> ## Documentation Index
> Fetch the complete documentation index at: https://developers.fd.xyz/llms.txt
> Use this file to discover all available pages before exploring further.

# End-to-End Flow

> Watch one complete purchase play out: an AI agent buys socks via ACP from discovery to on-chain settlement.

An AI agent buys one pair of "Classic Crew Socks - Black / M" (\$12.99) from an ACP merchant, paying in USDC on Base Sepolia. Each step shows what the agent sends, what the merchant does behind the scenes, and what Prism returns.

***

## 1. Discovery

The agent checks if the merchant supports ACP.

**Agent → Merchant**

```http theme={"theme":{"light":"github-light","dark":"one-dark-pro"}}
GET https://merchant.example/.well-known/acp.json
```

**Agent receives:**

```json theme={"theme":{"light":"github-light","dark":"one-dark-pro"}}
{
  "protocol": {
    "name": "acp",
    "version": "2026-04-17",
    "supported_versions": ["2026-04-17"]
  },
  "api_base_url": "https://merchant.example/acp/api",
  "transports": ["rest"],
  "capabilities": {
    "services": ["checkout", "product_feed", "orders"],
    "extensions": [],
    "supported_currencies": ["usd"],
    "supported_locales": ["en-US"]
  }
}
```

The agent confirms ACP is supported, extracts `api_base_url`, and proceeds.

***

## 2. Product Feed

The agent fetches the merchant's product feed and finds the item.

**Agent → Merchant**

```http theme={"theme":{"light":"github-light","dark":"one-dark-pro"}}
GET https://merchant.example/acp/api/product_feed
```

**Agent receives:**

```json theme={"theme":{"light":"github-light","dark":"one-dark-pro"}}
{
  "products": [
    {
      "id": "prod_socks_001",
      "title": "Classic Crew Socks",
      "description": "Comfortable cotton crew socks",
      "variants": [
        {
          "id": "var_socks_blk_m",
          "title": "Black / M",
          "price": { "amount": 1299, "currency": "usd" },
          "available": true,
          "options": { "color": "Black", "size": "M" }
        }
      ]
    }
  ]
}
```

The agent selects `var_socks_blk_m` and proceeds to checkout.

***

## 3. Create Checkout Session

The agent sends the item. The merchant calls Prism to get x402 payment requirements and returns them in the session.

**Agent → Merchant**

```http theme={"theme":{"light":"github-light","dark":"one-dark-pro"}}
POST https://merchant.example/acp/api/checkout_sessions
Authorization: Bearer <agent_token>
API-Version: 2026-04-17
Idempotency-Key: cs_create_socks_1
Content-Type: application/json

{
  "line_items": [
    {
      "product_id": "prod_socks_001",
      "variant_id": "var_socks_blk_m",
      "quantity": 1
    }
  ]
}
```

**Merchant → Prism** (background)

```http theme={"theme":{"light":"github-light","dark":"one-dark-pro"}}
POST https://prism-gw.fd.xyz/api/v2/merchant/acp/payment-requirements
X-API-Key: prism_live_...
Content-Type: application/json

{
  "amount": "12.99",
  "currency": "usd",
  "resource": {
    "url": "https://merchant.example/acp/api/checkout_sessions/cs_001",
    "description": "Purchase from Example Store"
  }
}
```

**Agent receives:**

```json theme={"theme":{"light":"github-light","dark":"one-dark-pro"}}
{
  "id": "cs_001",
  "status": "collecting_information",
  "currency": "usd",
  "capabilities": {
    "payment": {
      "handlers": [
        {
          "id": "x402",
          "name": "xyz.fd.prism_payment",
          "version": "2026-01-15",
          "spec": "https://prism-gw.fd.xyz/acp/spec.md",
          "requires_delegate_payment": false,
          "requires_pci_compliance": false,
          "psp": "prism",
          "config_schema": "https://prism-gw.fd.xyz/acp/config_schema.json",
          "instrument_schemas": [
            "https://prism-gw.fd.xyz/acp/instrument_schema.json"
          ],
          "config": {
            "x402Version": 2,
            "resource": {
              "url": "https://merchant.example/acp/api/checkout_sessions/cs_001",
              "description": "Purchase from Example Store"
            },
            "accepts": [
              {
                "scheme": "exact",
                "network": "eip155:84532",
                "amount": "12990000",
                "asset": "0x036c...cf7e",
                "payTo": "0x40a0...C4fD",
                "maxTimeoutSeconds": 300,
                "extra": { "name": "USDC", "version": "2" }
              }
            ]
          }
        }
      ]
    },
    "interventions": {
      "supported": [],
      "required": [],
      "enforcement": "conditional"
    },
    "extensions": []
  },
  "line_items": [
    {
      "product_id": "prod_socks_001",
      "variant_id": "var_socks_blk_m",
      "title": "Classic Crew Socks - Black / M",
      "quantity": 1,
      "unit_price": { "amount": 1299, "currency": "usd" }
    }
  ],
  "totals": {
    "subtotal": 1299,
    "tax": 0,
    "shipping": 0,
    "total": 1299
  },
  "required_fields": ["buyer.name", "buyer.email", "shipping_address"]
}
```

