Technology History | 500 Howard Street — The Sign on the Glass Still Said Slack

The photograph on this page was taken in January 2020, and the name across the glass is Slack's. The building is Foundry Square IV, ten storeys on the corner of Howard and First, and it has been Anthropic's headquarters since late 2023. Anthropic took it as a sublease of about 230,000 square feet from Slack, whose owner Salesforce is also an investor in Anthropic. Before that the company worked out of fewer than 18,000 square feet at 394 Pacific Avenue, a 1907 building in Jackson Square, which the San Francisco Chronicle calls its first office. Anthropic was almost three years old before it had a headquarters on this scale, and the jump between the two addresses is the plainest measure of what happened to it in 2023. It kept going. The company added floors at 505 Howard Street in September 2025, then at 400 Howard and 405 Howard, and in 2026 converted the 500 Howard sublease into a long-term direct lease from the landlord Heitman. Brokers began marketing the stretch as AI Alley, and CoStar reported that some people had started calling it Anthropic Row. In January 2026 Anthropic signed for the whole of 300 Howard Street, a twenty-five-storey tower that had sat empty under its old name of 199 Fremont, together with the smaller building at 342 Howard on the same parcel. The landlords' announcement puts that tower at 466,000 square feet and the Chronicle reported roughly 420,000, a gap nobody has reconciled in public. The Chronicle described the deal as one of the largest single-tenant office leases signed downtown since the pandemic. Anthropic intends to move there in 2027, which will turn 500 Howard into a former headquarters. Two things are visible from the pavement today. The Slack logo is gone from the glass, and the Salesforce Transit Center with its rooftop park stands a block away. Photo: HaeB, CC BY-SA 4.0, via Wikimedia Commons — 500 Howard Street, January 2020, while Slack was still the tenant. Sources: CoStar; San Francisco Chronicle; SF Standard; The Real Deal; DivcoWest and Blackstone Real Estate press release, 2 February 2026.