Borrowing it
Nothing to install: this file belongs to flightlesstux/prompt-caching. Take a copy, put it at the same path in your own repository, and replace the rules that are about this project with yours.
curl -O https://raw.githubusercontent.com/flightlesstux/prompt-caching/main/CLAUDE.mdgit clone --depth 1 https://github.com/flightlesstux/prompt-cachingWrote this? Show the measurements
A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.
[](https://agentmods.dev/instructions/flightlesstux/prompt-caching/claude-md)<a href="https://agentmods.dev/instructions/flightlesstux/prompt-caching/claude-md"><img src="https://agentmods.dev/badge/instructions/flightlesstux/prompt-caching/claude-md.svg" alt="Measured on agentmods" height="20"></a>What 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 5.1 | $0.01508 | $0.01508 |
| Opus 5 | $0.00754 | $0.00754 |
| Sonnet 5 | $0.00302 | $0.00302 |
| Haiku 4.5 | $0.00151 | $0.00151 |
Grade B, and why
prompt-caching CLAUDE.md scanned grade B with 2 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 8d 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.
Reads agent configuration directoriesmediumAgent snooping
.claude/, .codex/, .gemini/ hold keys, settings and other credentials a mod has no legitimate need for.
In `~/.claude/settings.json`: Runs shell commandslowCapability
Expected in a hook, worth knowing in a rule or an instructions file.
| Shell commands | `sh -c ...` | avoid; use Node `fs`/`child_process` with `shell: true` only when necessary | How it starts
The opening of the file, as written. The whole thing — 146 lines — stays where its author put it; the contents beside it link to each section on GitHub.
CLAUDE.md
This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
Project Overview
This is a Claude Code MCP plugin that automatically applies Anthropic's prompt caching to reduce token costs in coding workflows. It wraps the Anthropic SDK to inject cache_control markers intelligently and reports cache savings.
Core value proposition: In agentic coding sessions, large system prompts, tool definitions, and document context are re-sent on every turn. This plugin identifies these stable, reusable segments and marks them for caching, achieving up to 90% cost reduction on repeated content.
Engineering Mindset
All code in this repo follows BEST_PRACTICES.md. Read it before writing any code. Key rules that affect every file:
- stdout is the MCP wire — never
console.log()in tool handlers, usestderr - Every tool handler must catch all errors and return
{ isError: true, ... }— never throw - Every tool must define
outputSchema— no schema, no merge - The irony rule: this plugin must never flood context itself — tool responses must be small and structured
Cross-Platform Requirements
This plugin runs on Linux, macOS, and Windows. Every code path and every test must pass on all three. CI runs a 9-job matrix (3 OS × 3 Node versions).
Rules — always follow these
| Topic | Wrong | Correct |
|---|---|---|
| Path construction | dir + '/' + file |
path.join(dir, file) |
| Home directory | ~ or process.env.HOME |
os.homedir() |
| Temp directory | '/tmp' |
os.tmpdir() |
| Config path | hardcoded string | path.join(os.homedir(), '.claude', 'settings.json') |
| Line endings | \r\n |
\n (.gitattributes enforces LF on all platforms) |
| Process signals | SIGTERM only |
SIGINT + SIGTERM (SIGTERM is not available on Windows) |
| File permissions | fs.chmod(...) alone |
guard with if (process.platform !== 'win32') |
| Shell commands | sh -c ... |
avoid; use Node fs/child_process with shell: true only when necessary |
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.
- 8d ago First seen · 146 lines · 1,508 tokens per session scan B fa9c50735b64
prompt-caching CLAUDE.md is an instructions file published in the GitHub repository flightlesstux/prompt-caching (135 stars, last pushed 2mo ago), licensed MIT. It adds 1,508 tokens to every session, about $0.0075 per session on Opus 5. A static security scan graded it B with 2 findings (reads agent configuration directories, runs shell commands). No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-30.
Other instructions, from other repositories
codex AGENTS.md
AGENTS.md instructions for openai/codex, covering rust/codex-rs, the codex-core crate, code review rules, crate api surface and model visible context.
vscode buildNext.instructions.md
Working notes and architecture documentation for the new esbuild-based build system in build/next. Use when making changes to the new build pipeline (transpile/bundle commands, NLS plugin, source-map handling, resource copying, or self-hosting watch tasks).
next.js AGENTS.md
AGENTS.md instructions for vercel/next.js, covering next.js development guide, codebase structure, monorepo overview, core package: packages/next and other important packages.
langchain AGENTS.md
AGENTS.md instructions for langchain-ai/langchain, covering global development guidelines for the langchain monorepo, corridor security analysis, project architecture and context, monorepo structure and development tools & commands.
vscode oss-third-party-notices.instructions.md
Instructions for microsoft/vscode, covering vs code oss third-party-notices pipeline, architecture, pipeline flow in ci, applying the notice (cutover) and fallback chain (never fail the build).
spec-kit AGENTS.md
AGENTS.md instructions for github/spec-kit, covering agents.md, about spec kit and specify, quickstart — add a new integration in 5 steps, integration architecture and integrationmanifest — file tracking.