LONDON, Nov 12 : When AI firm Anthropic starts trading after its expected IPO next week, its share price in the first few weeks will largely be determined by who is forced to buy it rather than who wants to buy it. And investors are examining SpaceX’s successful June stock market debut for clues.

Elon Musk’s SpaceX altered the playbook for stock-market listing by behemoths, as major US index providers such as Nasdaq rewrote the rules around inclusion, ensuring a massive pool of passive money would buy the shares in the first 15 days of its listing.

Investors expect the same dynamic with Anthropic. The Claude chatbot creator aims to raise up to $64.4 billion by selling 825 million shares priced between $70 and $78 each, valuing the firm at…