The ceiling on a private guarantee is the guarantor's market cap

A private-company guarantee is only as large as the guarantor can borrow against, and the guarantor is only as large as the market says it is. Most read Nvidia's OpenAI backstop cut as a negotiation, a risk trim, or a boardroom compromise. The actual mechanism is simpler: Nvidia's share price is the ceiling on its promises, and the ceiling fell.

When Nvidia fell 5% on 27 July after the first Wall Street Journal report on the 10GW Ohio campus deal, the backstop ceiling fell with it — the reporting confirms the parties recognized the backstop would shrink after the market reaction, not that the 5% drop was the sole or confirmed cause of the final agreed size. OpenAI has no investment-grade credit rating, so the project debt was priced against Nvidia's balance sheet, not OpenAI's. The promise was Nvidia's to keep, and the market kept repricing it. By mid-August, what was billed as a $250bn financial backstop had been cut to roughly half, per WSJ's Anissa Gardizy. TheNextWeb's synthesis puts the total project cost, silicon included, at more than $500bn, with a first phase of roughly 800MW due in 2028 from developer SB Energy, a SoftBank subsidiary.

The reusable pattern: market cap is borrowing capacity, and borrowing capacity is the ceiling on any guarantee to a non-IG counterparty. A separate $350bn chip-purchase financing survives, where Nvidia helps arrange rather than guarantee outright. The tie-up is reshaped, not abandoned. The cap is what the trading day allows, not what the press release claims.