---
title: "An agent should never hold the key it's using"
date: 2026-07-03
summary: "You want an AI agent that can actually do things - call APIs, touch real data. You also don't fully trust it. The resolution isn't a better sandbox; it's making sure the agent never possesses a credential at all. A broker holds the keys, mints short-lived capabilities, and gates every write behind a human. Here's the pattern."
tags: ["security", "ai-agents", "architecture", "zero-trust", "homelab"]
draft: false
hero: "/blog/broker-pattern.webp"
heroAlt: "An untrusted agent reaches through a sealed gate to ask; on the far side, a guardian holds a ring of keys the agent can never touch."
---


The previous post put untrusted code in a hardware-isolated VM, and ended on a caveat: isolation contains an
*escape*, but it does nothing about an agent **misusing a tool it was legitimately given**. If you hand an AI
agent a database credential so it can be useful, a single bad decision - a prompt injection, a confused chain
of reasoning - spends that credential. The sandbox did its job perfectly and you still got robbed, through the
front door you built.

So the real question isn't "how do I isolate the agent?" It's "how does the agent get work done *without ever
holding a key*?"

## The agent holds a capability, not a credential

The pattern is to put every credential, every tool, and every model endpoint **behind a broker**, and give the
agent only a *capability to ask*. The agent never sees a token. It calls the broker; the broker holds the real
credential, decides whether the request is allowed, and - if it is - performs the action itself and returns the
result. The key never leaves the broker.

That one inversion changes the threat model completely. A fully compromised agent can now do exactly one thing:
**ask**. And asking is answered by something it can't reach, can't impersonate, and can't bypass.

<!-- DIAGRAM: untrusted sandbox -> cred-broker (mint capability) -> mcp-broker (verify + run tool with held cred) -> human-approval gate -> downstream. Agent never touches the credential. -->
![The two-tier broker flow: capability in, action out, credential never exposed](/diagrams/broker-pattern.svg)

## Two clusters, two brokers

The trust boundary is physical, not just logical. The **untrusted executor** (the Kata sandboxes) lives on one
cluster; the **trusted tier** (the brokers, the model gateway, the real credentials) lives on a *separate*
cluster. A total compromise of the executor still can't reach the brokers' secrets except across a policed
network link - there's no shared kernel, no shared API server, nothing to pivot through.

On the trusted side there are two brokers, deliberately split:

- A **credential broker** validates the sandbox's identity (a short-lived, signed token unique to the task) and
  mints a **capability** - a cryptographically signed, scoped, single-use, expiring grant. Not a credential. A
  *permission to ask for one specific thing*.
- A **tool broker** takes that capability, verifies the signature, the scope, and the one-time nonce, and only
  then runs the requested tool - using a credential *it* holds. The result comes back; the credential doesn't.

## A human gates every write

Reads are one thing. For anything that *changes the world* - creating, deleting, sending - the broker doesn't
just decide on policy. It **stages** the action and pings a human: an Approve/Deny prompt on my phone, carrying
a one-time token bound to the exact task, method, and arguments. Tap approve and it executes; tap deny, or
ignore it, and it doesn't. The gate is **fail-closed**: a timeout is a denial, a replayed token is rejected, an
unknown method is rejected. The default, always, is *no*.

The load-bearing principle underneath all of it: **enforcement that has to survive a compromised agent lives at
the broker, never at the agent or the orchestrator.** Approval, scope, budgets, write-authority - none of it
lives anywhere the agent's reasoning can touch. The agent can be wrong, jailbroken, or outright hostile, and the
worst-case is still just *a request that gets refused*.

## Useful and safe at the same time

It's tempting to think you have to choose: give the agent real power and accept real risk, or lock it down so
hard it can't do anything. The broker pattern is how you get both. The agent is *useful* - it can call real
tools against real systems. It's *safe* - it never holds a key, every write waits on a human, and the moment
something goes wrong, the blast radius is a denied request, not a spent credential.

Run agents like you'd run any other untrusted input: assume it's compromised, and make sure that assumption is
*boring*.

*Live as the trusted tier of a two-cluster AI-agent platform: a credential broker and a tool broker holding the
keys, a phone-based human-approval gate on every write, and an agent that - by construction - never sees a
secret.*
