The Biden administration has revised the nutrition standards of the food stamp program and prompted the largest permanent increase to benefits in the program’s history, a move that will give poor people more power to fill their grocery carts but add billions of dollars to the cost of a program that feeds one in eight Americans. At the same time, a temporary 15 percent increase in benefits as part of pandemic relief, is set to expire on Sept. 30. The $3.5 billion boost approved earlier this year provides about $27 more per person per month, or over $100 more a month for a household of four, in additional food stamp benefits. Under rules to be announced on Monday and put in place in October, average benefits will rise more than 25 percent from pre-pandemic levels. All 42 million people in the program will receive additional aid. The move does not require congressional approval, and unlike the large pandemic-era expansions, which are starting to expire, the changes are intended to last.