I Ran 300 LLM Drift Checks: Here's the Distribution of Failure Patterns I Found

After running 300 automated drift checks on production LLM deployments, I have enough data to say something statistically meaningful about where models fail. The Dataset 300 drift checks across 5 different LLM-powered production systems Checks run every 6 hours over 6 weeks Models tested: GPT-4o, Claude 3.5 Sonnet, Gemini 1.5 Pro, GPT-4o-mini Metrics: Cosine similarity against baseline, exact string match rate, JSON parse success rate What I Found Category 1: Format Drift (47% of failures) JSON outputs break most often. The model decides to add a preamble, change indentation, or re-order fields. Example of actual drift caught: Baseline: {"status": "ok", "value": 42} Drifted: "Based on the analysis, the result is: {\"status\": \"ok\", \"value\": 42}" This is the silent killer. The JSON still parses, but downstream code expects the clean format. Category 2: Verbosity Drift (31% of failures) Responses getting longer for no reason. More hedging language, more caveats, longer explanations.