Anthropic’s initial public offering (IPO), expected no earlier than mid-October, will be a test of just how hungry the financial markets still are for exposure to artificial intelligence (AI). The S&P 500 (^GSPC +0.17%) is going to react depending on how that test goes. Its backers want a valuation of $2 trillion or more, which would top Space Exploration Technologies as the biggest IPO ever if it happens.
Don’t expect S&P 500 funds to buy it immediately, though. The road for Anthropic entering the S&P 500 is going to be slow, and it could test some nerves along the way.
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Anthropic can’t join the S&P 500 for at least 12 months
By definition, index funds tracking the S&P 500 must buy whatever the S&P 500 adds to its list of stocks.
Anthropic probably won’t get special treatment, given that on June 4, S&P Dow Jones Indices declined to loosen its rules for mega-cap listings. Those rules still require 12 months as a public company, at least 10% of shares being publicly held, and reported positive earnings under generally accepted accounting principles (GAAP).
While SpaceX joined the Nasdaq-100 list of stocks within weeks of its June 2026 IPO, it still needs to wait outside the S&P 500. If Anthropic has its offering in late 2026 as planned, its earliest shot at inclusion in the S&P 500 will occur in late 2027.
Today’s Change
(0.17%) +12.74
Index Level
7,650.50
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Day’s Range
7,610.52 – 7,657.17
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Nonetheless, two of the index’s largest companies already own Anthropic, which means that holders of the index funds have some exposure already as well. For instance, Amazon reported $16.8 billion of pretax gains on its Anthropic position in the first quarter of 2026. Alphabet also has a stake in Anthropic, so a strong debut could boost it and Amazon.
Should you expect a slump after the Anthropic IPO?
Per finance professor Jay Ritter at the University of Florida, across 9,343 U.S. IPOs from 1980 to 2025, the average first-day gain was 19%. The pain came later. Measured from their first close, IPOs from 2012 to 2024 underperformed the market by 25.5% over three years, on average.
SpaceX shows how bumpy the ride can get. It priced its June 2026 IPO at $135. It then jumped to $225.64 before falling to $104.83, and was $151.10 on Sept. 16. That peak-to-trough plunge was probably quite frustrating for its shareholders.
Another factor here is that AI stocks were responsible for more than 80% of the S&P 500’s 2026 gains through mid-May, per the Jefferies investment bank. A lukewarm reception for the year’s biggest IPO stock would signal fading AI appetite, and AI-heavy index funds would feel it.
So, while over the longer term the Anthropic IPO could do well for those who buy it — as well as for any indexes it’s ultimately included in — in the near term, it’s still a risk to the S&P 500 because of what it symbolizes.
