New Delhi, Nov. 4 -- Wall Street is starting to face what could be the most important challenge to the AI investment boom, which has delivered the lion's share of S&P 500 gains since the bull market began four years ago and is single-handedly keeping the domestic economy afloat despite quickening inflation and a moribund job market.

It's the same challenge that has undone many of the biggest booms in American economic history, and it's tied to the bond market.

The flood of AI investment has shifted this year from one financed by the cash generated by the world's biggest tech companies to one that's become increasingly reliant on their borrowing.

So much so, in fact, that the share of debt issuance this year from the tech sector alone h...