diff --git a/internal/lsp/server.go b/internal/lsp/server.go
index 165b0972db..e057e28d61 100644
--- a/internal/lsp/server.go
+++ b/internal/lsp/server.go
@@ -468,6 +468,10 @@ var handlers = sync.OnceValue(func() handlerMap {
 
 func registerNotificationHandler[Req any](handlers handlerMap, info lsproto.NotificationInfo[Req], fn func(*Server, context.Context, Req) error) {
 	handlers[info.Method] = func(s *Server, ctx context.Context, req *lsproto.RequestMessage) error {
+		if s.session == nil && req.Method != lsproto.MethodInitialized {
+			return lsproto.ErrServerNotInitialized
+		}
+
 		var params Req
 		// Ignore empty params; all generated params are either pointers or any.
 		if req.Params != nil {
@@ -486,6 +490,10 @@ func registerRequestHandler[Req, Resp any](
 	fn func(*Server, context.Context, Req, *lsproto.RequestMessage) (Resp, error),
 ) {
 	handlers[info.Method] = func(s *Server, ctx context.Context, req *lsproto.RequestMessage) error {
+		if s.session == nil && req.Method != lsproto.MethodInitialize {
+			return lsproto.ErrServerNotInitialized
+		}
+
 		var params Req
 		// Ignore empty params.
 		if req.Params != nil {