Your APIs Assumed One Human at a Time. Parallel Agents Broke the Contract.
A backend engineer I know spent a Tuesday afternoon staring at a Datadog graph that had never spiked before: the per-user 429 counter on their internal calendar service. The customer complaining had not changed their behavior. They had simply turned on the assistant feature, which now spawned eight planning threads in parallel against the same calendar API every time the user said "find me time next week." The rate limiter — a perfectly reasonable 60 requests per minute per user, written years ago against a UI that physically could not click that fast — was firing within the first three seconds of every request and silently corrupting half the assistant's responses.
The rate limit was not the bug. The contract was the bug. That backend, like most internal services written before 2024, had a quietly enforced assumption baked into every layer: one user means one stream of activity, paced by a human's reaction time, with one cookie jar, one CSRF token, and one set of credentials that could be re-prompted if anything went wrong. Agents shred all five of those assumptions at once, and the failures show up as a constellation of unrelated incidents — 429 storms, last-write-wins corruption, audit logs you can't subpoena, re-auth loops that hang headless workers — that nobody connects until the pattern is named.
The shorthand I have been using with platform teams is this: every backend you own has an undocumented contract with its callers, and that contract was negotiated with humans. Agents are now showing up to renegotiate. You can either do the renegotiation deliberately, in code review, or you can do it during your next incident.
