A century-old Mission District office building that once housed OpenAI and Elon Musk's xAI is now on the market, even as a new artificial intelligence startup prepares to move into its halls. The Pioneer Building, long considered a launching pad for some of San Francisco's best-known AI companies, is for sale as the city's AI-fueled office boom continues to reshape its commercial real estate landscape.
According to CoStar, the historic building is coming to market just as startup Exa Labs prepares to move in. The property sits at 3180 18th Street, according to Harvest Properties, which describes the building as sitting in the heart of the Mission District.
The building's roots go back more than a century. Harvest Properties states the Pioneer Building was originally constructed in 1902 and repositioned as a creative-office asset. Harvest Properties identifies the building as a former headquarters for Stripe and OpenAI.
From OpenAI to Musk's xAI
OpenAI vacated the Pioneer Building in 2024 after signing a major lease in Mission Bay, the San Francisco Chronicle reported. The building later housed Musk's xAI company. The rotating cast of occupants has turned the Pioneer Building into something of a symbol for San Francisco's AI-driven office resurgence.
Now it's Exa's turn. The AI search startup has taken the roughly 40,000-square-foot building on a lease with a minimum term of seven years, the Chronicle reported.
A Tight but Thriving AI Office Market
San Francisco logged 12.1 million square feet of office leasing through the third quarter of 2026 — a 49% jump over the same period the year before — with 3.9 million square feet signed in the third quarter alone, according to Bisnow.
That leasing surge hasn't erased the city's vacancy problem so much as exposed its unevenness. Bisnow reported that overall office vacancy sits at 28%, though panelists cautioned the figure includes obsolete space unlikely to ever lease again. Vacancy among top-tier Class-A buildings was around 12% in the second quarter of 2026, compared with more than 31% for Class-B properties — and Class-A rents commanded a 54% premium over older, lower-class space, per the same outlet's reporting.
Startups Racing for Space
Exa's seven-year lease gives it a long-term commitment to the Pioneer Building.
Other AI players have taken varied paths into San Francisco office space. The Chronicle has reported that OpenAI's footprint in Mission Bay alone includes a 486,000-square-foot takeover of two former Uber buildings in 2023, a 315,000-square-foot lease at 550 Terry A. Francois Boulevard, and a 280,000-square-foot sublease at 1800 Owens. Elsewhere, Cognition took over a former Cruise building in SoMa on a five-year term, while Anthropic initially leased space from Slack before converting to a long-term deal, according to the same reporting.
For now, the Pioneer Building's next chapter hinges on who buys it — a detail that remains unresolved as the listing hits the market. What's clear is that the century-old structure, once a trunk factory and now a symbol of San Francisco's AI gold rush, is about to welcome yet another startup eager to claim a piece of its history.
