Block uses Claude Fable to orchestrate code migrations of thousands of pull requests, with two humans approving every deploy

Bradley Axen, Head of AI Capabilities at Block, told Anthropic that AI is now a major part of how Block runs all large-scale code migrations, which can mean "hundreds or thousands of individual pull requests across multiple repos and millions of lines of code." Claude Fable 5 orchestrates the work across many sessions while a human engineer steers: Fable handles higher-level design and directs "up to dozens of smaller, more cost-effective models like Opus or Sonnet" to make the file edits and run tests. Agents can open PRs, review code and query logs, but merges to main and deploys to production stay with people, and "two humans have to approve the deploy." Block also runs agents that do "daily or weekly passes" over a codebase of around a million lines to stop tech debt growing.