Stop AI is an American activist organization founded in 2024 and based in Oakland, California, that seeks to permanently ban the development of artificial general intelligence (Source: theregister.com). The group is known for civil-disobedience actions targeting OpenAI, including blockades of the company's San Francisco headquarters that led to member arrests and criminal trials (Source: kqed.org). The November 2025 disappearance of co-founder Sam Kirchner, and the group's proximity to the April 2026 attack on Sam Altman's home by an unaffiliated assailant, have made Stop AI a recurring reference point in coverage of the hard-line wing of the anti-AI movement (Source: wsj.com).
Founding and goals
Stop AI was launched in 2024 by Sam Kirchner, an electrical engineering technician then in his mid-twenties, and Guido Reichstadter, a longtime civil-disobedience activist; both had previously been involved in PauseAI (Source: fortune.com). PauseAI US founder Holly Elmore has said she expelled the two over tactical disagreements: Stop AI's founders favored civil disobedience involving law-breaking, which PauseAI explicitly rejects (Source: fortune.com). Where PauseAI calls for a moratorium on frontier development, Stop AI's stated goal is a permanent ban on the development of artificial general intelligence, grounded in the position that AGI poses an extinction risk to humanity (Source: theregister.com).
The group states that it adheres to nonviolent activism. After the April 2026 Altman attack it wrote that "the current leadership of Stop AI is deeply committed to nonviolence in both actions and statements" (Source: fortune.com). As of April 2026 the organization was run by five co-leaders, including Valerie Sizemore (Source: fortune.com).
Activities
Stop AI's actions have concentrated on frontier-lab headquarters in San Francisco. Members have been arrested for blocking the doors of OpenAI's offices in protest, and the resulting criminal cases have continued into 2026: retired engineering professor Wynd Kaufmyn stood trial in mid-2026 over a blockade of OpenAI's headquarters, saying she was prepared to serve time to spread the group's warnings (Source: sf.gazetteer.co). Reichstadter staged a hunger strike outside Anthropic's headquarters, and in early December 2025 was booked into San Francisco County Jail for allegedly violating a judge's order barring him from OpenAI premises following a previous arrest (Source: kqed.org).
In November 2025 the group drew national attention when a member of its defense team served a subpoena on OpenAI chief executive Sam Altman while he was on stage at San Francisco's Sydney Goldstein Theater with Golden State Warriors coach Steve Kerr; the subpoena was sought by the San Francisco Public Defender's Office in connection with the ongoing trial of Stop AI members (Source: sfgate.com). The group also participated in a "Stop the AI Race" protest in San Francisco in March 2026 (Source: fortune.com).
Kirchner disappearance
According to reporting in Fortune and the Atlantic, Kirchner allegedly assaulted Stop AI leader Matthew Hall during a late-2025 internal dispute in which Kirchner reportedly suggested the group abandon nonviolence; he was expelled from the organization (Source: fortune.com). On November 21, 2025 he disappeared, after — according to a Wall Street Journal account — telling a colleague that "the ship may have sailed on nonviolence" (Source: wsj.com). Callers notified police that Kirchner had threatened to go to several OpenAI offices in San Francisco to "murder people," and OpenAI told employees at its headquarters to shelter in place during a lockdown (Source: sfstandard.com). Stop AI has stated that both Kirchner and Reichstadter were removed from the group in 2025 (Source: fortune.com).
Kirchner remained missing as of July 2026. A July 12, 2026 Wall Street Journal profile described the disappearance as leaving the Bay Area's anti-AI movement "on edge," and situated the group within a broader movement of young professionals leaving careers for think tanks, nonprofits, and street protest against AI development (Source: wsj.com). Members told the Daily Californian the group was rethinking strategy after the disappearance (Source: dailycal.org).
Scrutiny after the April 2026 Altman attack
After the April 10, 2026 attempted firebombing of Sam Altman's San Francisco home, allegedly by 20-year-old Daniel Moreno-Gama (see AI-Driven Political Violence), Stop AI and PauseAI both condemned the violence and said the suspect was never a member of either organization (Source: fortune.com). Stop AI stated that Moreno-Gama had joined its public online forum, asked whether "speaking about violence" would get him banned, and ceased activity after being told yes — several months before his alleged crimes (Source: fortune.com).
Commentators disagree about the group's relationship to radicalization. Researcher Nirit Weiss-Blatt argues that apocalyptic AI rhetoric can radicalize "young, anxious followers" even without explicit calls to violence, citing the November 2025 OpenAI lockdown as a warning sign; University of Bath lecturer Mauro Lubrano, author of Stop the Machines, cautions against conflating groups seeking regulation or a pause with violent anti-technology extremism, drawing a comparison to the Luddites' unheard grievances (Source: fortune.com). Politico described Stop AI in December 2025 as a group "at the edge of the AI debate" (Source: politico.com).
Relationships
- related: PauseAI — organizational predecessor of both co-founders; split over civil-disobedience tactics
- related: AI-Driven Political Violence — the April 2026 Altman attack and the framework debate over anti-AI radicalization
- related: Public Opinion on AI — organized opposition within broader AI sentiment
- related: OpenAI — primary protest target; Anthropic — hunger-strike site
- related: Sam Altman — subpoena and protest target