Borrowing it
Nothing to install: this file belongs to btafoya/mailsandbox. 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/btafoya/mailsandbox/main/CLAUDE.mdgit clone --depth 1 https://github.com/btafoya/mailsandboxWrote 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/btafoya/mailsandbox/claude-md)<a href="https://agentmods.dev/instructions/btafoya/mailsandbox/claude-md"><img src="https://agentmods.dev/badge/instructions/btafoya/mailsandbox/claude-md/github.svg" alt="Measured on agentmods" height="20"></a>Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.
<a href="https://agentmods.dev/instructions/btafoya/mailsandbox/claude-md"><img src="https://agentmods.dev/badge/instructions/btafoya/mailsandbox/claude-md.svg" alt="Reviewed on agentmods" width="80" 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 | $0.01392 | $0.01392 |
| Opus 5.5 | $0.00557 | $0.00557 |
| Sonnet 5.5 | $0.00278 | $0.00278 |
| Haiku 4.5 | $0.00139 | $0.00139 |
Grade A, and why
mailsandbox CLAUDE.md 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 2mo 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 — 157 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
MailSandbox is an email and SMTP testing tool for developers. It captures emails via SMTP (port 1025), provides a web UI (port 8025) for viewing/testing, and includes a REST API for automation.
Tech Stack: Go 1.24.3 backend, Vue 3 + Bootstrap 5 frontend, SQLite storage, WebSockets for real-time updates
Essential Commands
Running MailSandbox
# Run locally with default settings
go run main.go
# Run with custom ports
go run main.go --smtp 0.0.0.0:1025 --ui 0.0.0.0:8025
Development Workflow
# Frontend development (watch mode)
npm install
npm run watch
# Build for production
npm run package
CGO_ENABLED=0 go build -ldflags "-s -w" -o mailsandbox
Testing
# Run Go tests for specific packages
go test ./internal/storage ./server -v
# Run all tests (use -p 1 to avoid parallel execution issues)
go test -p 1 ./internal/storage ./server ./internal/smtpd ./internal/pop3 ./internal/tools ./internal/html2text ./internal/htmlcheck ./internal/linkcheck -v
# Run a single test
go test ./internal/storage -v -run TestSpecificFunction
Code Quality
# Format Go code (REQUIRED before commits)
gofmt -s -w .
# Check Go formatting
gofmt -s -d .
# Lint JavaScript/Vue
npm run lint
# Fix JS/Vue linting issues
npm run lint-fix
Architecture
/internal/ - Private packages (core business logic)
/smtpd/ - SMTP server implementation
/storage/ - Database layer (SQLite/RQLite)
/auth/ - Authentication logic
/htmlcheck/ - HTML compatibility checking
/linkcheck/ - Link validation
/pop3/ - POP3 server (optional)
/server/ - HTTP server and API
/apiv1/ - REST API v1 endpoints
/ui-src/ - Vue.js source code
/ui/ - Compiled/embedded UI assets
/websockets/ - WebSocket handlers for real-time updates
/config/ - Application configuration
main.go - Application entry point
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.
- 2mo ago First seen · 157 lines · 1,392 tokens per session scan A 3fb72e217e81
mailsandbox CLAUDE.md is an instructions file published in the GitHub repository btafoya/mailsandbox (8 stars, last pushed 1y ago), licensed MIT. It adds 1,392 tokens to every session, about $0.0056 per session on Opus 5.5. 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 2026-08-31.
Other instructions, from other repositories
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.
vscode oss-third-party-notices.instructions.md
Instructions for microsoft/vscode, covering vs code oss third-party-notices pipeline, architecture, pipeline flow in ci, cache eligibility and rollout and applying the notice (cutover).
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 copilot-instructions.md
Copilot instructions for microsoft/vscode, covering vs code copilot instructions, project overview, root folders, core architecture (src/ folder) and built-in extensions (extensions/ folder).
deno copilot-instructions.md
Copilot instructions for denoland/deno, covering deno development guide for github copilot, network access, high level overview, key directories and key files to understand first.