2.5 — Prompt Reuse: Caching, Modular Prompts, and Skills

CCAR-P Domain 2 · Task 2.5

What You Need to Know

Three mechanisms for three reuse questions:

Mechanism	Question it answers
Prompt caching	Stop a repeated prefix costing full price each time
Modular prompts	Stop ten features each keeping a drifting copy of the same policy
Skills	Occasional deep procedure without paying for it on every request

Prefix Mechanics

Order: tools → system → messages. Front-load stability: static system/policy/examples first, dynamic content last. A timestamp early in the prompt defeats the entire cache.

Cache Economics

• Writes carry a premium; longer TTL costs more
• Reads earn a steep discount
• Break-even within first read or two at volume
• “Cache everything” is not always correct (sporadic traffic can make write premium exceed savings)

Caching is not persistence, window relief, or a quality lever.

Modular Prompts

Versioned sections assembled from a shared library. One change propagates. Shared stable sections form common cacheable prefixes across features.

Skills & Progressive Disclosure

1. Metadata only (tiny fixed footprint) at startup
2. SKILL.md body loads only when triggered
3. Bundled resources cost nothing until read

Standing policy → cached system prompt. On-demand expertise → Skills. Per-request data → messages (after the cached prefix).

Exam Traps

• Marking content that changes every request as cached
• Offering “enable caching” for window overflow or quality problems
• Trimming few-shot or downsizing model to cut spend on a repeated static prefix
• Stuffing entire procedure library into every system prompt