The Idiom Your Coding Agent Wrote Around Instead Of Using
A senior engineer on a payments team I work with told me a story that I think every team running coding agents will eventually live through. Their codebase has a Result<T, E> wrapper — homegrown, sits in a single core/result.ts file, used in roughly two hundred call sites across the service. New code is expected to thread Result through every function that can fail; throwing is reserved for genuinely unexpected states. It's not enforced by a lint rule. It is the dialect.
Six months into shipping with a coding agent, they audited the diffs the agent had merged. About a third of the new functions ignored Result entirely. The agent had reached for try/catch, returned T | null, thrown Error subclasses with descriptive messages — every one of those choices is correct in some imagined codebase. None of them was correct in this one. The code typechecked. The tests passed. Reviewers approved it because nothing in it looked wrong line by line. But the file the agent touched no longer fit the file it lived next to, and the team had quietly grown a second dialect inside their own service.
This is the failure mode I want to talk about: not bugs, not hallucinations, not lint violations — idiomatic drift. The agent ships code that compiles, runs, and passes tests, in a style your codebase does not speak. Over enough merges, the codebase bifurcates into agent-style zones and human-style zones, and the cost shows up in places no dashboard is watching.
