# Structuring Prompts for Caching — Prompt & Context Engineering

Source: https://www.skillbyai.com/en/prompt-context-engineering/c-cache

> Stable content first, variable content last.

## Prefix caching rewards stable prefixes

Several providers offer **prompt caching**: a repeated, identical **prefix** of the prompt is processed once and reused, which lowers cost and latency on later calls. Caching works on the **exact prefix**, so a single changing value near the top (a timestamp, a user name, a request ID) breaks it for everything after. Put system rules, tool definitions and fixed examples first, then per-request content such as documents, the date and the question. Check your provider's rules for minimum length, how long a cache entry lives and how cache hits are reported.

## Fast, parseable, reproducible

Structure the prompt so it can be cached, the output can be checked, and every change can be traced.

![Three ideas: cache, contract, version.](assets/figures/prompt-context-engineering/section-6-map.svg) — Figure 6.1 — Cache, contract and version.

## How a timestamp at the top breaks the shared prefix, run

I ran this with plain Python 3 (standard library only); the data is made-up example data. Two prompts with a timestamp first share only 10 of 2,462 characters; moving the timestamp after the stable instructions and examples makes them share 2,450 characters.

```python
import os
def common_prefix(a, b):
    return len(os.path.commonprefix([a, b]))
system = "You are a support assistant. Rules: ... " * 40
examples = "Example 1 ... Example 2 ... " * 30
bad_1 = "Time: 10:01\n" + system + examples + "Q: refund?"
bad_2 = "Time: 10:02\n" + system + examples + "Q: shipping?"
good_1 = system + examples + "Time: 10:01\nQ: refund?"
good_2 = system + examples + "Time: 10:02\nQ: shipping?"
print("dynamic first : shared prefix", common_prefix(bad_1, bad_2), "of", len(bad_1), "chars")
print("stable first  : shared prefix", common_prefix(good_1, good_2), "of", len(good_1), "chars")
```

Output:

```
dynamic first : shared prefix 10 of 2462 chars
stable first  : shared prefix 2450 of 2462 chars
```

## Watch the cache-hit metric

Log the cached-token count the API reports; a sudden drop usually means someone added dynamic text to the top of the prompt.

**Quiz:** Where should a per-request timestamp go to keep caching effective?

- [ ] Inside every example
- [ ] At the very top of the prompt
- [x] After the stable instructions and examples
- [ ] In the tool definitions

*Answer:* After the stable instructions and examples. Caching reuses the exact identical prefix.