SpaceX and Anthropic are about to go public -- and your 401(k) may be forced to buy in

Two of the most valuable companies in history are about to go public, and because of their sheer size, they may fundamentally alter what sits inside millions of Americans' retirement accounts. With SpaceX's IPO also sparking index providers to change the rules on how stocks are added to major stock market indexes (like Nasdaq or the S&P 500), you may soon feel the effects of the IPO much faster than you would have otherwise.

Index funds are usually the backbone of most 401(k)s, and because they're obligated to buy whatever is in the index, changing the rules may be the mechanism that forces one's exposure to a new IPO, such as SpaceX's, and eventually Anthropic's. But simply because SpaceX and Anthropic are so enormous at their debut (SpaceX at $1.77 trillion as of Wednesday and Anthropic expected at nearly $1 trillion), index providers can't necessarily leave them out. So they've shortened or even eliminated the seasoning period, meaning your 401(K) will reflect their presence in the stock market that much sooner.

While market bulls eagerly await their opportunity to purchase stocks the second they become available, others warn that the safeguards were in place for a reason, and without them, there could be a serious threat to people's retirement nest eggs.

"We put in place guardrails after the dot-com bubble for a reason," Elizabeth Wilkins, the President and CEO of the Roosevelt Institute the Roosevelt Institute, told Fortune. "Because we remembered that there's real downside risk to tying retiree savings to the fortunes of not only corporate America generally, but specifically the tech sector."

A major IPO filing

Anthropic, which confidentially filed its IPO prospectus with the SEC on Monday, was most recently valued at $965 billion following a $65 billion fundraising round in late May that even topped rival OpenAI. Elon Musk's SpaceX, meanwhile, launched its roadshow this week, at a valuation of $1.77 trillion, which would make it one of the most valuable companies on earth essentially overnight. Companies of that magnitude are impossible for index providers to ignore, which, in turn, recognizing they cannot afford to leave two near-trillion-dollar companies sitting on the sidelines, are already rewriting the rules to pull them in faster.

Several major stock market index providers, including Nasdaq and FTSE Russell, have recently changed or adopted fast-entry rules that could allow companies like SpaceX to be added to major indexes much sooner than they typically would, after as few as five trading days under FTSE Russell's new standard, or 15 under Nasdaq's. Even S&P Dow Jones Indices has reportedly been weighing similar changes. The practical consequence is that the funds that track those indexes may be forced to buy shares in both companies shortly after their debuts, meaning ordinary retirement savers could end up with significant exposure to both whether they chose it or not.