WASHINGTON, Aug. 12 -- Consumer prices rose 3.4 percent in July from a year ago, a headline rate held in check almost entirely by an energy reprieve that expires Sunday when the United States-Iran ceasefire deadline passes.

The Bureau of Labor Statistics reported Tuesday that the Consumer Price Index climbed just 0.1 percent from June to July on a seasonally adjusted basis, the slowest monthly gain since January. Energy prices, up 14.7 percent over the past year, fell 1.5 percent in the month, the only category to deliver meaningful downward pressure on a reading that might otherwise have looked considerably worse.

That drop in energy costs traced directly to the Iran ceasefire, a tentative framework reached in late July that eased Stra...