Inspiration
AI agents are starting to spend real money. An agent books the flight, orders the supplies, renews the subscription, and when something is wrong the buyer gets two things: a charge, and the agent's word. The payment record proves money moved. It does not prove what the agent did, what it meant to buy, or whether the thing delivered matches the thing ordered. Agent work has no receipt. That is the gap Zambo exists to close, and this project takes it to the checkout: every agent purchase should end with a verifiable receipt for the work, not just a record of the money.
What it does
An AI agent buys a digital good through a PayPal Sandbox checkout, and the whole job runs inside Zambo, the verifiable work layer for AI. The agent's calls bind into one job. It creates the PayPal order and logs the order id into the job. It captures the order and logs the capture id, the amount, and the item into the same job. When the job wraps, Zambo issues an AER-1 receipt whose record binds that PayPal capture to everything the agent did.
The buyer gets one link. Here is a real one, from a recorded run of this exact demo: https://zambo.dev/run/e340c33c-ccec-458f-aa9d-fe98f4243969
Anyone can open it, read what the agent bought and for how much, and check that the saved record has not changed since it was saved. The capture id in the record can be cross-checked against the original PayPal record. One honest note: the recorded run is a rehearsal, so its PayPal leg is simulated and the receipt record says so. The job and the receipt are fully real, and with free PayPal Sandbox credentials the same single command binds a live sandbox capture instead. The code supports both modes and labels which one ran.
This is not a mockup bolted onto a slide. Zambo is a live production platform: 153 tools behind one MCP endpoint at https://zambo.dev/mcp, more than 108,000 public receipts issued, more than 186,000 downloads across 56 published packages, and 12 verified paying customers. On zambo.dev, humans check out by card or PayPal, and agents pay $0.99 in USDC on Base for a 24 hour Day Pass. The free tier gives every user 20 calls plus 1 wrap-up free every day, and picking up work in another AI is always free.
How I built it
- PayPal Orders API (Sandbox): OAuth client credentials flow, order create, order capture, using a published sandbox test card so any judge can reproduce the run without real money.
- Zambo MCP at https://zambo.dev/mcp: one session id binds the agent's calls into a job; the journal_log tool records the PayPal actions that happened outside Zambo, each with its own receipt; the handover tool wraps the job and issues the final AER-1 receipt.
- AER-1 as the receipt format: the open standard I authored, an IETF Internet-Draft closed at revision -26, with a published conformance corpus of 143 vectors and 146 checks that independent implementers have run.
- The demo itself is one readable Python file (demo.py) on purpose. The pattern is the product: any agent framework can journal its PayPal calls into a Zambo job the same way and get the same receipt at the end.
Challenges I ran into
The hard part is not the payment, it is the binding. A receipt is only worth something if it commits to the exact record of the job that produced the payment. A Zambo wrap holds only receipts from its own job; receipts from side calls are refused as not job-owned. So the PayPal steps cannot be bolted on afterward. They have to run inside the job they belong to, in order, and that discipline is what makes the final receipt mean anything. Getting the simulated rehearsal mode honest took the same care: the run output, the receipt record, and the video narration all state which mode ran, because a receipt system that blurs that line proves nothing.
What I learned
Agent commerce does not need a new payment rail. PayPal already moves the money, for millions of people, every day. What it needs is a work record a stranger can verify before anyone argues about a charge. Receipt-first design turns "the agent said it bought the right thing" into "here is the record, check it yourself". That is the sentence this whole platform is built on: get me a zambo receipt.
Accomplishments I am proud of
- A working end to end pattern, in one file, that any agent builder can copy: PayPal checkout in, verifiable receipt out.
- The receipt is real and public today, not rendered for the demo: https://zambo.dev/run/e340c33c-ccec-458f-aa9d-fe98f4243969
- The underlying platform is already in production use, with more than 108,000 receipts issued and 12 verified paying customers.
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