Debt markets are tightening access for AI companies, with several firms already receiving junk-rated credit assessments despite significant investor gains of 100% to 200%. John D'Agostino from Coinbase warns that market participants are reluctant to be "the last person on this train," signaling growing caution amid the AI investment boom. The shift in debt market conditions reflects increasing scrutiny of AI sector valuations and creditworthiness as investors reassess risk profiles after substantial returns.