How do agent loops fail?

Agent loops fail in a small number of recognisable ways, and all five are avoidable once you know their shape. Knowing them is more useful than any individual loop, because every new loop you write will fail in one of these directions first.

Runaway cost. The loop has no budget ceiling, hits a case it cannot resolve, and retries until something external stops it. The fix is a hard cap the loop checks before each cycle, plus a warning threshold below it. A loop that can spend without limit will eventually find the case that makes it.

Scope creep. The agent notices an adjacent problem and fixes that too. The output is larger than the brief, and now the diff is too big to review properly, which means it gets approved unread. The fix is a prohibition list in the goal prompt and a verifier that fails when the work touches anything outside its scope.

The checker that rubber-stamps. The most common failure and the hardest to notice, because everything looks like it is working. A verifier asked to judge quality will approve, and a verifier run in the same session as the maker will agree with its own reasoning. The fix is a fresh session, mechanical checks only, and a deliberately broken test case proving the checker can fail.

The silent no-op. The loop runs, finds nothing to do, and reports success. Over weeks this looks identical to a loop that is working, and the difference only surfaces when someone checks the output by hand. The fix is to have the loop report what it examined, not only what it changed: “13 pages checked, 0 needed changes” is a different statement from “done”.

The satisfiable stopping condition. The loop stops early because the condition was loose enough for the agent to meet immediately. “Improve the page” is met by any change at all. The fix is a condition naming a count, a comparison, or an exhaustion, which the agent cannot satisfy by deciding it is finished.

Four of the five are failures of the loop file, not the model. A better model runs a bad loop more confidently. This is why the file is the artifact worth improving, and why every loop in the catalog publishes its verifier prompt alongside its goal prompt: the checker is the part that decides whether the rest can be trusted.

VibeMarketers publishes one verified marketing agent system every week, with the installable files.