A recent Wall Street tech rally saw the S&P 500 tech sector soar 11.4 per cent in five days, with Nvidia shares jumping 16 per cent in a week. Investor enthusiasm included over 4 million call options purchased in a single day, a historical high, and hedge funds recording one of their biggest buying weeks since 2008. Microsoft significantly contributed to this sentiment after its June quarter results, with its stock surging almost 25 per cent, adding US$450 billion to its market capitalisation. Growth was propelled by a 43 per cent expansion in its Azure cloud computing division, with revenue surpassing US$100 billion for the first time. However, analysis suggests up to 70 per cent of Microsoft’s AI revenue—and 7 per cent of its total—originates from one client: OpenAI, which contributed US$24.1 billion to its June quarter revenue. This dependency extends across the sector, with Barclays estimating that OpenAI and rival Anthropic will account for 73 per cent of Amazon’s AI revenues in 2026-2027, and UBS projecting 28 per cent of Google’s cloud revenue in 2026 will come from these two, potentially rising to over 48 per cent by 2027. The rally raises questions about reliance on companies whose economic models are yet to be proven sustainable.