Anthropic's CI system broke three times before agentic coding forced a redesign

Malhotra describes how Anthropic's CI job volume grew 25x over six months, driven by a codebase where the number of tests grew 10x while headcount barely moved. Engineers there now ship 8x as much code per quarter as they did between 2021 and 2025, and Claude authors 80% of that code. The service that decides which tests run on each pull request broke three times. Doubling the machine's core count bought time, though Malhotra writes he knew from the start it would be "fleeting." Sharding the service by code package, with Claude generating the split, bought 29 days. Daily restarts bought less than a day and quietly let some test results go unrecorded, so the service ended up selecting tests from stale data. The team then rebuilt the service as stateless and horizontally scalable, backed by a journal, a redesign that took three weeks for one engineer; a year earlier, Malhotra writes, the same project would have taken closer to a quarter.

Why it matters: Anthropic's growth rates show how code output can outpace the test infrastructure around it. Malhotra recommends planning for 10 to 20x the expected load and building stateless services that can scale horizontally.