# Prompt caching: what qualifies and which TTL to pick

1. Cache static, repeated content: tool definitions, the system parameter, few-shot examples, long reference documents. These belong before the breakpoint.
2. Do not try to cache the varying tail: the current user message, per-request data, anything with a timestamp. Only the stable prefix is eligible.
3. Cache the top-level block, not nested content. Anthropic's docs give citations as the example: citation content blocks themselves cannot be cached directly; cache the top-level document blocks that serve as the citation source material instead.
4. Default TTL is 5 minutes, refreshed at no cost on every use. Use it for conversational or bursty workloads where the same prefix repeats within minutes.
5. The 1-hour TTL (`{"cache_control": {"type": "ephemeral", "ttl": "1h"}}`) is priced at 2x the base input token price. Use it only for prefixes reused across a long session (long documents, persistent system instructions) where the 5-minute window keeps expiring.
6. Combining explicit breakpoints with automatic caching is supported: the automatic behavior handles the rest while your explicit breakpoints pin the sections you care about.

Failure modes this prevents: paying 2x for a 1-hour TTL on a prefix used twice; trying to cache nested citation blocks and getting no hits; caching per-request data that invalidates the prefix every call.