Long-term U.S. borrowing costs are climbing to levels not seen in decades as federal debt approaches $40 trillion and AI hyperscalers increase bond issuance to fund infrastructure spending. The combination points to growing supply in long-dated debt markets, a dynamic that can push yields higher as investors demand more compensation to absorb new bonds. The development matters beyond government finance because higher benchmark yields can raise funding costs across the economy, including for companies financing large-scale AI buildouts.