What is Anchor drift?

Anchor drift is the shift in which sentence inside a rerank-surviving chunk wins the anchor slot across sessions, even when the chunk itself has not been edited. Mid-2026 cohort: roughly 28% of anchor sentences on rerank-surviving chunks shift within a rolling 8-week window across the general-purpose engines, with Perplexity running the highest drift rate (roughly 38% within 8 weeks) and the other engines running 18–24%. Four observable causes require different fixes: substrate re-embedding (the engine re-scores sentence relevance under a revised embedding model), query-intent drift (the head-query's intent distribution shifts and re-weights sentence-level scores), competitor sharpening (a competitor's sharper anchor sentence pulls click-through weight on the rendered answer surface without shifting the within-chunk selection), and Perplexity-style anchor rotation (some engines actively rotate anchor sentences to test user engagement). The anchor-drift audit captures anchor sentence identity weekly per priority sub-query per engine, computes the drift rate on a rolling 4-week window, and diagnoses the cause before scoping the fix — anchor-slot losses attributed to the wrong cause produce zero lift and burn editorial bandwidth.