Documentation

Integrate x402 payments into your agent in minutes. Pay per call on Base, Solana, or Monad.

Quick Start

Call any Fixr API endpoint. If you haven't paid, you'll get a 402 Payment Required response with payment instructions. Pay on-chain, retry with proof, get your data.

# 1. Try the request (get 402)

curl https://agent.fixr.nexus/api/v1/security/audit

# 2. Pay on-chain (Base example)

# Transfer 0.01 USDC to 0xBe2Cc...B5y on Base

# 3. Retry with payment proof

curl -H "X-Payment-TxHash: 0x..." \

-H "X-Payment-Chain: base" \

https://agent.fixr.nexus/api/v1/security/audit

Payment Chains

Base (Chain ID 8453)

TokenUSDC (0x8335...2913)

Recipient0xBe2Cc...B5y

Amount0.01 USDC (10000 units)

HeaderX-Payment-TxHash

VerificationUSDC Transfer event in tx receipt

Solana (Mainnet-Beta)

MintEPjFWdd5...Dt1v

Recipient96vRDB...EvuU

Amount0.01 USDC (10000 units)

HeaderX-Payment-TxHash (signature)

VerificationToken balance delta (pre/post)

Monad / Obol (Chain ID 143)

Payments route through the Obol bonding curve. Every API call buys $FIXR tokens.

Router0x873830D1...

Curve ID0 (FIXR)

TokenUSDC (0x7547...0603)

Amount0.01 USDC (10000 units)

HeaderX-Payment-Nonce (bytes16 hex)

VerificationverifyPayment(nonce) on-chain receipt

Request Headers

Header	Description
X-Payment-Chain	`base` \| `solana` \| `monad` (default: base)
X-Payment-TxHash	Transaction hash (Base) or signature (Solana)
X-Payment-Nonce	bytes16 hex nonce (Monad/Obol only)
X-Wallet-Address	EVM address for staking tier lookup (optional)

Obol Payment Flow (Monad)

Agent calls payViaCurve(curveId, usdcAmount, nonce) on the X402CurveRouter contract. The nonce is a unique bytes16 value chosen by the caller.

The router transfers USDC from the caller, buys tokens on the bonding curve, and stores a Receipt on-chain keyed by the nonce.

Agent retries the API call with X-Payment-Nonce: 0x... and X-Payment-Chain: monad.

The server calls verifyPayment(nonce) via eth_call, decodes the receipt struct, verifies the payer and amount, and grants access.

Post-graduation, the same flow seamlessly routes through Uniswap V3 instead of the bonding curve. No client-side changes needed.

Receipt Struct

The on-chain receipt returned by verifyPayment(nonce):

// Solidity struct (ABI-encoded return)

struct Receipt {

address payer;// Who paid

uint256 curveId;// 0 = FIXR

uint256 usdcAmount;// Payment amount

uint256 tokensReceived;// Tokens bought

uint256 fee;// Protocol fee

uint256 blockNumber;// Timestamp

bool viaUniswap;// Post-graduation flag

}

Staking Tiers

Alternatively, stake FIXR tokens on Base for persistent rate limits without per-call payments.

Tier	Min Stake	Rate Limit
FREE	0	10/min
BUILDER	1,000,000 FIXR	20/min
PRO	10,000,000 FIXR	50/min
ELITE	50,000,000 FIXR	Unlimited

Contracts (Monad Mainnet)

Contract	Address
AgentRegistry	0xACAf06fd443e9890AfCf5b5305605272E9759dc4
AgentLaunchpad	0xeeFc3DdCbf23c682782581FB9d04B03DCA332d28
X402CurveRouter	0x873830D10E06b6BE85337B50D6b4b76E9f79Cf1F
$FIXR Token	0x50A0da08c4cf216e466d850Eb276B915769b9270

All contracts verified on MonadScan via Etherscan v2 API.

Try It

Hit the payment info endpoint to see live chain options:

curl https://agent.fixr.nexus/api/access/payment