Improve HttpSseFilterChainFactory: Cannot Configure HTTP Path and Host for MCP Servers

[bettercallsaulj]

Issue Details

Problem Description

The HttpSseFilterChainFactory had hardcoded values for HTTP path and host, making it impossible to:

1. Connect to different endpoints: All clients were forced to use /rpc path, but MCP servers may use /sse, /events, /api/mcp, etc.
2. Specify actual server host: The host was hardcoded to localhost, preventing connections to remote MCP servers
3. Configure per-connection settings: Each factory instance couldn't be customized for different server configurations

Root Cause

The factory constructor only accepted three parameters:

HttpSseFilterChainFactory(event::Dispatcher& dispatcher,
                          McpProtocolCallbacks& message_callbacks,
                          bool is_server = true)

The http_path and http_host values used by the internal filters were either:

• Hardcoded as /rpc and localhost
• Not passed through from the factory to the filter chain

Technical Flow (Before Fix)

// User wants to connect to https://mcp.example.com/api/sse
auto factory = std::make_shared<HttpSseFilterChainFactory>(
    dispatcher, callbacks, false /* client mode */);

// Factory creates filters with hardcoded values:
// - http_path_ = "/rpc"      ❌ Should be "/api/sse"
// - http_host_ = "localhost" ❌ Should be "mcp.example.com"

// Result: Client sends request to wrong endpoint
// GET /rpc HTTP/1.1
// Host: localhost

Technical Flow (After Fix)

// User wants to connect to https://mcp.example.com/api/sse
auto factory = std::make_shared<HttpSseFilterChainFactory>(
    dispatcher, callbacks,
    false,              // client mode
    "/api/sse",         // custom path ✅
    "mcp.example.com"); // custom host ✅

// Factory creates filters with configured values
// Result: Client sends correct request
// GET /api/sse HTTP/1.1
// Host: mcp.example.com

---

How to Reproduce

Prerequisites

• gopher-mcp client code
• An MCP server running on a non-default endpoint

Steps to Reproduce

1. Create factory for remote MCP server:

#include "mcp/filter/http_sse_filter_chain_factory.h"

// Before fix: No way to specify path/host
auto factory = std::make_shared<HttpSseFilterChainFactory>(
    dispatcher, callbacks, false /* client mode */);
// Path is hardcoded to "/rpc"
// Host is hardcoded to "localhost"

2. Attempt to connect to server at different endpoint:

// Server is at: https://mcp.example.com:8080/api/events
// But factory will send requests to: /rpc on localhost

3. Observe the failure:

Expected HTTP Request:
  GET /api/events HTTP/1.1
  Host: mcp.example.com:8080

Actual HTTP Request:
  GET /rpc HTTP/1.1
  Host: localhost

Result: Connection fails or connects to wrong server

Minimal Reproduction Test

TEST(HttpSseFilterChainFactory, CannotConfigureEndpoint) {
  MockMcpCallbacks callbacks;
  auto dispatcher = createDispatcher();

  // Before fix: Constructor doesn't accept path/host
  // This would fail to compile or use wrong defaults
  auto factory = std::make_shared<HttpSseFilterChainFactory>(
      *dispatcher, callbacks, false, "/sse", "server.example.com:8080");

  // Before fix: Compilation error - constructor only takes 3 params
  // After fix: Compiles and uses custom path/host
}

TEST(HttpSseFilterChainFactory, DefaultsToWrongEndpoint) {
  MockMcpCallbacks callbacks;
  auto dispatcher = createDispatcher();

  auto factory = std::make_shared<HttpSseFilterChainFactory>(
      *dispatcher, callbacks, false);

  // Create filter chain and inspect HTTP requests
  // Before fix: Uses /rpc and localhost regardless of server
  // After fix: Uses configurable defaults, can be overridden
}

---

Key Changes in the Fix

Component	Change
Constructor signature	Added http_path and http_host parameters with defaults
Member variables	Added http_path_ and http_host_ storage
Include directive	Added <string> for std::string member types
Default values	http_path="/rpc", http_host="localhost" for backward compatibility

Updated Constructor

// Before
HttpSseFilterChainFactory(event::Dispatcher& dispatcher,
                          McpProtocolCallbacks& message_callbacks,
                          bool is_server = true)

// After
HttpSseFilterChainFactory(event::Dispatcher& dispatcher,
                          McpProtocolCallbacks& message_callbacks,
                          bool is_server = true,
                          const std::string& http_path = "/rpc",
                          const std::string& http_host = "localhost")

Usage Examples

// Server mode with defaults
auto server_factory = std::make_shared<HttpSseFilterChainFactory>(
    dispatcher, callbacks, true);

// Client mode for Claude Desktop MCP
auto claude_factory = std::make_shared<HttpSseFilterChainFactory>(
    dispatcher, callbacks, false, "/sse", "localhost:3000");

// Client mode for remote MCP server
auto remote_factory = std::make_shared<HttpSseFilterChainFactory>(
    dispatcher, callbacks, false, "/api/events", "mcp.example.com:8080");

// Client mode for custom enterprise MCP
auto enterprise_factory = std::make_shared<HttpSseFilterChainFactory>(
    dispatcher, callbacks, false, "/v2/mcp/stream", "internal.corp.com:443");