Support SSE protocol with fixes of mcp client#211
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The MCP spec requires sending notifications/initialized before any subsequent requests like tools/list. Without this, some servers reject the tools/list request.
The Connection header is prohibited in HTTP/2 (RFC 9113 Section 8.2.2) and causes Traefik v3 to fail when translating HTTP/1.1 backend responses to HTTP/2 for external clients. HTTP/1.1 defaults to keep-alive without an explicit header.
The raw socket approach for sendHttpPost failed due to SSL handshake issues (EAGAIN dropping Client Hello) and HTTP/1.1 vs HTTP/2 mismatch with servers that prefer h2. Using libcurl handles TLS negotiation and protocol selection correctly. Runs POST in a detached thread to avoid blocking the event loop.
Rename http_sse_path default from /events to /sse for clarity. Add external_url field to McpServerConfig for constructing absolute SSE callback URLs when running behind a reverse proxy.
Pass http_sse_path, http_rpc_path, and external_url from McpServerConfig to the HttpSseFilterChainFactory so the filter chain uses the configured endpoint paths instead of hardcoded defaults. Defer active_connections_ removal to the next event loop iteration via dispatcher post to prevent use-after-free when a connection is destroyed during its own close callback. Add response logging for debugging request/response flow.
Extend HttpSseFilterChainFactory constructor to accept configurable sse_path, rpc_path, and external_url parameters. These are passed through to the protocol filter for SSE server transport support.
Add full MCP SSE server transport support:
- SseSessionRegistry tracks active SSE connections by session ID
- GET /sse opens SSE stream, registers session, sends endpoint event
with absolute callback URL (supports reverse proxy deployments)
- POST /callback/{session_id} sends 202 Accepted and routes the
JSON-RPC response through the corresponding SSE stream
- POST /mcp continues to work as Streamable HTTP (direct response)
- Configurable SSE/RPC paths passed from server config
- SSE session cleanup on connection close via filter destructor
- Updated /info endpoint to reflect configured paths
- CORS registration for configurable endpoint paths
❌ Code Formatting Check FailedSome files in this PR are not properly formatted according to the project's clang-format rules. To fix this issue: make formatThen commit and push the changes. |
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Add SSE server transport and fix critical thread-safety crashes
Summary
Adds SSE server transport, fixes critical thread-safety crashes in McpClient/McpServer, and replaces fragile raw socket HTTP POST with libcurl.
Changes
New Features
GET /sseto open a long-lived stream andPOST /callback/{session_id}to receive JSON-RPC requests, with responses routed back through the SSE stream via a session registry. Supportsexternal_urlconfig for absolute callback URLs behind reverse proxies (Traefik, nginx).http_sse_pathandhttp_rpc_pathare now wired throughMcpServerConfigto the filter chain factory.McpConnectionManager::sendHttpPost(). The raw socket approach was failing due to EAGAIN on SSL handshake and HTTP/1.1 vs HTTP/2 mismatches.Bug Fixes
Crashes / use-after-free
src/client/mcp_client.cc) —reconnectInternal()was destroying the oldMcpConnectionManagerimmediately while libevent callbacks were still queued in the current event-loop iteration. Now moves it to adead_connection_managers_list cleared on the next iteration.src/mcp_connection_manager.cc) — Replaces deferred-via-dispatcher destruction with aclosed_connections_vector. CallsreadDisable(true)first to unregister libevent fd events before retiring the connection, preventing stale callbacks from firing.src/server/mcp_server.cc) — Fixed map erase race inonConnectionEvent()by removing redundantconnection_sessions_erase that duplicatedSessionManager::removeSessionByConnection().Timer lifetime / resource leaks
src/server/mcp_server.cc) —startBackgroundTasks()was assigning timers to localunique_ptrvariables that went out of scope immediately, so session cleanup never ran. Timers are now stored insession_cleanup_timer_andresource_update_timer_member variables.stopBackgroundTasks()explicitly disables and resets them.src/filter/http_codec_filter.cc) — The 60s body timeout was firing on client SSE connections (where the body is an infinite stream), then crashing inHttpCodecStateMachine::executeTransition(). Body timeout is now0(disabled) for client mode.Routing / proxy compatibility
src/filter/http_sse_filter_chain_factory.cc) — Old code requiredpath.find("/callback/") == 0, which failed when behind a reverse proxy that forwarded a prefixed path like/v1/mcp/gateways/{id}/callback/client_1. Now usespath.rfind("/callback/")to match anywhere in the path.Connectionhop-by-hop header — Should not be set by the application layer.notifications/initializedafter protocol init as required by MCP spec.Files Changed
include/mcp/client/mcp_client.hdead_connection_managers_membersrc/client/mcp_client.ccnotifications/initializedinclude/mcp/mcp_connection_manager.hclosed_connections_membersrc/mcp_connection_manager.ccinclude/mcp/server/mcp_server.hsrc/server/mcp_server.ccinclude/mcp/filter/http_sse_filter_chain_factory.hsrc/filter/http_sse_filter_chain_factory.ccsrc/filter/http_codec_filter.ccTesting
Verified locally with Docker against production MCP backends:
POST /mcp(Streamable HTTP) andGET /sse+POST /callback/{id}transports workexternal_urlconfig