How AI Agents Tripled Our Test Coverage on a 1.8M-Line iOS Codebase

Duolingo's iOS app has nearly 1.8M executable lines and, until recently, test coverage that lagged far behind. We built an autonomous pipeline that writes unit tests and gets them to a merged state. Scheduled Temporal workflows pick untested code, Claude Code agents open real pull requests, and a lifecycle manager monitors CI, self-heals failures, runs AI review, and merges. Humans act only as final reviewers. The pipeline has merged over 85,000 lines of test code and tripled unit-test coverage of our MVVM components. This talk walks through the architecture, the guardrails that keep agent-written code trustworthy, the failure modes we hit, and how we are using the same skeleton for additional migrations across the company. You will walk away with a concrete blueprint for agents that ship code in a large production codebase.