mcp: add Session.ID

Support retrieving the session ID from client and server sessions.

For modelcontextprotocol#65.

Author: jba

mcp/client.go

@@ -142,6 +142,18 @@ type ClientSession struct {
 	client           *Client
 	initializeResult *InitializeResult
 	keepaliveCancel  context.CancelFunc
+	sessionIDFunc    func() string
+}
+
+func (cs *ClientSession) setSessionIDFunc(f func() string) {
+	cs.sessionIDFunc = f
+}
+
+func (cs *ClientSession) ID() string {
+	if cs.sessionIDFunc == nil {
+		return ""
+	}
+	return cs.sessionIDFunc()
 }
 
 // Close performs a graceful close of the connection, preventing new requests

mcp/server.go

@@ -496,13 +496,25 @@ func (ss *ServerSession) NotifyProgress(ctx context.Context, params *ProgressNot
 type ServerSession struct {
 	server           *Server
 	conn             *jsonrpc2.Connection
+	sessionIDFunc    func() string
 	mu               sync.Mutex
 	logLevel         LoggingLevel
 	initializeParams *InitializeParams
 	initialized      bool
 	keepaliveCancel  context.CancelFunc
 }
 
+func (ss *ServerSession) setSessionIDFunc(f func() string) {
+	ss.sessionIDFunc = f
+}
+
+func (ss *ServerSession) ID() string {
+	if ss.sessionIDFunc == nil {
+		return ""
+	}
+	return ss.sessionIDFunc()
+}
+
 // Ping pings the client.
 func (ss *ServerSession) Ping(ctx context.Context, params *PingParams) error {
 	_, err := handleSend[*emptyResult](ctx, ss, methodPing, params)

mcp/shared.go

@@ -36,6 +36,9 @@ type methodHandler any // MethodHandler[*ClientSession] | MethodHandler[*ServerS
 // A Session is either a ClientSession or a ServerSession.
 type Session interface {
 	*ClientSession | *ServerSession
+	// ID returns the session ID, or the empty string if there is none.
+	ID() string
+
 	sendingMethodInfos() map[string]methodInfo
 	receivingMethodInfos() map[string]methodInfo
 	sendingMethodHandler() methodHandler

mcp/sse.go

@@ -263,6 +263,9 @@ type sseServerConn struct {
 	t *SSEServerTransport
 }
 
+// TODO(jba): get the session ID. (Not urgent because SSE transports have been removed from the spec.)
+func (s sseServerConn) sessionID() string { return "" }
+
 // Read implements jsonrpc2.Reader.
 func (s sseServerConn) Read(ctx context.Context) (JSONRPCMessage, error) {
 	select {
@@ -518,6 +521,9 @@ type sseClientConn struct {
 	done   chan struct{} // closed when the stream is closed
 }
 
+// TODO(jba): get the session ID. (Not urgent because SSE transports have been removed from the spec.)
+func (c *sseClientConn) sessionID() string { return "" }
+
 func (c *sseClientConn) isDone() bool {
 	c.mu.Lock()
 	defer c.mu.Unlock()

mcp/streamable.go

@@ -161,6 +161,10 @@ func NewStreamableServerTransport(sessionID string) *StreamableServerTransport {
 	}
 }
 
+func (t *StreamableServerTransport) sessionID() string {
+	return t.id
+}
+
 // A StreamableServerTransport implements the [Transport] interface for a
 // single session.
 type StreamableServerTransport struct {
@@ -331,7 +335,7 @@ func (t *StreamableServerTransport) servePOST(w http.ResponseWriter, req *http.R
 		http.Error(w, fmt.Sprintf("malformed payload: %v", err), http.StatusBadRequest)
 		return
 	}
-	var requests = make(map[JSONRPCID]struct{})
+	requests := make(map[JSONRPCID]struct{})
 	for _, msg := range incoming {
 		if req, ok := msg.(*JSONRPCRequest); ok && req.ID.IsValid() {
 			requests[req.ID] = struct{}{}
@@ -624,20 +628,26 @@ func (t *StreamableClientTransport) Connect(ctx context.Context) (Connection, er
 }
 
 type streamableClientConn struct {
-	url       string
-	sessionID string
-	client    *http.Client
-	incoming  chan []byte
-	done      chan struct{}
+	url      string
+	client   *http.Client
+	incoming chan []byte
+	done     chan struct{}
 
 	closeOnce sync.Once
 	closeErr  error
 
-	mu sync.Mutex
+	mu         sync.Mutex
+	_sessionID string
 	// bodies map[*http.Response]io.Closer
 	err error
 }
 