The agent reads `status: "collecting_information"` and `required_fields`. Buyer info and a shipping address are needed.

***

## 4. Session Update

The agent provides buyer info and the shipping address. The merchant recalculates totals and re-fetches the Prism config with updated amounts.

**Agent → Merchant**

```http theme={"theme":{"light":"github-light","dark":"one-dark-pro"}}
PUT https://merchant.example/acp/api/checkout_sessions/cs_001
Authorization: Bearer <agent_token>
API-Version: 2026-04-17
Idempotency-Key: cs_001_update_1
Content-Type: application/json

{
  "buyer": {
    "name": "Alice Smith",
    "email": "alice@example.com"
  },
  "shipping_address": {
    "line1": "123 Main St",
    "city": "San Francisco",
    "state": "CA",
    "postal_code": "94105",
    "country": "US"
  }
}
```

**Merchant → Prism** (background, re-fetch with updated total)

```http theme={"theme":{"light":"github-light","dark":"one-dark-pro"}}
POST https://prism-gw.fd.xyz/api/v2/merchant/acp/payment-requirements
X-API-Key: prism_live_...
Content-Type: application/json

{
  "amount": "20.02",
  "currency": "usd",
  "resource": {
    "url": "https://merchant.example/acp/api/checkout_sessions/cs_001",
    "description": "Purchase from Example Store"
  }
}
```

**Agent receives:**

```json theme={"theme":{"light":"github-light","dark":"one-dark-pro"}}
{
  "id": "cs_001",
  "status": "ready_for_payment",
  "currency": "usd",
  "capabilities": {
    "payment": {
      "handlers": [
        {
          "id": "x402",
          "name": "xyz.fd.prism_payment",
          "version": "2026-01-15",
          "config": {
            "x402Version": 2,
            "resource": {
              "url": "https://merchant.example/acp/api/checkout_sessions/cs_001",
              "description": "Purchase from Example Store"
            },
            "accepts": [
              {
                "scheme": "exact",
                "network": "eip155:84532",
                "amount": "20020000",
                "asset": "0x036c...cf7e",
                "payTo": "0x40a0...C4fD",
                "maxTimeoutSeconds": 300,
                "extra": { "name": "USDC", "version": "2" }
              }
            ]
          }
        }
      ]
    }
  },
  "line_items": [
    {
      "product_id": "prod_socks_001",
      "variant_id": "var_socks_blk_m",
      "title": "Classic Crew Socks - Black / M",
      "quantity": 1,
      "unit_price": { "amount": 1299, "currency": "usd" }
    }
  ],
  "totals": {
    "subtotal": 1299,
    "tax": 104,
    "shipping": 599,
    "total": 2002
  }
}
```

Status is now `ready_for_payment`. Total: \$20.02. The `config.accepts` amount has been updated to `"20020000"` (20.02 USDC).

***

## 5. Payment Authorization

The agent extracts `config` from the handler and passes it to the wallet. No HTTP request goes out; everything happens locally in the wallet.

```
Wallet receives:  config — x402 paymentRequirements with accepts[]
Wallet selects:   eip155:84532 (Base Sepolia), USDC — sufficient balance
Wallet signs:     ERC-3009 transferWithAuthorization inside the secure enclave
Wallet returns:   complete signed x402 payment object
```

The wallet output contains:

* `paymentPayload.accepted`: the option the wallet chose (USDC on Base Sepolia)
* `paymentPayload.payload.signature`: EIP-712 signature over the authorization
* `paymentPayload.payload.authorization`: EIP-3009 params (exact value, exact recipient, time-limited)
* `paymentRequirements`: echo of the chosen `accepts[]` item

The private key never leaves the enclave. Only the signed authorization exits.

