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.

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.