Earlier this month, the team behind Hydro developed a new way to check that a distributed system still works correctly when it resends messages after a failure.

This produced what Shadaj Laddad described to me as “like a 5000 line PR.” Hydro is an open-source Rust framework for distributed systems, the software that runs across many machines at once, and agents now write most of it with humans driving. The PR was difficult to split into smaller pieces as its components worked together. 

That’s a lot of code to review for a tool developers count on to catch bugs. Laddad, research lead for the Hydro team at AWS, says it’s something the team spends a lot of time thinking about. I sat down with him last week at AGNTCon + MCPCon in San Jose. He was set to give a keynote the next day where he was going to argue code review can’t keep pace with how agents write code.

Agents have let the team aim higher, Laddad told me, saying PRs arrive faster and run longer. “You can have the feature work well in isolation, then you mix it with something else, and suddenly the guarantees break down,” he said. And you can’t just trust the tests. In most agent workflows, he says, the agent writes both the code and the unit tests. “Agents are great, but they’re unpredictable,” Laddad said. That unpredictability is part of what makes them creative, and it makes it hard for a human to review all those tests and confirm they do what the team wants.

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