Mega IPOs are creating a new trading logic: passive capital driven by index inclusion could become a major force behind the early share price performance of giant listings.
According to a recent Reuters report, Anthropic plans to issue 825 million shares at a price of $70 to $78 each, aiming to raise up to $64.4 billion, which would correspond to a valuation of as much as $1.4 trillion. Similar to SpaceX's listing in June this year, Anthropic is expected to be included in at least six major indices within 5 to 15 trading days after listing, including the FTSE USA Index, the S&P Total Market Index, and the Nasdaq 100 Index, thereby triggering concentrated buying by passive funds.
This arrangement means that Anthropic's early share price performance after listing may not depend entirely on spontaneous market demand, but will be significantly influenced by index inclusion expectations and passive capital inflows. Investors may position themselves ahead of the official index inclusion, further amplifying early buying.
Index rebalancing analysis firm Intropic estimates that demand for Anthropic shares from passive funds tracking the relevant indices could equal 28% of the shares issued in this offering, amounting to about $19 billion. SpaceX's listing has already demonstrated this model, and now Anthropic is poised to become another case, creating new opportunities for index arbitrage traders as well.
Index rules change, passive capital enters earlier
This shift stems from adjustments by major U.S. index providers such as Nasdaq to the rules for constituent inclusion.
In the past, large companies typically had to wait weeks or even months after listing before entering major benchmark indices. The new rules allow eligible mega IPOs to complete index inclusion within 15 days of listing, bringing passive fund demand forward to the early post-listing period.
Carsten Hesse, head of index events at Berenberg in London, said Anthropic will be a mirror image of the SpaceX IPO, facing the same index-driven factors within 15 days of listing.
Ruben Pathmanathan, an equity analyst at Intropic, noted that index inclusion, passive fund demand, and a highly concentrated free float combine to influence the pricing and trading performance of SpaceX in its early listing phase.
Because passive funds need to allocate to the stock after index inclusion, active investors may buy ahead of time to capture the price advantage before passive money enters.
SpaceX rose 50% in three days after listing
SpaceX's listing experience provides a real-world reference for this model.
SpaceX opened at $150 per share, and by the third trading day the stock had risen 50% from its opening price, remaining above its $135 IPO issue price throughout the first 15 days after listing.
The third trading day was also exactly when stock options trading began. Derivatives traders bought the underlying shares to hedge their positions, further increasing buying pressure. Analysts expect similar early positioning and hedging purchases may occur when Anthropic lists.
Hesse said market participants will not wait until the 5th, 10th, or 15th day after listing to act, but will position ahead of time around index inclusion expectations.
The offering structure also played a role. SpaceX allocated about one-fifth of its $75 billion offering size to retail investors, and Anthropic has adopted a similar arrangement. During SpaceX's listing, index funds and retail investors together absorbed nearly half of the issued shares, and combined with other fund managers positioning early, this further amplified the supply-demand imbalance in the initial free float.
How Anthropic is different
Although the two companies' listing paths are similar, Anthropic's share supply arrangement still differs.
This IPO involves only about 6% of Anthropic's equity, leaving a relatively limited initial free float. However, employees holding about 15% of the equity are permitted to sell some shares on the first day of listing, which could increase early supply and put some pressure on the share price.
In addition, Anthropic has also raised $26 billion from cornerstone investors through a separate private placement channel, with participants including Alphabet's Google, Nvidia, and AMD. These shares have no lock-up period, adding another variable to potential post-listing circulating supply.
Therefore, while potential demand from passive funds is considerable, the final share price performance still depends on whether buying can absorb the additional supply.
A rare opportunity for index arbitrage
The combination of mega IPOs and new rules for rapid index inclusion has also created a rare opportunity for funds that specialize in trading index constituent changes.
Brian Hartigan, global head of ETFs and index investments at Invesco, called this phenomenon an evolution of the market, arguing that behind it is the enormous scale of private equity that mega companies have accumulated before listing.
Celia Fseil, head of equity market neutral strategies at Candriam, said such opportunities are exactly what arbitrage traders favor, because the risk of overcrowding is relatively limited. Traditional index rebalancing trades are often constrained by IPO size, with limited passive fund demand, and active money front-running can easily cause crowded trades; mega IPOs significantly expand the potential scale of passive buying, providing more room for arbitrage trades.
According to Bloomberg, two teams at Millennium Management focused on index change trading recorded $3.7 billion in gains in June alone.
Bill Park, chief investment officer at Connecticut-based Altisma Capital Management, said that for investors engaged in index rebalancing trades, such events may be one of the most important trading opportunities of a lifetime.
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