlock()
	s.resources[resource.URI] = resource
}

func (s *StatelessMCPServer) HandleRequest(ctx context.Context, req MCPRequest) MCPResponse {
	metaCtx := extractContext(req.Meta)

	switch req.Method {
	case "mcp.list_tools":
		return s.handleListTools(req, metaCtx)
	case "mcp.call_tool":
		return s.handleCallTool(req, metaCtx)
	case "mcp.list_resources":
		return s.handleListResources(req, metaCtx)
	case "mcp.read_resource":
		return s.handleReadResource(req, metaCtx)
	default:
		return MCPResponse{
			ID: req.ID,
			Error: &MCPError{
				Code:    -32601,
				Message: fmt.Sprintf("Method not found: %s", req.Method),
			},
		}
	}
}

func extractContext(meta map[string]interface{}) map[string]interface{} {
	if meta == nil {
		return make(map[string]interface{})
	}
	return meta
}

func (s *StatelessMCPServer) handleListTools(req MCPRequest, ctx map[string]interface{}) MCPResponse {
	s.mu.RLock()
	defer s.mu.RUnlock()
	tools := make([]MCPTool, 0, len(s.tools))
	for _, tool := range s.tools {
		tools = append(tools, tool)
	}
	return MCPResponse{
		ID: req.ID,
		Result: map[string]interface{}{"tools": tools},
	}
}

func (s *StatelessMCPServer) handleCallTool(req MCPRequest, ctx map[string]interface{}) MCPResponse {
	name, _ := req.Params["name"].(string)
	arguments, _ := req.Params["arguments"].(map[string]interface{})
	s.mu.RLock()
	tool, exists := s.tools[name]
	s.mu.RUnlock()
	if !exists {
		return MCPResponse{
			ID: req.ID,
			Error: &MCPError{
				Code:    -32602,
				Message: fmt.Sprintf("Tool not found: %s", name),
			},
		}
	}
	result := executeTool(tool, arguments, ctx)
	return MCPResponse{
		ID: req.ID,
		Result: map[string]interface{}{"content": result},
	}
}

func (s *StatelessMCPServer) handleListResources(req MCPRequest, ctx map[string]interface{}) MCPResponse {
	s.mu.RLock()
	defer s.mu.RUnlock()
	resources := make([]MCPResource, 0, len(s.resources))
	for _, resource := range s.resources {
		resources = append(resources, resource)
	}
	return MCPResponse{
		ID: req.ID,
		Result: map[string]interface{}{"resources": resources},
	}
}

func (s *StatelessMCPServer) handleReadResource(req MCPRequest, ctx map[string]interface{}) MCPResponse {
	uri, _ := req.Params["uri"].(string)
	s.mu.RLock()
	resource, exists := s.resources[uri]
	s.mu.RUnlock()
	if !exists {
		return MCPResponse{
			ID: req.ID,
			Error: &MCPError{
				Code:    -32602,
				Message: fmt.Sprintf("Resource not found: %s", uri),
			},
		}
	}
	_ = resource
	content := readResourceContent(uri, ctx)
	return MCPResponse{
		ID: req.ID,
		Result: map[string]interface{}{
			"contents": []map[string]interface{}{
				{"uri": uri, "text": content},
			},
		},
	}
}

func executeTool(tool MCPTool, args map[string]interface{}, ctx map[string]interface{}) []map[string]interface{} {
	return []map[string]interface{}{
		{"type": "text", "text": fmt.Sprintf("Executed %s with args: %v", tool.Name, args)},
	}
}

func readResourceContent(uri string, ctx map[string]interface{}) string {
	return fmt.Sprintf("Content for %s", uri)
}

// HTTPServer wrapper
type HTTPServer struct {
	server *StatelessMCPServer
}

func (h *HTTPServer) ServeHTTP(w http.ResponseWriter, r *http.Request) {
	mcpMethod := r.Header.Get("Mcp-Method")
	mcpName := r.Header.Get("Mcp-Name")
	body, _ := io.ReadAll(r.Body)
	
	var req MCPRequest
	json.Unmarshal(body, &req)
	
	if req.Meta == nil {
		req.Meta = make(map[string]interface{})
	}
	if mcpMethod != "" {
		req.Meta["_transport_method"] = mcpMethod
	}
	if mcpName != "" {
		req.Meta["_transport_name"] = mcpName
	}
	
	resp := h.server.HandleRequest(context.Background(), req)
	w.Header().Set("Content-Type", "application/json")
	json.NewEncoder(w).Encode(resp)
}

func main() {
	server := NewStatelessMCPServer()
	server.RegisterTool(MCPTool{
		Name: "search_web",
		Description: "Search the web for information",
		InputSchema: map[string]interface{}{
			"type": "object",
			"properties": map[string]interface{}{
				"query": map[string]interface{}{
					"type": "string", "description": "Search query",
				},
			},
			"required": []string{"query"},
		},
	})
	httpServer := &HTTPServer{server: server}
	log.Fatal(http.ListenAndServe(":8080", httpServer))
}

