Why Claude Code Token Costs Explode: The Definitive Context Engineering & Prompt Caching Practical Guide for Opus 5.5 and Fable 5.1
An authoritative September 2026 technical guide analyzing Anthropic’s Claude Opus 5.5, Sonnet 5, and Fable 5.1 within Claude Code, contrasting with OpenAI’s GPT-6 Sol, dissecting KV cache vs API prompt caching, introducing Cache Diagnostics, and presenting field context engineering protocols.