With the next time to buy health plans under the Affordable Care Act starting in less than a week, the Obama administration is expressing confidence that HealthCare.gov is no longer the rickety online insurance marketplace that exasperated consumers a year ago.

Behind the scenes, however, federal health officials and government contractors are scrambling, according to confidential documents and federal and outside experts familiar with this work. They have been making contingency plans in case the information technology or other aspects prove less sturdy than the administration predicts. And some preparations are coming down to the wire.

The confidential documents written in recent weeks hint at elaborate backup planning that undercuts the administration's predictions that an improved HealthCare.gov will be able to handle everyone who wants to sign up.

One document from late October, for instance, describes a new system known as "throttling," which will be deployed if the number of people trying to use HealthCare.gov at the same time strains the Web site's enlarged capacity.

This throttling could send groups of people using different parts of the site into separate online "waiting rooms."

Other confidential CMS documents show that federal health officials drafted contingency plans involving notices for people who have insurance through the exchanges. The notices provide important information, such as whether enrollees appear eligible for federal subsidies for the coming year.

The notices were supposed to be in consumers' hands by Nov. 1. But by the third week of October, a document says, fewer than 1 million of 7.6 million notices were ready to be mailed or e-mailed. Officials considered sending some late to certain groups, sending skimpier notices or reducing the size of ones delivered electronically. In the end, technicians fixed a computer problem, and the notices are going out in batches that are due to arrive by Saturday, when enrollment begins.