"""
MCP Stateless Gateway Implementation

Supports zero-body-inspection routing based on Mcp-Method and Mcp-Name headers
"""
import asyncio
import json
import logging
import time
from typing import Dict, List, Optional, Tuple
from dataclasses import dataclass, field
from enum import Enum


class MCPMethod(Enum):
    LIST_TOOLS = "mcp.list_tools"
    CALL_TOOL = "mcp.call_tool"
    LIST_RESOURCES = "mcp.list_resources"
    READ_RESOURCE = "mcp.read_resource"
    PING = "mcp.ping"


@dataclass
class MCPGatewayConfig:
    backend_urls: List[str]
    health_check_interval: int = 10
    circuit_breaker_threshold: int = 5
    circuit_breaker_timeout: int = 30
    rate_limit_per_second: int = 1000


@dataclass
class BackendInstance:
    url: str
    healthy: bool = True
    active_requests: int = 0
    circuit_breaker_count: int = 0
    circuit_breaker_open_until: float = 0
    total_requests: int = 0
    total_latency: float = 0.0


class MCPStatelessGateway:
    """
    Stateless MCP Gateway
    
    Key features:
    - Header-based routing without body parsing
    - Round-robin load balancing (no sticky sessions needed)
    - Circuit breaker protection
    - Rate limiting
    - Health checking
    """
    
    def __init__(self, config: MCPGatewayConfig):
        self.config = config
        self.backends = [BackendInstance(url=url) for url in config.backend_urls]
        self._next_backend = 0
        self._lock = asyncio.Lock()
    
    async def route_request(self, method: str, name: str,
                           body: bytes, headers: Dict[str, str]) -> Dict:
        """Route request to backend using header-based routing"""
        backend = await self._select_backend()
        if backend is None:
            return {"error": {"code": -32001, "message": "No healthy backends"}}
        
        start = time.time()
        try:
            response = await self._forward(backend, method, name, body, headers)
            async with self._lock:
                backend.total_requests += 1
                backend.total_latency += time.time() - start
                backend.circuit_breaker_count = 0
            return response
        except Exception as e:
            async with self._lock:
                backend.circuit_breaker_count += 1
                if backend.circuit_breaker_count >= self.config.circuit_breaker_threshold:
                    backend.circuit_breaker_open_until = time.time() + \
                        self.config.circuit_breaker_timeout
                    backend.healthy = False
            return {"error": {"code": -32002, "message": str(e)}}
    
    async def _select_backend(self) -> Optional[BackendInstance]:
        """Round-robin load balancing"""
        async with self._lock:
            healthy = [b for b in self.backends if b.healthy]
            if not healthy:
                return None
            idx = self._next_backend % len(healthy)
            self._next_backend += 1
            return healthy[idx]
    
    async def _forward(self, backend: BackendInstance,
                       method: str, name: str,
                       body: bytes, headers: Dict[str, str]) -> Dict:
        """Forward request to backend"""
        forward_headers = {
            "Content-Type": "application/json",
            "Mcp-Method": method,
            "Mcp-Name": name,
        }
        await asyncio.sleep(0.01)
        request_data = json.loads(body)
        return {
            "jsonrpc": "2.0",
            "id": request_data.get("id"),
            "result": {"status": "ok", "backend": backend.url}
        }


async def main():
    config = MCPGatewayConfig(
        backend_urls=[
            "http://mcp-server-1:8080",
            "http://mcp-server-2:8080",
            "http://mcp-server-3:8080",
        ],
    )
    gateway = MCPStatelessGateway(config)
    
    print("=" * 60)
    print("MCP Stateless Gateway Performance Test")
    print("=" * 60)
    
    # Test header-based routing
    print("\nTest: Header-based routing (no body parsing)")
    for method in ["mcp.list_tools", "mcp.call_tool", "mcp.list_resources"]:
        body = json.dumps({"jsonrpc": "2.0", "id": "1", "method": method})
        response = await gateway.route_request(
            method=method, name="test-server",
            body=body.encode(), headers={"X-Request-Id": "test-001"}
        )
        print(f"  {method} -> {response['result']['backend']}")
    
    # Test load balancing distribution
    print("\nTest: 100 request round-robin distribution")
    for i in range(100):
        body = json.dumps({"jsonrpc": "2.0", "id": str(i), "method": "mcp.ping"})
        await gateway.route_request(
            method="mcp.ping", name="test-server",
            body=body.encode(), headers={"X-Request-Id": f"test-{i:03d}"}
        )
    
    total = sum(b.total_requests for b in gateway.backends)
    print(f"  Total requests: {total}")
    for b in gateway.backends:
        print(f"  {b.url}: {b.total_requests} requests")


if __name__ == "__main__":
    asyncio.run(main())