Architecture Development Notes

Sep 4, 2026 · Artificial Intelligence

75% Cheaper Cache Reads: Why Long-Running Agent Costs Now Depend on Prefix Stability

Anthropic's Fable 5.1 reduces cache read pricing from $1 to $0.25 per million tokens, shifting long-running agent cost bottlenecks from output to repeated stable prefix reads, making prefix stability, cache breakpoint placement, TTL tuning, and hit-rate observability critical architectural levers for cost control.