+func (c *streamableClientConn) sessionID() string {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	return c._sessionID
+}
+
 // Read implements the [Connection] interface.
 func (s *streamableClientConn) Read(ctx context.Context) (JSONRPCMessage, error) {
 	select {
@@ -658,7 +668,7 @@ func (s *streamableClientConn) Write(ctx context.Context, msg JSONRPCMessage) er
 		return s.err
 	}
 
-	sessionID := s.sessionID
+	sessionID := s._sessionID
 	if sessionID == "" {
 		// Hold lock for the first request.
 		defer s.mu.Unlock()
@@ -681,7 +691,7 @@ func (s *streamableClientConn) Write(ctx context.Context, msg JSONRPCMessage) er
 
 	if sessionID == "" {
 		// locked
-		s.sessionID = gotSessionID
+		s._sessionID = gotSessionID
 	}
 
 	return nil
@@ -753,7 +763,7 @@ func (s *streamableClientConn) Close() error {
 		if err != nil {
 			s.closeErr = err
 		} else {
-			req.Header.Set("Mcp-Session-Id", s.sessionID)
+			req.Header.Set("Mcp-Session-Id", s._sessionID)
 			if _, err := s.client.Do(req); err != nil {
 				s.closeErr = err
 			}

mcp/streamable_test.go

@@ -69,7 +69,10 @@ func TestStreamableTransports(t *testing.T) {
 		t.Fatalf("client.Connect() failed: %v", err)
 	}
 	defer session.Close()
-
+	sid := session.ID()
+	if sid == "" {
+		t.Error("empty session ID")
+	}
 	// 4. The client calls the "greet" tool.
 	params := &CallToolParams{
 		Name:      "greet",
@@ -79,6 +82,9 @@ func TestStreamableTransports(t *testing.T) {
 	if err != nil {
 		t.Fatalf("CallTool() failed: %v", err)
 	}
+	if g := session.ID(); g != sid {
+		t.Errorf("session ID: got %q, want %q", g, sid)
+	}
 
 	// 5. Verify that the correct response is received.
 	want := &CallToolResult{

mcp/transport.go

@@ -50,6 +50,7 @@ type Connection interface {
 	Read(context.Context) (JSONRPCMessage, error)
 	Write(context.Context, JSONRPCMessage) error
 	Close() error // may be called concurrently by both peers
+	sessionID() string
 }
 
 // A StdioTransport is a [Transport] that communicates over stdin/stdout using
@@ -94,6 +95,7 @@ type binder[T handler] interface {
 
 type handler interface {
 	handle(ctx context.Context, req *JSONRPCRequest) (any, error)
+	setSessionIDFunc(func() string) // so Sessions can get the session ID
 }
 
 func connect[H handler](ctx context.Context, t Transport, b binder[H]) (H, error) {
@@ -124,6 +126,7 @@ func connect[H handler](ctx context.Context, t Transport, b binder[H]) (H, error
 		},
 	})
 	assert(preempter.conn != nil, "unbound preempter")
+	h.setSessionIDFunc(conn.sessionID)
 	return h, nil
 }
 
@@ -200,6 +203,8 @@ type loggingConn struct {
 	w        io.Writer
 }
 
+func (c *loggingConn) sessionID() string { return c.delegate.sessionID() }
+
 // loggingReader is a stream middleware that logs incoming messages.
 func (s *loggingConn) Read(ctx context.Context) (JSONRPCMessage, error) {
 	msg, err := s.delegate.Read(ctx)
@@ -285,6 +290,8 @@ func newIOConn(rwc io.ReadWriteCloser) *ioConn {
 	}
 }
 
+func (c *ioConn) sessionID() string { return "" }
+
 // addBatch records a msgBatch for an incoming batch payload.
 // It returns an error if batch is malformed, containing previously seen IDs.
 //