Endogenous Yardstick Drift
Endogenous yardstick drift occurs when the system that is being optimized can influence the ruler by which future versions are measured. The phrase is report-derived terminology used here as an analytical label.
How drift appears
Drift can occur when evaluator prompts change, benchmark weights move, hidden tests leak, model judges share training data with candidates, release pressure weakens thresholds, or success metrics overfit to a shortcut. It does not require malicious intent. It only requires a feedback loop where measurement changes in the direction favored by optimization.
Why it is dangerous
A stable model with a stable test can be compared over time. A changing model evaluated by a changing evaluator produces a moving baseline. Apparent improvement may be real, or it may be the ruler changing.
Control pattern
Keep frozen reference suites for regression comparison alongside live adaptive tests. Preserve evaluator versions, hidden-test custody, deterministic validators, negative controls, independent evidence stores, and human-owned hard constraints. The evaluator must have its own rollback path.
Reference control
Use frozen reference suites alongside evolving evaluators so improvement claims can be compared against a stable ruler.