Repository: modelcontextprotocol/go-sdk

Issue:
streamable session persists past client disconnect
StreamableHTTPHandler holds a map of StreamableServerTransports. Currently, an entry from that map is freed only when the client sends a DELETE. We should also free it when the client stops responding to keepalive pings. If clients don't send DELETEs, the server memory will grow without bound.

Discussion:
@findleyr: IMO this should be an implementation detail of the session store. The memory store should have an eviction policy.@jba: TCP connections don't have an eviction policy; they stay open until the peer explicitly closes them or keepalive fails. I don't see why we shouldn't do the same. You should be able to walk away from your machine that has an MCP client, come back the next day, and continue work.@findleyr: @h9jiang this might be something you could pick up.@samthanawalla: Duplicate of #190 ?@h9jiang: @samthanawalla  This focus on the unbounded map 

```
type StreamableHTTPHandler struct {
  transports map[string]*StreamableServerTransport // keyed by IDs (from Mcp-Session-Id header)
}
```

and should fix by deleting the map entry once keep alive failed. 

Need @jba for confirmation. @jba: @h9jiang that is correct, but it may not be sufficient. Is there anything else in a StreamableServerTransport that needs to be shut down before it dropped? If it has a Close method, that should be called, probably.

Solution patch:
diff --git a/mcp/client.go b/mcp/client.go
index 3b1741b..1ed3b04 100644
--- a/mcp/client.go
+++ b/mcp/client.go
@@ -76,9 +76,9 @@ type ClientOptions struct {
 
 // bind implements the binder[*ClientSession] interface, so that Clients can
 // be connected using [connect].
-func (c *Client) bind(mcpConn Connection, conn *jsonrpc2.Connection, state *clientSessionState) *ClientSession {
+func (c *Client) bind(mcpConn Connection, conn *jsonrpc2.Connection, state *clientSessionState, onClose func()) *ClientSession {
 	assert(mcpConn != nil && conn != nil, "nil connection")
-	cs := &ClientSession{conn: conn, mcpConn: mcpConn, client: c}
+	cs := &ClientSession{conn: conn, mcpConn: mcpConn, client: c, onClose: onClose}
 	if state != nil {
 		cs.state = *state
 	}
@@ -130,7 +130,7 @@ func (c *Client) capabilities() *ClientCapabilities {
 // server, calls or notifications will return an error wrapping
 // [ErrConnectionClosed].
 func (c *Client) Connect(ctx context.Context, t Transport, _ *ClientSessionOptions) (cs *ClientSession, err error) {
-	cs, err = connect(ctx, t, c, (*clientSessionState)(nil))
+	cs, err = connect(ctx, t, c, (*clientSessionState)(nil), nil)
 	if err != nil {
 		return nil, err
 	}
@@ -173,6 +173,8 @@ func (c *Client) Connect(ctx context.Context, t Transport, _ *ClientSessionOptio
 // Call [ClientSession.Close] to close the connection, or await server
 // termination with [ClientSession.Wait].
 type ClientSession struct {
+	onClose func()
+
 	conn            *jsonrpc2.Connection
 	client          *Client
 	keepaliveCancel context.CancelFunc
@@ -208,7 +210,13 @@ func (cs *ClientSession) Close() error {
 	if cs.keepaliveCancel != nil {
 		cs.keepaliveCancel()
 	}
-	return cs.conn.Close()
+	err := cs.conn.Close()
+
+	if cs.onClose != nil {
+		cs.onClose()
+	}
+
+	return err
 }
 
 // Wait waits for the connection to be closed by the server.
diff --git a/mcp/server.go b/mcp/server.go
index eef6a5a..75632f7 100644
--- a/mcp/server.go
+++ b/mcp/server.go
@@ -698,9 +698,9 @@ func (s *Server) Run(ctx context.Context, t Transport) error {
 
