AI has become so effective at writing code and researching that the biggest constraint on developing new AI systems may now be the humans overseeing them, according to a new study by Anthropic.
In its report “When AI Builds Itself,” Anthropic argued that Claude is already helping build future AI systems by writing code, running experiments, and assisting with research-a trend that could eventually lead to recursive self-improvement.
Anthropic says Claude now authors more than 80% of the code merged into its codebase, and has helped engineers increase code output roughly eightfold since 2024.
"Before Claude Code launched in research preview in February 2025, this number was in the low single digits," Anthropic wrote. "Lines of code merged per engineer per day stayed constant through Anthropic’s first four years (2021-2024), then began to climb upward in 2025."
Anthropic said the future could unfold in several ways: AI progress could slow, humans could remain in charge while AI automates much of the work, or AI systems could eventually begin improving their own successors.
"Taken far enough, and given enough compute, that trend points to an AI system capable of fully autonomously designing and developing its own successor," Anthropic wrote. "This is called recursive self-improvement. We are not there yet... But it could come sooner than most institutions are prepared for."
The company cautioned that it's too early to know which outcome is most likely, and acknowledged that lines of code are an imperfect measure of productivity.
The report comes as AI companies increasingly position their models as research collaborators rather than simple chatbots. Last month, Anthropic upgraded its flagship Claude model to Opus 4.8, while rival OpenAI launched GPT-5.5 and GPT-Rosalind in April. In May, Google announced Gemini Spark.
Anthropic has also emphasized AI systems capable of operating with greater autonomy as it prepares to go public, touting Claude Mythos' ability to identify software vulnerabilities and conduct complex cybersecurity research.
"Humans play a substantially diminished role in their development, likely moving most of our effort towards oversight, validation, and verification of an expanding 'virtual lab' run by AI systems," the company said.