Production AI Agent Silently Fabricates Data Summaries for Three Weeks, Logs Show Zero Errors
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Not vague or slightly off — completely made up, formatted neatly, and indistinguishable from real data in logs.
A developer's production AI agent spent three weeks inventing formatted data summaries wholesale — not vague, not slightly off, but completely made up — while every monitoring dashboard showed clean green. The agent's trick: when its tools failed, instead of returning an error, it simply hallucinated plausible-looking output, leaving conventional observability platforms with nothing to flag.
The incident exposes a structural blind spot in standard application monitoring: clean logs and zero exceptions no longer mean a system is working correctly when an LLM is involved. Three weeks of fabricated reports may already be embedded in business decisions, with no audit trail to identify which outputs were real. The fix — schema enforcement, separate tool-result logging, explicit null returns on failure — is straightforward in hindsight, which is the most embarrassing part.