 // bind implements the binder[*ServerSession] interface, so that Servers can
 // be connected using [connect].
-func (s *Server) bind(mcpConn Connection, conn *jsonrpc2.Connection, state *ServerSessionState) *ServerSession {
+func (s *Server) bind(mcpConn Connection, conn *jsonrpc2.Connection, state *ServerSessionState, onClose func()) *ServerSession {
 	assert(mcpConn != nil && conn != nil, "nil connection")
-	ss := &ServerSession{conn: conn, mcpConn: mcpConn, server: s}
+	ss := &ServerSession{conn: conn, mcpConn: mcpConn, server: s, onClose: onClose}
 	if state != nil {
 		ss.state = *state
 	}
@@ -727,6 +727,8 @@ func (s *Server) disconnect(cc *ServerSession) {
 // ServerSessionOptions configures the server session.
 type ServerSessionOptions struct {
 	State *ServerSessionState
+
+	onClose func()
 }
 
 // Connect connects the MCP server over the given transport and starts handling
@@ -739,10 +741,12 @@ type ServerSessionOptions struct {
 // If opts.State is non-nil, it is the initial state for the server.
 func (s *Server) Connect(ctx context.Context, t Transport, opts *ServerSessionOptions) (*ServerSession, error) {
 	var state *ServerSessionState
+	var onClose func()
 	if opts != nil {
 		state = opts.State
+		onClose = opts.onClose
 	}
-	return connect(ctx, t, s, state)
+	return connect(ctx, t, s, state, onClose)
 }
 
 // TODO: (nit) move all ServerSession methods below the ServerSession declaration.
@@ -809,6 +813,8 @@ func newServerRequest[P Params](ss *ServerSession, params P) *ServerRequest[P] {
 // Call [ServerSession.Close] to close the connection, or await client
 // termination with [ServerSession.Wait].
 type ServerSession struct {
+	onClose func()
+
 	server          *Server
 	conn            *jsonrpc2.Connection
 	mcpConn         Connection
@@ -1043,7 +1049,13 @@ func (ss *ServerSession) Close() error {
 		//    Close is idempotent and conn.Close() handles concurrent calls correctly
 		ss.keepaliveCancel()
 	}
-	return ss.conn.Close()
+	err := ss.conn.Close()
+
+	if ss.onClose != nil {
+		ss.onClose()
+	}
+
+	return err
 }
 
 // Wait waits for the connection to be closed by the client.
diff --git a/mcp/streamable.go b/mcp/streamable.go
index 99fbe42..f56b708 100644
--- a/mcp/streamable.go
+++ b/mcp/streamable.go
@@ -40,6 +40,8 @@ type StreamableHTTPHandler struct {
 	getServer func(*http.Request) *Server
 	opts      StreamableHTTPOptions
 
+	onTransportDeletion func(sessionID string) // for testing only
+
 	mu sync.Mutex
 	// TODO: we should store the ServerSession along with the transport, because
 	// we need to cancel keepalive requests when closing the transport.
@@ -283,6 +285,19 @@ func (h *StreamableHTTPHandler) ServeHTTP(w http.ResponseWriter, req *http.Reque
 			connectOpts = &ServerSessionOptions{
 				State: state,
 			}
+		} else {
+			// Cleanup is only required in stateful mode, as transportation is
+			// not stored in the map otherwise.
+			connectOpts = &ServerSessionOptions{
+				onClose: func() {
+					h.mu.Lock()
+					delete(h.transports, transport.SessionID)
+					h.mu.Unlock()
+					if h.onTransportDeletion != nil {
+						h.onTransportDeletion(transport.SessionID)
+					}
+				},
+			}
 		}
 
 		// Pass req.Context() here, to allow middleware to add context values.
diff --git a/mcp/transport.go b/mcp/transport.go
index 2bcd8d7..fac640a 100644
--- a/mcp/transport.go
+++ b/mcp/transport.go
@@ -122,7 +122,8 @@ func NewInMemoryTransports() (*InMemoryTransport, *InMemoryTransport) {
 }
 
 type binder[T handler, State any] interface {
-	bind(Connection, *jsonrpc2.Connection, State) T
+	// TODO(rfindley): the bind API has gotten too complicated. Simplify.
+	bind(Connection, *jsonrpc2.Connection, State, func()) T
 	disconnect(T)
 }
 
