# Anthropic — Prompt Caching Best Practices

**Source**: https://docs.anthropic.com/en/docs/build-with-claude/prompt-caching
**Fetched**: 2026-03-05

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## How It Works

1. On first request: system processes full prompt and caches the prefix once the response begins.
2. On subsequent requests with same prefix: uses cached version (much cheaper + faster).
3. Cache is checked against a cryptographic hash of the prefix content.

## Two Caching Modes

### Automatic Caching (recommended for multi-turn)
Add `cache_control: {"type": "ephemeral"}` at the **top level** of the request body.
- System automatically caches all content up to the last cacheable block.
- Moves cache breakpoint forward as conversation grows.
- Best for multi-turn conversations.

### Explicit Cache Breakpoints
Place `cache_control` directly on individual content blocks.
- Finer control over exactly what gets cached.
- Use when you want to cache specific blocks (e.g., a large document) but not others.

## Cache Lifetimes

| Duration | Cost | Availability |
|----------|------|-------------|
| 5 minutes (default) | 1.25x base input price for write | All models |
| 1 hour | 2x base input price for write | Available at additional cost |

- Cache **reads** cost 0.1x (10%) of base input price.
- Cache is refreshed for **no additional cost** each time cached content is used.
- Default TTL: **5 minutes** (refreshed on each use within TTL).

## Pricing Per Million Tokens (relevant models)

| Model | Base Input | 5m Write | 1h Write | Cache Read | Output |
|-------|-----------|----------|----------|------------|--------|
| Claude Opus 4.6 | $5 | $6.25 | $10 | $0.50 | $25 |
| Claude Sonnet 4.6 | $3 | $3.75 | $6 | $0.30 | $15 |
| Claude Haiku 4.5 | $1 | $1.25 | $2 | $0.10 | $5 |

> Note: We use Copilot subscription (flat rate), so per-token cost doesn't apply directly. But quota burn follows similar relative proportions — caching still saves quota by reducing re-processing of identical prefixes.

## Supported Models
- Claude Opus 4.6, 4.5, 4.1, 4
- Claude Sonnet 4.6, 4.5, 4
- Claude Haiku 4.5, 3.5

**Not supported**: Non-Claude models (GPT, GLM, Gemini) — caching is Anthropic-only.

## What Gets Cached
Prefix order: `tools` → `system` → `messages` (up to the cache breakpoint).

The full prefix is cached — all of tools, system, and messages up to and including the marked block.

## Key Best Practices

1. **Put static content first**: Instructions, system prompts, and background context should come before dynamic/user content.
2. **Use 1-hour cache for long sessions**: Default 5-minute TTL means cache expires between turns if idle > 5 min. Use 1h for agents with longer gaps.
3. **Automatic caching for multi-turn**: Simplest approach, handles the growing message history automatically.
4. **Minimum size**: Cache only activates for content > a certain token threshold (details not specified, but system prompts qualify easily).
5. **Privacy**: Cache stores KV representations and cryptographic hashes, NOT raw text. ZDR-compatible.

## For Our Setup (OpenClaw)
- Main session system prompt is large (~15-20k tokens) and mostly static → ideal caching candidate.
- Heartbeat turns are the same every 25-30min → if using 1h cache, heartbeats keep cache warm for free.
- OpenClaw's `cacheRetention` config likely maps to this `cache_control` setting.
- Applies to: `litellm/copilot-claude-*` models only. Does NOT apply to GLM, GPT-4o, Gemini.
