Payment Schemes

T402 supports multiple payment schemes to accommodate different billing models and use cases.

What is a Scheme?

A scheme defines how payments are authorized, verified, and settled. Each scheme specifies:

Authorization format (how clients sign payment requests)
Verification requirements (what the facilitator checks)
Settlement mechanism (how funds are transferred on-chain)

Available Schemes

Scheme	Description	Use Case
`exact`	Fixed amount payments	One-time purchases, subscriptions
`upto`	Usage-based billing	AI tokens, API calls, streaming

Exact Scheme

The exact scheme transfers a fixed amount specified upfront.

{
  "scheme": "exact",
  "network": "eip155:8453",
  "amount": "1000000",
  "asset": "0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913",
  "payTo": "0x..."
}

Characteristics:

Amount known before request
Full amount transferred on settlement
Simpler implementation
Uses EIP-3009 TransferWithAuthorization (EVM)

Best for:

Fixed-price API calls
Content purchases
Subscription payments
Any scenario where cost is predetermined

Up-To Scheme

The upto scheme authorizes transfer of up to a maximum amount.

{
  "scheme": "upto",
  "network": "eip155:8453",
  "maxAmount": "1000000",
  "minAmount": "10000",
  "asset": "0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913",
  "payTo": "0x...",
  "extra": {
    "unit": "token",
    "unitPrice": "100"
  }
}

Characteristics:

Client authorizes maximum exposure
Settlement based on actual usage
Server tracks and reports consumption
Uses EIP-2612 Permit (EVM)

Best for:

LLM token generation
Metered API usage
Streaming content
Compute resources

Learn more about the upto scheme →

Scheme Comparison

Feature	`exact`	`upto`
Amount field	`amount`	`maxAmount`
Settlement	Fixed amount	Variable (≤ max)
Client risk	Exact cost known	Maximum exposure known
Server complexity	Lower	Higher (usage tracking)
EVM implementation	EIP-3009	EIP-2612 + Router

Choosing a Scheme

Use `exact` when:

Price is known before the request
Simple pay-per-call model
Fixed content or service pricing
Lower implementation overhead desired

Use `upto` when:

Cost depends on request outcome
Usage-based or metered billing
Variable-length operations (streaming, generation)
Partial refunds may be needed

Network Support

Network	`exact`	`upto`
EVM (Base, Ethereum, Arbitrum, etc.)	✅ EIP-3009	✅ EIP-2612
Solana	✅ SPL Transfer	—
TON	✅ Jetton Transfer	—
TRON	✅ TRC-20 Transfer	—
NEAR	✅ NEP-141 Transfer	—
Aptos	✅ Fungible Asset Transfer	—
Tezos	✅ FA2 Transfer	—
Polkadot	✅ assets.transfer	—
Stacks	✅ SIP-010 Transfer	—
Cosmos (Noble)	✅ Bank MsgSend	—

SDK Support

All T402 SDKs support both schemes:

// TypeScript - exact scheme
import { ExactEvmScheme } from '@t402/evm';
 
// TypeScript - upto scheme
import { UptoEvmScheme } from '@t402/evm';

# Python - exact scheme
from t402.schemes import ExactEvmClientScheme
 
# Python - upto scheme
from t402.schemes import UptoEvmClientScheme

// Go - exact scheme
import "github.com/t402-io/t402/sdks/go/mechanisms/evm/exact"
 
// Go - upto scheme
import "github.com/t402-io/t402/sdks/go/mechanisms/evm/upto"

// Java - exact scheme
import io.t402.model.ExactSchemePayload;
 
// Java - upto scheme
import io.t402.schemes.upto.UptoPaymentRequirements;
import io.t402.schemes.evm.upto.UptoEIP2612Payload;

Micro-Payments Protocol Compatibility