***

## 6. Complete Checkout

The agent submits the signed wallet output as the credential, adding `type: "default"` discriminators.

**Agent → Merchant**

```http theme={"theme":{"light":"github-light","dark":"one-dark-pro"}}
POST https://merchant.example/acp/api/checkout_sessions/cs_001/complete
Authorization: Bearer <agent_token>
API-Version: 2026-04-17
Idempotency-Key: cs_001_complete_1
Content-Type: application/json

{
  "payment_data": {
    "handler_id": "x402",
    "instrument": {
      "type": "default",
      "credential": {
        "type": "default",
        "x402Version": 2,
        "paymentPayload": {
          "x402Version": 2,
          "resource": {
            "url": "https://merchant.example/acp/api/checkout_sessions/cs_001",
            "description": "Purchase from Example Store",
            "mimeType": null
          },
          "accepted": {
            "scheme": "exact",
            "network": "eip155:84532",
            "amount": "20020000",
            "payTo": "0x40a0...C4fD",
            "maxTimeoutSeconds": 300,
            "asset": "0x036c...cf7e",
            "extra": { "name": "USDC", "version": "2" }
          },
          "payload": {
            "signature": "0xef55...a71c",
            "authorization": {
              "from": "0xC565...0573",
              "to": "0x40a0...C4fD",
              "value": "20020000",
              "validAfter": "1776664729",
              "validBefore": "1776665029",
              "nonce": "0xf1c1...1b55"
            }
          },
          "extensions": null
        },
        "paymentRequirements": {
          "scheme": "exact",
          "network": "eip155:84532",
          "amount": "20020000",
          "payTo": "0x40a0...C4fD",
          "maxTimeoutSeconds": 300,
          "asset": "0x036c...cf7e",
          "extra": { "name": "USDC", "version": "2" }
        }
      }
    }
  }
}
```

**Merchant → Prism** (background)

The merchant extracts `paymentPayload` and `paymentRequirements` from the credential (stripping the ACP `type` fields) and forwards them to Prism:

```http theme={"theme":{"light":"github-light","dark":"one-dark-pro"}}
POST https://prism-gw.fd.xyz/api/v2/payment/settle
X-API-Key: prism_live_...
Content-Type: application/json

{
  "paymentPayload": { "...signed authorization..." },
  "paymentRequirements": { "...chosen accepts[] item..." }
}
```

Prism executes the ERC-3009 `transferWithAuthorization` on Base Sepolia, verifies the transfer, and returns:

```json theme={"theme":{"light":"github-light","dark":"one-dark-pro"}}
{
  "success": true,
  "payer": "0xC565...0573",
  "transaction": "0x7a3b...1a2b",
  "network": "eip155:84532"
}
```

**Agent receives:**

```json theme={"theme":{"light":"github-light","dark":"one-dark-pro"}}
{
  "id": "cs_001",
  "status": "completed",
  "order": {
    "id": "ord_001",
    "checkout_session_id": "cs_001",
    "order_number": "#1001",
    "permalink_url": "https://merchant.example/orders/ord_001",
    "status": "confirmed",
    "confirmation": {
      "confirmation_number": "0x7a3b...1a2b",
      "receipt_url": "https://sepolia.basescan.org/tx/0x7a3b...1a2b",
      "confirmation_email_sent": true
    },
    "line_items": [
      {
        "product_id": "prod_socks_001",
        "variant_id": "var_socks_blk_m",
        "title": "Classic Crew Socks - Black / M",
        "quantity": 1,
        "unit_price": { "amount": 1299, "currency": "usd" }
      }
    ],
    "totals": {
      "subtotal": 1299,
      "tax": 104,
      "shipping": 599,
      "total": 2002
    }
  }
}
```

The socks are ordered. 20.02 USDC has settled on-chain to the merchant's wallet on Base Sepolia.

***

## Testing

Use the Prism gateway with your own account and small amounts to validate the full flow end-to-end against real on-chain settlement.

<CardGroup cols={2}>
  <Card title="Merchant Guide" icon="store" href="/prism/integrations/acp/merchants">
    Step-by-step implementation for the merchant server
  </Card>

  <Card title="Agent Guide" icon="robot" href="/prism/integrations/acp/agents">
    Step-by-step implementation for the agent
  </Card>
</CardGroup>