Sliding-Window Spend Guard: the $47K Loop Per-Call Caps Miss

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 In November 2025 a team woke up to a $47,000 bill from a single agent deployment. Four LangChain agents, talking to each other over A2A, and two of them — an Analyzer and a Verifier — got into a ping-pong. Analyzer hands work to Verifier, Verifier kicks it back, repeat. For 264 hours. The cost didn't spike. It escalated , week over week: $127, then $891, then $6,240, then $18,400. The author of the postmortem, Gabriel Anhaia, describes the root cause in a way I keep coming back to: the dashboard was green for eleven days, and there was no step cap, no per-conversation USD budget, no orchestrator deciding when the work was done ( dev.to/gabrielanhaia, Nov 2025 ). The dashboard showed the number. It just showed it after each call, never before the next one. A follow-up teardown by the Waxell team sharpened the line into the title of their piece: