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One payment, step by step

7 minutes. Checked 14 October.

In this lesson

The program asks, gets a 402 with the price, signs a payment and sends the same request again.
A checker, called a facilitator, verifies the payment and moves the money, so the seller runs no blockchain software.
Money moves only at the end, once the data is ready. If the server fails, the program pays nothing.
The signature fixes the amount and who gets paid, works once and soon expires.

A farmer sells rainfall

In this lesson you play a program buying one rainfall reading, and see exactly when the money moves.

A farmer logs rainfall every hour and sells each reading for $0.001. A weather program planning irrigation for another farm needs the latest one. It has never visited the farmer’s site, has no account there and holds no API key.

Who takes part

Every x402 payment has the same four parties.

You, the client: here, the weather program. You ask for the reading and pay.
The site, or resource server: the farmer’s server. It sets the price and serves the reading. A paid tool that an AI assistant calls over MCP can play this part too, and is paid the same way.
The checker: a service the site uses to check payments and move the money, so the farmer runs no blockchain software. Its real name is the facilitator. Coinbase runs the one that handles most payments on Base.
Base: the network where the USDC actually moves.

Buy one reading yourself

You’re the weather program, with a small USDC budget.

Money moved so far$0.000

You ask for the reading
The agent asks for the latest rainfall reading near a farm. It sends an ordinary request, with no payment and no API key.
Show the real messageHide the real message
```
GET /v1/rainfall?lat=52.62&lon=1.29 HTTP/1.1
Host: rain.example.com
Accept: application/json
```
Header names and message shapes follow the x402 v2 spec. Long lines are cut short. The seller, wallets, signature and transaction hash are made-up examples; the USDC contract address and the Base network ID are real.

The site answers 402, with the price

The server answers 402 Payment Required. The PAYMENT-REQUIRED header carries the price in base64, and the body shows the same details in plain JSON: 1,000 units of USDC (USDC counts in millionths of a dollar, so that is $0.001) on Base, paid to the seller’s wallet.

Show the real messageHide the real message

HTTP/1.1 402 Payment Required
Content-Type: application/json
PAYMENT-REQUIRED: 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

{
  "x402Version": 2,
  "error": "PAYMENT-SIGNATURE header is required",
  "resource": {
    "url": "https://rain.example.com/v1/rainfall?lat=52.62&lon=1.29",
    "description": "Latest rainfall reading: millimetres in the last hour",
    "mimeType": "application/json"
  },
  "accepts": [
    {
      "scheme": "exact",
      "network": "eip155:8453",
      "amount": "1000",
      "asset": "0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913",
      "payTo": "0x2222222222222222222222222222222222222222",
      "maxTimeoutSeconds": 60,
      "extra": {
        "name": "USD Coin",
        "version": "2"
      }
    }
  ]
}

Header names and message shapes follow the x402 v2 spec. Long lines are cut short. The seller, wallets, signature and transaction hash are made-up examples; the USDC contract address and the Base network ID are real.

You check your budget
Before signing anything, the agent’s payment software checks the offer against the limits its owner set: which networks and tokens it may use, and how much it may pay per request. The x402 reference client refuses any single payment over $1 unless the owner raises that cap. $0.001 is well inside it.
You sign the payment
The agent signs a message with its wallet’s private key: move exactly 1,000 units of USDC from my wallet to the seller’s, valid for the next 60 seconds, with a one-time random number so it can never be reused. Signing moves no money. It is a permission slip only the agent’s key can write.

You ask again, with the payment

The agent repeats the same request. This time it adds a PAYMENT-SIGNATURE header holding the signed permission slip, packed as base64.

Show the real messageHide the real message

GET /v1/rainfall?lat=52.62&lon=1.29 HTTP/1.1
Host: rain.example.com
Accept: application/json
PAYMENT-SIGNATURE: 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

The checker verifies
The seller’s server passes the payment to a facilitator, here Coinbase’s. The facilitator checks the signature, the amount, the recipient, the time window and the agent’s USDC balance. Checking costs nothing and moves nothing. Once it says the payment is valid, the server looks up the reading.
Show the real messageHide the real message
```
POST /platform/v2/x402/verify HTTP/1.1
Host: api.cdp.coinbase.com
Authorization: Bearer …
Content-Type: application/json

{
  "x402Version": 2,
  "paymentPayload": { "x402Version": 2, "accepted": { … }, "payload": { "signature": "0x5e5e…", "authorization": { … } } },
  "paymentRequirements": { "scheme": "exact", "network": "eip155:8453", "amount": "1000", … }
}
```
The checker answers:
```
HTTP/1.1 200 OK
Content-Type: application/json

{"isValid": true, "payer": "0x1111111111111111111111111111111111111111"}
```
Header names and message shapes follow the x402 v2 spec. Long lines are cut short. The seller, wallets, signature and transaction hash are made-up examples; the USDC contract address and the Base network ID are real.

The checker settles

With the reading ready, the server asks the facilitator to settle. The facilitator submits the signed authorization to Base and reports back with the transaction hash, the public ID of the transfer.

