Agent Context Cost Review: Accounting for Every Tool Call
This review ignores capability and runs the numbers instead, on the context cost of agentic long-context and multi-turn trajectories (explicitly scoped apart from our 8-26 image-model capability review and the batch-22 image cost piece). It opens with a reproducible per-turn cost formula and makes the point that the resident prefix is the portion you re-pay on every single turn. It then compares five levers - prefix caching, KV Cache compression and sparse attention, context compression, tool-output trimming, and switching trajectory replay to incremental commit - across payoff magnitude, implementation cost, risk and fit, with a five-lever comparison table plus a cost-structure table for three scenarios (a ten-tool-call single task, a long-trajectory coding agent, and batch offline work), then ranks the levers by scale: individual, small team, and bulk. Every unit price is either symbolic (P_in / P_out / P_cache) or marked "refer to the official pricing page"; magnitude judgments are labeled engineering estimates, never passed off as benchmarks. Cold take: a vendor's "cost down X%" is usually the optimum under one specific workload - cache hit rate, context distribution and tool-output length decide your bill, so instrument your own stack rather than trusting launch numbers.