U.S. factory production unexpectedly increased in December amid a surge in primary metals output that offset a decline at motor vehicle assembly plants, but activity contracted in the fourth quarter against the backdrop of challenges from import tariffs. Some improvement in manufacturing is expected this year as the drag from President Donald Trump's sweeping import duties eases and tax legislation, which made bonus depreciation permanent among other perks, takes effect. An artificial intelligence investment boom is also seen providing support, though some economists warned the environment for manufacturers remained fragile, citing weak factory surveys.