The AI agent can run `echo $API_KEY` all it wants, but the value is only a placeholder which is useless outside the system, and only the proxy service which the agent cannot directly access, will replace the placeholder with the real value and return the result of the network call. Furthermore, the replacement will happen within the proxy service itself, it does not expose the replaced value to memory or files that the agent can access.

It's a bit like taking a prepaid voucher to a food truck window. The cashier receives the voucher, checks it against their list of valid vouchers, records that the voucher was used so they can be paid, and then gives you the food you ordered. You as the customer never get to see the exchange of money between the cashier and the payment system.