Getting it into your agent
One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.
npx agentmods add skills/archive228/loopkit/prompt-cachingnpx skills add Archive228/loopkit --skill prompt-cachinggit clone --depth 1 https://github.com/Archive228/loopkitWhat it costs to keep this loaded
Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.
| Model | Per session | Once invoked |
|---|---|---|
| Fable 5 | $0.00044 | $0.00688 |
| Opus 5 | $0.00022 | $0.00344 |
| Sonnet 5 | $0.00009 | $0.00138 |
| Haiku 4.5 | $0.00004 | $0.00069 |
Grade A, and why
prompt-caching scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 2d ago.
A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.
Nothing flagged
None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.
How it starts
The opening of the file, as written. The whole thing — 56 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Prompt Caching
Every turn of a Plan→Act→Verify loop resends the same system prompt, the same tool definitions, and (usually) the same reference docs. Without cache breakpoints you pay full input price on all of it, every turn. With them, cached reads cost ~10% of the write.
Where to put breakpoints
Cache from the top of the prompt down. The cache is prefix-matched — a break in the middle invalidates everything after it.
- System prompt — mark the end of it as a breakpoint if it's >1024 tokens (Sonnet/Opus) or >2048 (Haiku).
- Tool definitions — cache immediately after, if the tool set is stable across the loop.
- Large stable docs — repo map, style guide, spec — before any turn-specific user text.
- User message stem — only if the same preamble repeats every turn.
Everything past the last breakpoint is billed fresh every turn. That's fine — that's where the changing content goes.
TTL choice
- 5-minute cache (default) — for tight loops where turns are seconds apart. Free to write.
- 1-hour cache — for slow loops (human in the loop, background jobs). Write cost is higher; break-even is ~2 hits.
Pick 5m unless you know turns are minutes apart.
Staleness rules — cache invalidation is silent
- Any byte change above the breakpoint invalidates the cache from that point.
- Reordering tools or messages counts as change.
- Trailing whitespace counts as change.
- A different model version counts as change.
If cost isn't dropping, log the cache-hit metric. Do not assume.
When NOT to cache
- Prompt <1024 tokens — below the minimum block size, no savings.
- One-shot calls — no reuse, cache write is wasted.
- Highly dynamic system prompt (per-user templating) — cache misses will exceed hits.
Red flags
- Cost graph flat after adding breakpoints — you're invalidating on every turn. Diff two consecutive requests byte-for-byte above the breakpoint.
- Breakpoint after user message — pointless; the user message changes every turn.
- Four+ breakpoints — max is four; extras are ignored silently.
- Caching a prompt that gets edited mid-session — one edit above the cut wipes all downstream savings.
What this file has done since we first saw it
Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.
- 2d ago First seen · 56 lines · 44 tokens per session scan A d9b254b4c21b
prompt-caching is a skill published in the GitHub repository Archive228/loopkit (753 stars, last pushed 1mo ago), licensed MIT. It adds 44 tokens to every session and 688 once invoked, about $0.0002 per session on Opus 5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-30.
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