Sliding-Window Spend Guard for AI Agents: Catch the $47K Loop Per-Call Caps Miss
A sliding-window spend guard sums what your agent has spent over the last N minutes and refuses the next call before it dispatches — which is the thing a per-call cap can't do. A per-call cap asks "is this one call too expensive?" The runaway loops that empty a budget are built from calls that each pass that check. The damage lives in the sum, not in any single call.
In short: a sliding-window spend guard tracks a trailing window of calls and blocks the next one when cumulative spend or a repeated near-identical call breaches a per-window rule. In my run it stopped an Analyzer-Verifier ping-pong at call 12, $45.80 in, after a naive per-call $5 cap let all 12 through. Stdlib, keyless, runs in seconds.
AI disclosure: I wrote window_guard.py with AI assistance and ran it myself before publishing. Every number in the output blocks below is pasted from a real run of that script on a fixture I'll show you. The $47K incident is someone else's, and I link the postmortem next to it. I label which is which.
A $47K agent loop where every single call was fine
