Agent payment check
Monitoring that knows 402 means open for business
Uptime monitoring for endpoints that sell to AI agents over x402 or the Machine Payments Protocol. Sentinel treats a 402 with a valid payment envelope as healthy, reads the price, network and pay-to address on every check, and alerts the moment any of them changes.
GET /v1/signals · from Nuremberg
HTTP/2 402 Payment Required
payment-required: eyJ4NDAyVmVyc2lvbiI6Miwi…
{ "x402Version": 2,
"accepts": [{ "network": "eip155:8453",
"amount": "10000",
"payTo": "0x325b…d430" }] }How it works
One request, one envelope, three assertions
Add the agent payment check to any HTTP monitor, on any plan. Sentinel requests the paid route from four regions on your interval, without paying. The response has to be an HTTP 402 carrying a payment envelope Sentinel can parse, or the check fails.
Optionally set the expected price in USD, the network, and the pay-to address. Each check compares the live envelope against them. The envelope itself is stored with the monitor, so you always know what the endpoint was advertising at the moment it broke.
- x402 v1 and v2, and Stripe's Machine Payments Protocol
- Expected amount, network and pay-to, each optional
- Sentinel never pays; it verifies the challenge only
Payment check settings
- Healthy status
- 402
- Expected amount
- $0.01
- Expected network
- eip155:8453
- Expected pay-to
- 0x325b…d430
- On mismatch
- Open incident + alert
Features
Built for endpoints that charge per call
402 is the healthy answer
A normal uptime check calls a paid endpoint down the moment it asks for payment. The agent payment check flips that: HTTP 402 with a valid envelope is up, and a 200, a 500 or a timeout is the outage.
The envelope is read, not assumed
Every check parses the payment challenge: x402 v2 in the payment-required header, x402 v1 in the body, or an MPP challenge in WWW-Authenticate. No envelope, or one that doesn't parse, fails the check.
Price, network and pay-to, asserted
Tell Sentinel what the endpoint should charge, on which network, to which address. A silent price change or a rotated pay-to address opens an incident from four regions, before an agent pays the wrong amount to the wrong wallet.
Why it matters
A paid endpoint fails in ways a 200 can't show
An agent-payable API is only working when it asks for the right money, on the right network, to the right wallet. A plain check can't see any of that: it either calls the 402 an outage or, once you tell it 402 is fine, stops looking at what the 402 says. This check reads the envelope every time, so the thing that changes silently is the thing you're told about.
402
The status code that counts as up. Anything else opens an incident.
3 rails
x402 v2 (header), x402 v1 (body) and MPP (WWW-Authenticate) all parse.
$0.00
Cost per check. Sentinel reads the challenge and never settles it.
4 regions
Every check runs from Ashburn, Portland, Nuremberg and Singapore.
You sell to agents
Your x402 or MPP endpoint is the storefront. A deploy that drops the envelope, a config change that moves the pay-to address, or a facilitator outage all look like uptime to a plain check. This one catches them.
Your agent depends on a paid API
Pin the price you agreed to. If the seller raises it, changes network or rotates the wallet, you find out from Sentinel before the next call settles at the new terms.
You run a registry or marketplace
Every service in Sentinel's own agent directory is verified this way, and the observed price is published beside the stated one. The same check is available on your account for your own list.
What is the agent payment check?
A sub-check for HTTP monitors that understands machine payments. Instead of treating a 402 Payment Required response as a failure, it treats a 402 with a valid payment envelope as the healthy state and verifies the envelope's contents. It exists because agent-payable endpoints are supposed to answer 402 to an unpaid request, and a normal uptime check gets that exactly backwards.
Which payment protocols does it understand?
x402 version 2, where the envelope travels base64-encoded in the payment-required response header; x402 version 1, where the envelope is the JSON body; and Stripe's Machine Payments Protocol, where the challenge is a WWW-Authenticate: Payment header. The check tries them in that order and reports which one it found.
Does Sentinel pay the endpoint?
No. The check sends an ordinary unpaid request, reads the 402 challenge, and stops there. Nothing is signed, nothing is settled, and the endpoint sees the same request an agent would send before deciding whether to pay.
What can I assert about the envelope?
Three things, each optional: the expected price in USD (atomic USDC amounts are normalized for you), the network the payment must be on, such as eip155:8453 for Base, and the pay-to address, compared case-insensitively. Leave them blank and the check only asserts that a 402 with a parseable envelope came back.
What happens when the price changes?
An incident opens with the reason, for example expected $0.01 but the envelope now says $0.05, and it alerts through every channel the monitor routes to at critical severity. When the envelope matches again, the incident resolves. The stored envelope on the monitor shows what was advertised at the time.
Is it available on the free plan?
Yes. The agent payment check is available on every plan, including free, on any HTTP monitor at the interval your plan allows. It can't be combined with keyword or JSON assertions on the same monitor, since the 402 body isn't the page an agent would receive.
Verify your paid endpoint from four regions
Free on every plan. Add the check to any HTTP monitor.
No card for the free plan · 14-day trial on paid plans · Cancel anytime