Transports

Transport	Client	Server	Description
Stdio	Yes	Yes	stdin/stdout pipes, subprocess communication
Streamable HTTP	Yes	Yes	HTTP POST with streaming server-sent events
SSE	Yes	Yes¹	Server-Sent Events for push notifications
WebSocket	Yes	No	TCP-based bidirectional (libhv WebSocketClient)
InMemory	Yes	Yes	In-process for testing

Streamable HTTP

The Streamable HTTP transport implements the MCP Streamable HTTP specification. Each session uses an internal StreamableHttpSessionTransport (wrapping ITransport) with its own message channel and background thread.

Request flow: Client sends a JSON-RPC message via HTTP POST. The server validates Mcp-Method and Mcp-Name headers against the body (SEP-2243), echoes them back on the response, and extracts Mcp-Param-* headers into _meta.x-mcp-headers. If the response is JSON, the server responds synchronously in stateless mode (with pending-response correlation via std::promise) or returns 202 Accepted in session mode. If the response is SSE (text/event-stream), the server keeps the connection open and streams events.

SSE read loop: On the client side, when the POST response has Content-Type: text/event-stream, a background thread (SseReadLoop) reads chunks, splits on \n\n delimiters, parses data: lines, and enqueues JsonRpcMessage into the MessageChannel.

Headers (StreamableHttpClientTransport):

• MCP-Protocol-Version: 2026-07-28 — always sent
• Mcp-Method — dynamic, derived from JSON-RPC body method field
• Mcp-Param-* — primitive params extracted for middleware routing (strings, integers, booleans only)
• Accept: application/json, text/event-stream — allows server to pick response mode

Transport Interfaces

ITransport (session connection)
  └── TransportBase (3-state: Initial → Connected → Disconnected)
      ├── StdioServerTransport
      ├── StdioClientSessionTransport (internal)
      ├── InMemoryTransportImpl
      ├── SseClientSessionTransport (internal)
      ├── WebSocketSessionTransport (wraps hv::WebSocketClient)
      ├── StreamableHttpServerTransport
      └── StreamableHttpSessionTransport (internal)

IClientTransport (connection factory)
  ├── StdioClientTransport (PlatformIO)
  ├── SseClientTransport (libhv HttpClient + requests::post)
  ├── StreamableHttpClientTransport (libhv requests::post / WinHTTP)
  └── WebSocketClientTransport (libhv WebSocketClient)

Key Types

HttpTransportMode

Controls how the HTTP client transport connects:

Value	Description
AutoDetect	Probe server/discover first; fall back to SSE POST
StreamableHttp	Use Streamable HTTP directly (requires 2026-07-28+)
Sse	Use legacy SSE POST only

StdioClientTransportOptions

Field	Type	Default	Description
command	std::string	required	Subprocess command
arguments	std::vector<std::string>	{}	Command-line arguments
name	std::string	"stdio"	Transport name
working_directory	std::string	""	Subprocess working directory
inherit_environment_variables	bool	true	Inherit parent environment
environment_variables	std::map<std::string, std::string>	{}	Additional env vars

HttpClientTransportOptions

Field	Type	Default	Description
endpoint	std::string	required	Server endpoint URL
transport_mode	HttpTransportMode	AutoDetect	Connection mode
name	std::string	""	Transport name
known_session_id	std::string	""	Session ID for resumption
additional_headers	std::map<std::string, std::string>	{}	Extra HTTP headers

StreamableHttpServerOptions

Field	Type	Default	Description
port	uint16_t	3001	HTTP server port
endpoint	std::string	"/mcp"	HTTP endpoint path
stateless	bool	false	Enable 2026-07-28 stateless mode
enable_legacy_sse	bool	true	Serve SSE stream on GET
event_store	std::shared_ptr<EventStore>	nullptr	Event store for resumption
server_name	std::string	"mcp-server"	Server name for discovery
server_version	std::string	"0.1.0"	Server version for discovery

InMemoryTransport::Pair

struct Pair {
    std::shared_ptr<ITransport> client;
    std::shared_ptr<ITransport> server;
};

¹ SSE server-side is provided through StreamableHttpServerTransport when enable_legacy_sse is true (default), which serves an SSE stream on the same endpoint via HTTP GET.

Stateless Mode

StreamableHttpServerTransport supports stateless mode controlled by StreamableHttpServerOptions::stateless (default false). When true, IsStateless() returns true and:

• No sessions: Each request is independent; the response is correlated synchronously via std::promise with a 30-second timeout.
• No SSE: Server-initiated notifications are delivered inline via JSON response; EventStore append is skipped.
• MRTR disabled: The server disables InputRequiredResult — requests must carry full context via _meta and requestState (opaque token for cross-request state recovery).