@@ -130,7 +131,7 @@ type handler interface {
 	handle(ctx context.Context, req *jsonrpc.Request) (any, error)
 }
 
-func connect[H handler, State any](ctx context.Context, t Transport, b binder[H, State], s State) (H, error) {
+func connect[H handler, State any](ctx context.Context, t Transport, b binder[H, State], s State, onClose func()) (H, error) {
 	var zero H
 	mcpConn, err := t.Connect(ctx)
 	if err != nil {
@@ -143,7 +144,7 @@ func connect[H handler, State any](ctx context.Context, t Transport, b binder[H,
 		preempter canceller
 	)
 	bind := func(conn *jsonrpc2.Connection) jsonrpc2.Handler {
-		h = b.bind(mcpConn, conn, s)
+		h = b.bind(mcpConn, conn, s, onClose)
 		preempter.conn = conn
 		return jsonrpc2.HandlerFunc(h.handle)
 	}



Test patch:
diff --git a/mcp/streamable_test.go b/mcp/streamable_test.go
index c9b00ed..52f6172 100644
--- a/mcp/streamable_test.go
+++ b/mcp/streamable_test.go
@@ -8,6 +8,7 @@ import (
 	"bytes"
 	"context"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"io"
 	"maps"
@@ -332,6 +333,76 @@ func testClientReplay(t *testing.T, test clientReplayTest) {
 	}
 }
 
+func TestServerTransportCleanup(t *testing.T) {
+	server := NewServer(testImpl, &ServerOptions{KeepAlive: 10 * time.Millisecond})
+
+	nClient := 3
+
+	var mu sync.Mutex
+	var id int = -1 // session id starting from "0", "1", "2"...
+	chans := make(map[string]chan struct{}, nClient)
+
+	handler := NewStreamableHTTPHandler(func(*http.Request) *Server { return server }, &StreamableHTTPOptions{
+		GetSessionID: func() string {
+			mu.Lock()
+			defer mu.Unlock()
+			id++
+			if id == nClient {
+				t.Errorf("creating more than %v session", nClient)
+			}
+			chans[fmt.Sprint(id)] = make(chan struct{}, 1)
+			return fmt.Sprint(id)
+		},
+	})
+
+	handler.onTransportDeletion = func(sessionID string) {
+		chans[sessionID] <- struct{}{}
+	}
+
+	httpServer := httptest.NewServer(handler)
+	defer httpServer.Close()
+
+	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
+	defer cancel()
+
+	// Spin up clients connect to the same server but refuse to ping request.
+	for range nClient {
+		client := NewClient(testImpl, nil)
+		pingMiddleware := func(next MethodHandler) MethodHandler {
+			return func(
+				ctx context.Context,
+				method string,
+				req Request,
+			) (Result, error) {
+				if method == "ping" {
+					return &emptyResult{}, errors.New("ping error")
+				}
+				return next(ctx, method, req)
+			}
+		}
+		client.AddReceivingMiddleware(pingMiddleware)
+		clientSession, err := client.Connect(ctx, &StreamableClientTransport{Endpoint: httpServer.URL}, nil)
+		if err != nil {
+			t.Fatalf("client.Connect() failed: %v", err)
+		}
+		defer clientSession.Close()
+	}
+
+	for _, ch := range chans {
+		select {
+		case <-ctx.Done():
+			t.Errorf("did not capture transport deletion event from all session in 10 seconds")
+		case <-ch: // Received transport deletion signal of this session
+		}
+	}
+
+	handler.mu.Lock()
+	if len(handler.transports) != 0 {
+		t.Errorf("want empty transports map, find %v entries from handler's transports map", len(handler.transports))
+	}
+	handler.mu.Unlock()
+}
+
 // TestServerInitiatedSSE verifies that the persistent SSE connection remains
 // open and can receive server-initiated events.
 func TestServerInitiatedSSE(t *testing.T) {