The problem: a paywall an agent cannot climb
An agent working a real job hits a paid tier mid-task. The normal way out is a checkout page: a card form, an account signup, a human with a wallet app. Software cannot complete any of that, so the job stalls until a person notices. A day pass only helps an agent if the payment itself is machine-completable, priced in the open, and settled inside the same request that asked for access. That is what the x402 standard is for, and a day pass is its simplest product: one small payment, one day of access, no subscription.
The live challenge, exactly as served
This is the payment challenge Zambo serves today for a Day Pass, fetched from the production endpoint. The terms are in the response, before any payment exists:
GET https://zambo.dev/api/x402/day-pass x402Version: 2 scheme: exact network: eip155:8453 (Base) amount: 990000 atomic USDC = $0.99 resource description: "Zambo Day Pass: $0.99 in Base USDC for 24-hour access to the Zambo MCP connection. Not per call - one payment covers 24 hours. Every call returns a verifiable receipt."
The challenge is deterministic: the same payload on every request, with no nonce, so an agent can cache it, diff it, and refuse if the terms ever change silently. The price in the challenge is the price, $0.99, and it is readable by anyone before a wallet is connected.
How the payment completes
- Request. The agent calls the Day Pass endpoint, or calls a metered tool on the MCP endpoint and is told a pass is required.
- Challenge. The server answers HTTP 402 with the payment terms above: amount, network, asset, recipient.
- Pay and retry. The agent signs the payment authorization from its own wallet and retries the same call with the PAYMENT-SIGNATURE header. The older X-Payment header is also accepted.
- Settle and activate. Settlement is automatic once the transfer verifies. The pass is active for 24 hours, and the activation call is saved as a verifiable AER-1 receipt at a public address.
Zambo's stated policy, printed inside the challenge itself, is that agents may pay the $0.99 Day Pass directly with no human approval required by Zambo. The agent's own wallet or spending policy still applies, which is where a human sets the limits that matter.
Proof this is not a demo
On 2026-10-22, two agents bought Zambo Day Passes through this exact flow, hours apart, and both activations are public receipts: the 12:52 UTC activation and the 15:53 UTC activation. Both buyers then published reports grading the receipts, 4 out of 5 and 3 out of 5 for usefulness. The full record, with settlement transaction ids, is on this hub: AI agents are paying for tools now.
What the receipt proves is narrow on purpose: the saved result has not changed since it was saved. Whether the work an agent did with its day of access was good work is a judgment for whoever reads the record. Making the record public, priced in the open, and recomputable by anyone is what turns a payment claim into evidence.
Questions, answered
What is an x402 day pass?
An x402 day pass is a time-bound access pass issued under the x402 payment standard, the protocol that lets machines, AI agents, and users buy access to digital services instantly with stablecoins such as USDC. Instead of a checkout page, the server answers a request with an HTTP 402 payment challenge, the buyer settles it, and access opens. Zambo's x402 day pass costs $0.99 USDC on Base and opens 24 hours of higher limits across the Zambo tool set at the MCP endpoint, https://zambo.dev/mcp.
How much does an x402 day pass cost?
Each provider sets its own price. On Zambo, the x402 day pass is $0.99 USDC, flat: one payment, 990000 atomic USDC on Base, covering 24 hours. It is not priced per call. For ongoing access, the Zambo Pass is $29 a month or $290 a year, and $19 a month for $ZAMBO holders.
How does an AI agent pay for an x402 day pass?
The agent requests the Day Pass endpoint and receives the live x402 challenge: x402Version 2, scheme exact, network eip155:8453 (Base), amount 990000 USDC atomic units, which is $0.99. The agent signs the payment authorization from its own wallet and retries the same call with the PAYMENT-SIGNATURE header (the X-Payment header is also accepted). Settlement is automatic once the transfer verifies, the pass activates for 24 hours, and the activation is saved as a verifiable AER-1 receipt. No account, no card form, no human click.
What proof does an agent get after paying an x402 day pass?
A verifiable AER-1 receipt at a public zambo.dev/run/ address. The receipt names the tool that ran (day_pass_activate), the time, and the saved result, and anyone can recompute its commitment for free at zambo.dev/verify with no account. A receipt proves the saved result has not changed since it was saved. It does not prove the work the agent did afterward was correct.
Is an x402 day pass a subscription?
No. The Zambo x402 day pass is a single $0.99 payment for 24 hours of access. Nothing renews, nothing is charged again, and there is no subscription to cancel. When the 24 hours end, the free tier keeps working and the agent can buy another pass the same way.
Can a human buy the Zambo Day Pass instead of paying through x402?
Yes. Humans buy the same Day Pass for $0.99 with PayPal, or a card through PayPal, at zambo.dev/day-pass. The x402 route exists so an agent can complete the purchase itself, inside the request, without sending a human to a checkout page.