Claude Cache Guard — Security skill for Claude Code

claude-cache-guard

A Claude Code skill that stops **prompt-cache thrash** — the hidden cost pattern where reading one screenshot inside a long conversation rewrites the entire conversation history at **12.5× the cache-read price**.

Measured results

Real image-heavy production workload, same project / model / day, comparable work batches, before vs after applying the rules:

Metric	Before	After	Δ
Images read in main context	40 (across 22 turns)	1
Cache writes (billed at 1.25×)	18.0M tok	0.74M tok	−96%
Output tokens (work produced)	139k	164k	+18%
Quota-weighted consumption	27.6M eq-tok	3.4M eq-tok	−88%
Estimated cost	$276.80	$33.65	−88%
Consumption per unit of output	199	20.5	≈10× efficiency

Same model, same image resolution, same output quality. (Honest caveat: a real-workload before/after, not a variable-controlled A/B; the "after" also starts from a fresh context — which is itself one of the rules, R6.)

What it does

When Claude is about to read images/screenshots in a session with history — visual QA, render checks, UI verification, diagram review — the skill loads rules that make it:

1. Batch all image reads into a single message (1 image or 10 = same rewrite cost)
2. Write findings to a notes file and never re-read an unchanged image
3. Delegate vision checks to a subagent with a tiny isolated context, so the main session's cached prefix is never invalidated
4. Keep large tool outputs (build logs, file dumps, base64) out of context
5. Avoid TTL-expiry wakes and mid-session model/tool changes that nuke the cache

Plus a read-only **audit script** that ranks your recent sessions by estimated cost and flags exactly which calls thrashed the cache and why.

Install

# macOS / Linux
git clone https://github.com/0x0funky/claude-cache-guard ~/.claude/skills/cache-guard

# Windows (PowerShell)
git clone http