Robert O'Callahan

In 2001, O'Callahan received a PhD in computer science from Carnegie Mellon University, supervised by Daniel Jackson and Jeannette Wing, with a dissertation on polymorphic type inference for context-sensitive alias analysis for Java code. From 2001 to 2004, he worked at IBM Research on dynamic program analysis tools; from 2000, he also moonlighted as a volunteer on the Mozilla open source project, which later became Firefox.⁠[1][1][1]

In December 2004, he announced on his blog that his family was moving back to New Zealand, citing reasons including being close to their extended family; he is a New Zealand citizen. Because New Zealand had no industrial computer science research labs, he decided to leave research work, parted amicably with IBM, and accepted a position with Novell’s Ximian division, continuing core Mozilla development such as graphics and layout from New Zealand.⁠[2][2][3][3]

In December 2006, he announced that he would leave Novell at the end of the year and work as a contractor for the Mozilla Corporation from January 2007; Mozilla also said it would support hiring additional developers in Auckland. He said that one of his long-term goals on returning home was to help develop New Zealand’s software industry. At Mozilla, he worked mainly on the Firefox browser engine, and Mozilla awarded him the title of Distinguished Engineer.⁠[4][4][4][1]

On March 26, 2014, he released version 1.0 of rr, a record-and-replay debugging tool developed by Mozilla. rr records the execution of a process tree on Linux; during replay, the contents of memory and registers are identical to those during recording, and through a gdbserver interface it lets gdb debug the replayed processes, helping track down hard-to-reproduce intermittent bugs. The project focused primarily on debugging Firefox, does not modify the operating system kernel, and was released as free software. He led the development of rr during his last few years at Mozilla, and in 2017 he was the first author of an extended technical report on rr.⁠[5][5][5][5][5][5][5][5][5][1][6]

On March 1, 2016, he announced that he was leaving Mozilla, with his last day on March 4, New Zealand time. He said he had worked on the Web platform for 16 years, 11 of them full-time, and was tired, and that realizing rr’s full potential required resources that Mozilla could not and should not provide, probably calling for a for-profit company. After leaving, he co-founded Pernosco, developing the debugger of the same name mainly with Kyle Huey: it analyzes rr recordings in the cloud, lets developers debug them through a web interface, and is offered as a paid cloud service.⁠[7][7][7][7][8][8][8][1]

On November 14, 2020, after working with Mozilla, the rr code repository was moved to an independent GitHub organization, rr-debugger, with no change in intellectual property ownership; the organization’s owners are O'Callahan, Kyle Huey, and Keno Fischer.⁠[9][9][9]

As of January 10, 2022, he left day-to-day work on Pernosco and joined Google’s New Zealand office. He said that after nearly 6 years working on Pernosco, his financial “runway” had run out; Google had announced it wanted to start engineering work in its Auckland office and reached out to him. Pernosco continued to be maintained and sold by Kyle Huey; he retained his stake in the company but would not take part in Pernosco’s development, sales, or operations while at Google.⁠[10][10][10][10][10]

In April 2023, he signed, in a personal capacity, the open letter calling for a pause on training giant AI models, noting that this did not represent Google’s position. He argued that AI capabilities were growing faster than people could learn new skills, that socioeconomic disruption, malicious use, and extinction risk from “alignment” failure all warranted great caution, and that it was absurd to regulate food and drug safety but not AI.⁠[11][11][11][11][11]

In November 2025, he wrote that he worked at Google DeepMindGoogle DeepMindGoogle DeepMind is Google’s artificial intelligence research organization, formed in April 2023 by merging DeepMind, which Google acquired in 2014, with the Brain team from Google Research, and led by CEO Demis Hassabis. It developed AlphaGo, AlphaFold, and the Gemini family of models, and Hassabis and John Jumper received the 2024 Nobel Prize in Chemistry for AlphaFold’s protein structure prediction work.Open the full entry and believed AI would most likely be disastrous for humanity. He had just returned from a three-week trip visiting GDM offices around the world, during which he discussed AI threats ranging from mass unemployment to human extinction with colleagues. He said he considered resigning almost every day, but at the time believed his work was far enough removed from AI capability improvements to stay for now.⁠[12][12][12][12][12]

On March 4, 2026, he publicly stated that he had signed the “We Will Not Be Divided” open letter, saying he agreed with Anthropic’s position that AI should not be used by a government for mass surveillance of its own citizens and should not be used to kill people without human oversight, that he hoped his employer, Google, would ask for the same restrictions, and that he considered it absurd for the US government to label Anthropic a “supply-chain risk.”⁠[13][13][13][13]

On July 30, 2026, he publicly stated that he had signed the “Pacing The Frontier” petition and responded to objections: he argued that refusing to cooperate on the grounds of “rogue labs” or China was running away from the challenge, that the Chinese Communist Party’s stated positions on AI safety and control were not bad, and that cooperation required verification mechanisms to build trust on both sides.⁠[14][14][14][14]

On September 24, 2026, he sent a resignation email to Google and published it in full on his blog. In the letter, he said he loved his colleagues and work environment, but his team’s ultimate goal was to make AI much cheaper and lower-latency, and he firmly believed AI progress was currently far too rapid, also having doubts about its destination; because of his ties to New Zealand, it was practically impossible for him to move to another Google project that would not accelerate AI. He acknowledged that his individual departure would have a very small impact, but argued that because some of his skills were rare, the impact was not zero; he had tried to exert a positive influence from within GDM, but the effect had not been strong. He wrote that turning a blind eye to the impact of his work would not be a Jesus-following thing to do.⁠[15][15][15][15][15][15][15][15]

In the same post, he explained that he did not work directly on AI capability research but on tools to improve hardware chip design; he had come to realize that the main impact of these tools would be to accelerate the design of a new breed of AI chips, making AI cheaper, faster, and more powerful. He said he had told his skip-level manager that he was reluctant to see the project succeed; the timing of his resignation was unrelated to the recent wave of other resignations and was set so he could leave before a long-planned hiking trip.⁠[15][15][15][15][15]

In the post, he argued that the existential risks from AI that many people are warning about deserve to be taken seriously, that the risk is real but uncertain, and that pursuing superintelligence in the near future is inherently irresponsible, and he called for national and international AI regulation; he also said that most Google DeepMind employees he had talked to had sincere and serious concerns about these issues. As for next steps, he said he would continue maintaining Pernosco and rr, investigate how AI agents debug code and whether debugging tools could help them, and wanted his future work to be “unambiguously pro-human.”⁠[15][15][15][15][15][15][15]

O'Callahan describes himself as a Christian and is an elder and occasional lay preacher in the English-speaking congregation of Auckland Chinese Presbyterian Church in Auckland’s inner city; the header of his personal blog, Eyes Above The Waves, describes him as “Christian. Kiwi. Software guy.” He has said that living in New Zealand places him outside both the tech industry bubble and the American bubble.⁠[15][16][15]

Eyes Above The Waves: Personal blog: https://robert.ocallahan.org/ (opens in a new window).⁠[1]

X: Public account: https://x.com/rocallahan (opens in a new window)⁠[1]

Bluesky: Public account: https://bsky.app/profile/rocallahan.bsky.social (opens in a new window)⁠[16]

GitHub: Public account: https://github.com/rocallahan (opens in a new window)⁠[1]

rr: Code repository: https://github.com/rr-debugger/rr (opens in a new window)⁠[9]