Fable 5 Prompt Caching: Slash 1M-Token Codebase Costs

Anthropic shipped Claude Fable 5 on June 9, 2026 with a 1,000,000-token context window and up to 128k output tokens. Stripe reported it ran a codebase-wide migration across a 50-million-line Ruby repo in a day, work they estimated at two months by hand. The catch nobody mentions in the hype threads: feeding a model that much context is expensive. At $10 per million input tokens, dumping a 400k-token codebase into every request burns $4 of input on each call before the model writes a single line.

Prompt caching is the fix. You cache the big, unchanging part of your prompt once, then every follow-up request reads it back at roughly one tenth of the input price. This guide shows you exactly how to wire that up against Fable 5: load a real codebase as context, set a cache breakpoint, reuse it across a loop of questions, read the usage counters to prove the cache hit, and handle Fable 5's Opus 4.8 safety fallback. You will finish with a working codebase Q&A agent and a clear picture of what it costs.