AI API cost review path for prompt caching and tool calls

Prompt caching helps only when repeated requests keep a long shared prefix stable enough for the provider to reuse it.

The first design rule is to put static material first. Product rules, response format, examples, tool definitions, and JSON schemas should stay at the top when they are reused. User-specific text, timestamps, fresh search results, and one-off instructions should come later. OpenAI describes cache hits as exact prefix matches, and Anthropic explains cache breakpoints over the prompt prefix. That means a small change near the beginning can break the useful match.

The second rule is to log what changed. A team should record prompt version, model, static prefix hash or version name, request purpose, prompt tokens, cached tokens if returned, output tokens, latency, and cost. Without a log, a cheaper month may be confused with lower traffic, shorter prompts, or a model change.

The third rule is to avoid accidental churn. Do not insert current time, random IDs, per-user notes, or changing examples before the stable section. If a request needs those details, place them after the reusable prefix. The same applies to tool lists: reordering or renaming tools can change the prefix.

The fourth rule is to separate cost from answer quality. Caching can reduce repeated input processing cost and latency, but it does not make a weak prompt more accurate. If the answer is wrong, inspect retrieval, instructions, examples, and evaluation cases separately.

A simple checklist is enough for most teams: stable prefix first, dynamic tail last, version the prompt, log cached tokens, compare latency, and keep a small test set. The goal is not to force caching into every request. The goal is to recognize repeated long prompts and stop paying full processing cost when the provider can reuse the shared prefix.

How to structure repeated AI API prompts so caching can actually help

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