Anthropic's IPO Could Reshape the S&P 500's AI Outlook

Anthropic's initial public offering, expected no earlier than mid-October, will test market appetite for AI, with backers seeking a valuation of $2 trillion or more, potentially making it the largest IPO ever. The S&P 500 will react to the IPO's reception, though Anthropic cannot join the index for at least 12 months due to rules requiring a year as a public company, 10% public float, and positive GAAP earnings. Amazon and Alphabet already hold stakes, giving index fund investors indirect exposure. Historically, IPOs underperform the market by 25.5% over three years on average, and AI stocks drove over 80% of the S&P 500's 2026 gains, so a weak debut could signal fading AI demand and pressure the index.