Show the real messageHide the real message

POST /platform/v2/x402/settle HTTP/1.1
Host: api.cdp.coinbase.com
Authorization: Bearer …
Content-Type: application/json

{
  "x402Version": 2,
  "paymentPayload": { "x402Version": 2, "accepted": { … }, "payload": { "signature": "0x5e5e…", "authorization": { … } } },
  "paymentRequirements": { "scheme": "exact", "network": "eip155:8453", "amount": "1000", … }
}

The checker answers:

HTTP/1.1 200 OK
Content-Type: application/json

{"success": true, "payer": "0x1111111111111111111111111111111111111111", "transaction": "0x7a7a7a7a7a7a7a7a7a7a7a7a7a7a7a7a7a7a7a7a7a7a7a7a7a7a7a7a7a7a7a7a", "network": "eip155:8453"}

USDC moves on Base
On Base, the USDC contract checks the signature and moves 0.001 USDC from the agent’s wallet to the seller’s. The facilitator pays the small network fee, called gas, so the agent needs no other token. The one-time number is now used up, so the same signature can never pay twice.
The reading comes back, with a receipt
The server sends the reading with 200 OK. The PAYMENT-RESPONSE header is the receipt: decoded, it says the payment succeeded and gives the transaction hash, which anyone can look up on a Base block explorer.
Show the real messageHide the real message
```
HTTP/1.1 200 OK
Content-Type: application/json
PAYMENT-RESPONSE: eyJzdWNjZXNzIjp0cnVlLCJ0cmFuc2FjdGlvbiI6IjB4N2E3YTdhN2E3YTdhN2E3YTdhN2E3YTdhN2E3YTdhN2E3YTdhN2E3YTdhN2E3YTdhN2E3YTdhN2E3YTdhN2E3YSIsIm5ldHdvcmsiOiJlaXAxNTU6ODQ1MyIsInBheWVyIjoiMHgxMTExMTExMTExMTExMTExMTExMTExMTExMTExMTExMTExMTExMTExIn0=

{
  "lat": 52.62,
  "lon": 1.29,
  "rainfall_mm_last_hour": 0.4,
  "observed_at": "2026-10-14T11:00:00Z"
}
```
Header names and message shapes follow the x402 v2 spec. Long lines are cut short. The seller, wallets, signature and transaction hash are made-up examples; the USDC contract address and the Base network ID are real.

One rainfall reading

rain.example.com

You ask for the reading$0.000

The site answers with a price$0.000

You check your budget$0.000

You sign the payment$0.000

You ask again, signed$0.000

The checker verifies it$0.000

The checker settles it$0.000

USDC moves on Base$0.001

The reading comes back$0.000

Money moved once. Charged to you: $0.001.

When does the money actually move?

Your choice is checked as soon as you pick it. With a keyboard, move with the arrow keys and press Space to choose.

When you signWhen the checker checks the paymentAt settlement, once the reading is readyWhen you first ask

Right.

Not quite.

Signing and checking move nothing. In the default flow the checker settles only once the site has the reading ready, so if the site fails first, you pay nothing. Some other schemes settle before the work is done.

What your signature protects

Your signed payment works like a cheque with four rules written in. It uses EIP-3009, a USDC feature that lets someone else submit a transfer you signed.

The amount and who gets paid are fixed. Change either and the signature stops matching, so not even the checker can.
It expires, here after 60 seconds.

What your signature protects

It works once. A one-time number is used up at settlement, when the money moves.
No account is needed. The site sees a wallet address and a signature, never a card or a key.

Careful. x402 doesn’t judge the reading. If the farmer’s number is wrong, there’s no refund button. Any fix is up to the seller.

The seller’s side of the bill

The farmer earns $0.001 a reading. Coinbase’s checker charges sellers nothing for the first 1,000 settlements each month and $0.001 for each one after that (14 Oct) source. Past the free tier, the fee is as large as the whole sale.

For prices this small, x402 also has a batch-settlement scheme, merged in April 2026, where many signed payments share one claim on the chain. Coinbase’s isn’t the only checker, and the build track compares them.

One question this leaves open: you paid from your own wallet, so where did that USDC come from? That’s the next lesson.

When your USDC moves on Base, who pays the network fee?

Your choice is checked as soon as you pick it. With a keyboard, move with the arrow keys and press Space to choose.

You, in another coin called ETHThe farmer’s other customersNobody, Base has no feesThe checker

Right.

Not quite.

The x402 spec for this kind of payment says the facilitator, your checker, pays the network fee, called gas. You only sign.

Lesson done.

x402 from zero

Your receipt on this device

Part 1: Understand it

0 of 6 done

What x402 is, one payment step by step, where an agent’s money comes from, and the real numbers.

Subtotal42 min

Done so far0 of 137 min

Charged$0.00

Price: about 7 minutes of your attention.

You can now say

The program asks, gets a 402 with the price, signs a payment and sends the same request again.
A checker, called a facilitator, verifies the payment and moves the money, so the seller runs no blockchain software.
Money moves only at the end, once the data is ready. If the server fails, the program pays nothing.
The signature fixes the amount and who gets paid, works once and soon expires.

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One payment, step by step

In this lesson

A farmer sells rainfall

Who takes part

Buy one reading yourself

You ask for the reading

The site answers 402, with the price

You check your budget

You sign the payment

You ask again, with the payment

The checker verifies

The checker settles

USDC moves on Base

The reading comes back, with a receipt

What your signature protects

What your signature protects

The seller’s side of the bill

Lesson done.

Part 1: Understand it

You can now say

Sources

This figure

Sources