Move GetSessionID closure into ServerOptions (#488)

Fixes: #478.

mcp/server.go

@@ -83,6 +83,16 @@ type ServerOptions struct {
 	// If true, advertises the tools capability during initialization,
 	// even if no tools have been registered.
 	HasTools bool
+
+	// GetSessionID provides the next session ID to use for an incoming request.
+	// If nil, a default randomly generated ID will be used.
+	//
+	// Session IDs should be globally unique across the scope of the server,
+	// which may span multiple processes in the case of distributed servers.
+	//
+	// As a special case, if GetSessionID returns the empty string, the
+	// Mcp-Session-Id header will not be set.
+	GetSessionID func() string
 }
 
 // NewServer creates a new MCP server. The resulting server has no features:
@@ -114,6 +124,11 @@ func NewServer(impl *Implementation, options *ServerOptions) *Server {
 	if opts.UnsubscribeHandler != nil && opts.SubscribeHandler == nil {
 		panic("UnsubscribeHandler requires SubscribeHandler")
 	}
+
+	if opts.GetSessionID == nil {
+		opts.GetSessionID = randText
+	}
+
 	return &Server{
 		impl:                    impl,
 		opts:                    opts,

mcp/streamable.go

@@ -50,16 +50,6 @@ type StreamableHTTPHandler struct {
 
 // StreamableHTTPOptions configures the StreamableHTTPHandler.
 type StreamableHTTPOptions struct {
-	// GetSessionID provides the next session ID to use for an incoming request.
-	// If nil, a default randomly generated ID will be used.
-	//
-	// Session IDs should be globally unique across the scope of the server,
-	// which may span multiple processes in the case of distributed servers.
-	//
-	// As a special case, if GetSessionID returns the empty string, the
-	// Mcp-Session-Id header will not be set.
-	GetSessionID func() string
-
 	// Stateless controls whether the session is 'stateless'.
 	//
 	// A stateless server does not validate the Mcp-Session-Id header, and uses a
@@ -92,9 +82,6 @@ func NewStreamableHTTPHandler(getServer func(*http.Request) *Server, opts *Strea
 	if opts != nil {
 		h.opts = *opts
 	}
-	if h.opts.GetSessionID == nil {
-		h.opts.GetSessionID = randText
-	}
 	return h
 }
 
@@ -233,7 +220,7 @@ func (h *StreamableHTTPHandler) ServeHTTP(w http.ResponseWriter, req *http.Reque
 		if sessionID == "" {
 			// In stateless mode, sessionID may be nonempty even if there's no
 			// existing transport.
-			sessionID = h.opts.GetSessionID()
+			sessionID = server.opts.GetSessionID()
 		}
 		transport = &StreamableServerTransport{
 			SessionID:    sessionID,

mcp/streamable_test.go

@@ -409,15 +409,14 @@ 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{
+	server := NewServer(testImpl, &ServerOptions{
+		KeepAlive: 10 * time.Millisecond,
 		GetSessionID: func() string {
 			mu.Lock()
 			defer mu.Unlock()
@@ -430,6 +429,7 @@ func TestServerTransportCleanup(t *testing.T) {
 		},
 	})
 
+	handler := NewStreamableHTTPHandler(func(*http.Request) *Server { return server }, nil)
 	handler.onTransportDeletion = func(sessionID string) {
 		chans[sessionID] <- struct{}{}
 	}
@@ -1199,8 +1199,6 @@ func TestStreamableStateless(t *testing.T) {
 		}
 		return &CallToolResult{Content: []Content{&TextContent{Text: "hi " + args.Name}}}, nil, nil
 	}
-	server := NewServer(testImpl, nil)
-	AddTool(server, &Tool{Name: "greet", Description: "say hi"}, sayHi)
 
 	requests := []streamableRequest{
 		{
@@ -1263,9 +1261,15 @@ func TestStreamableStateless(t *testing.T) {
 		}
 	}
 
-	sessionlessHandler := NewStreamableHTTPHandler(func(*http.Request) *Server { return server }, &StreamableHTTPOptions{
+	sessionlessHandler := NewStreamableHTTPHandler(func(*http.Request) *Server {
-		GetSessionID: func() string { return "" },
+		// Return a stateless server which never assigns a session ID.
-		Stateless:    true,
+		server := NewServer(testImpl, &ServerOptions{
+			GetSessionID: func() string { return "" },
+		})
+		AddTool(server, &Tool{Name: "greet", Description: "say hi"}, sayHi)
+		return server
+	}, &StreamableHTTPOptions{
+		Stateless: true,
 	})
 
 	// First, test the "sessionless" stateless mode, where there is no session ID.
@@ -1279,7 +1283,12 @@ func TestStreamableStateless(t *testing.T) {
 	// This can be used by tools to look up application state preserved across
 	// subsequent requests.
 	requests[0].wantSessionID = true // now expect a session ID for initialize
-	statelessHandler := NewStreamableHTTPHandler(func(*http.Request) *Server { return server }, &StreamableHTTPOptions{
+	statelessHandler := NewStreamableHTTPHandler(func(*http.Request) *Server {
+		// Return a server with default options which should assign a random session ID.
+		server := NewServer(testImpl, nil)
+		AddTool(server, &Tool{Name: "greet", Description: "say hi"}, sayHi)
+		return server
+	}, &StreamableHTTPOptions{
 		Stateless: true,
 	})
 	t.Run("stateless", func(t *testing.T) {