Add SSE server transport for HTTP+SSE filter chain

include/mcp/filter/http_sse_filter_chain_factory.h

@@ -1,3 +1,3 @@
 #pragma once
 
 #include <functional>
@@ -18,6 +18,7 @@
 namespace filter {
 class HttpRoutingFilter;
 class MetricsFilter;
+class SseSessionRegistry;
 }  // namespace filter
 }  // namespace mcp
 
@@ -66,19 +67,28 @@
    * @param http_host HTTP Host header value for client mode
    * @param use_sse True for SSE mode (GET /sse first), false for Streamable
    * HTTP (direct POST)
+   * @param sse_path Server-side SSE endpoint path (e.g., "/sse"). Only
+   *                 meaningful when is_server=true.
+   * @param rpc_path Server-side JSON-RPC endpoint path (e.g., "/mcp"). Only
+   *                 meaningful when is_server=true.
+   * @param external_url Absolute URL the server is reachable at from the
+   *                 client's perspective. Used to build the endpoint-event
+   *                 callback URL advertised on GET /sse. Leave empty to
+   *                 derive the URL from the incoming Host header.
    */
   HttpSseFilterChainFactory(event::Dispatcher& dispatcher,
                             McpProtocolCallbacks& message_callbacks,
                             bool is_server = true,
                             const std::string& http_path = "/rpc",
                             const std::string& http_host = "localhost",
-                            bool use_sse = true)
-      : dispatcher_(dispatcher),
-        message_callbacks_(message_callbacks),
-        is_server_(is_server),
-        http_path_(http_path),
-        http_host_(http_host),
-        use_sse_(use_sse) {}
+                            bool use_sse = true,
+                            const std::string& sse_path = "/sse",
+                            const std::string& rpc_path = "/mcp",
+                            const std::string& external_url = "");
+
+  // Destructor defined out-of-line so the unique_ptr<SseSessionRegistry>
+  // member can use the incomplete forward-declared type in this header.
+  ~HttpSseFilterChainFactory();
 
   /**
    * Create filter chain for the connection
@@ -167,11 +177,24 @@
   event::Dispatcher& dispatcher_;
   McpProtocolCallbacks& message_callbacks_;
   bool is_server_;
-  std::string http_path_;  // HTTP request path for client mode
-  std::string http_host_;  // HTTP Host header for client mode
-  bool use_sse_;           // True for SSE mode, false for Streamable HTTP
+  std::string http_path_;     // HTTP request path for client mode
+  std::string http_host_;     // HTTP Host header for client mode
+  bool use_sse_;              // True for SSE mode, false for Streamable HTTP
+  std::string sse_path_;      // Server-side SSE endpoint path (e.g., "/sse")
+  std::string rpc_path_;      // Server-side JSON-RPC endpoint path (e.g., "/mcp")
+  std::string external_url_;  // External URL for absolute SSE callback URLs
   mutable bool enable_metrics_ = true;  // Enable metrics by default
 
+  // SSE session registry — maps session IDs to the connections that are
+  // streaming SSE to each client, so POST /callback/{id} handlers can
+  // route a JSON-RPC response back through the correct stream. Lazily
+  // constructed on the first server-side filter chain creation; stays
+  // null for client-mode factories. Owned here (not a global singleton)
+  // so each McpServer instance has an isolated registry and lifetime is
+  // bounded by the factory, not process lifetime. Mutable because
+  // createFilterChain is const on the base class.
+  mutable std::unique_ptr<SseSessionRegistry> sse_registry_;
+
   // Store filters for lifetime management
   mutable std::vector<network::FilterSharedPtr> filters_;
 

include/mcp/server/mcp_server.h

@@ -88,8 +88,15 @@ struct McpServerConfig : public application::ApplicationBase::Config {
 
   // HTTP/SSE specific configuration
   std::string http_rpc_path = "/rpc";        // Path for JSON-RPC over HTTP
-  std::string http_sse_path = "/events";     // Path for SSE event stream
+  std::string http_sse_path = "/sse";        // Path for SSE event stream
   std::string http_health_path = "/health";  // Path for health check endpoint
+  // Absolute URL the server is reachable at from the client's perspective
+  // (scheme + host + port + optional path prefix). Used to build the
+  // endpoint-event callback URL advertised on GET /sse. Leave empty to
+  // have the server derive a URL from the incoming Host header; set
+  // explicitly when the server sits behind a reverse proxy that rewrites
+  // scheme or path so clients don't try to POST back to an internal URL.
+  std::string external_url;
 
   // Session management
   size_t max_sessions = 100;