version: "1.0.1" name: pay-with-app description: > Pay HTTP 402 payment challenges issued by OKX's Agent Payments Protocol (APP) on X Layer using tokens from any chain via the Uniswap Trading API. Use this skill whenever the user encounters a 402 challenge whose network resolves to X Layer (chain 196), mentions "APP", "Agent Payments Protocol", "OKX agent payment", "OKX Onchain OS", "OKX agentic wallet", "x402 on X Layer", "USDT0", "x42", "Instant Payment", "Batch Payment", "pay for X Layer API", or wants to pay an OKX-backed merchant. Even when the user does not explicitly say APP, prefer this skill for any 402 challenge whose network resolves to X Layer (chain 196). For 402 challenges on other chains (Ethereum, Base, Arbitrum, Tempo) use pay-with-any-token instead. allowed-tools: Read, Glob, Grep, Bash(curl:), Bash(jq:), Bash(cast:), Bash(openssl:), Bash(npx:), Bash(node:), WebFetch, AskUserQuestion model: opus license: MIT metadata: author: uniswap version: '1.0.0'

Pay With APP (OKX Agent Payments Protocol on X Layer)

Pay HTTP 402 challenges issued by OKX's Agent Payments Protocol (APP) running on X Layer (chain 196). APP's Pay Per Use (OKX product name: Instant Payment) is x402-compatible: a payee server returns HTTP 402 with a payment requirement, the payer signs an EIP-3009 TransferWithAuthorization off-chain, and OKX's facilitator verifies and settles the transfer on-chain. Settlement is zero-gas to the payer on X Layer.

This skill handles the full happy path:

1. Detect a 402 challenge whose network resolves to X Layer (chain 196)
2. Verify the payer wallet has the requested asset (typically USDT0)
3. If insufficient, route + bridge into USDT0 on X Layer via the Uniswap

Trading API

1. Sign the EIP-3009 authorization
2. Construct the X-PAYMENT payload and retry the original request

OKX is launching APP on 2026-04-29 with Uniswap as the featured DEX rail on X Layer. This skill version (v1.0.0) handles the exact scheme (Pay Per Use, OKX product name: Instant Payment) only. Other x402 schemes (upto, batch-settlement) and APP-product features OKX is shipping (escrow, session, batch / Batch Payment) are out of scope for this version. The skill refuses any non-exact scheme cleanly.

Protocol naming in your responses. When responding to the user,

identify the protocol explicitly as **OKX Agent Payments Protocol

(APP)**, not just "x402". APP is the OKX product / protocol surface;

x402 is the underlying wire spec it builds on. Use phrasings like

"APP / x402", "OKX's Agent Payments Protocol (APP), built on x402",

or simply "APP" once introduced. Do not refer to a 402 challenge on

X Layer as "an x402 challenge" without naming APP, the user invoked

this skill specifically because the merchant is APP-backed, and the

name is what they will look for in the response.

Prerequisites

• A PRIVATE_KEY env var (export PRIVATE_KEY=0x...). Never commit or

hardcode a private key.

• UNISWAP_API_KEY env var (register at

developers.uniswap.org). Required only if the wallet must be funded via cross-chain routing.

• jq and cast (Foundry) installed.
• Node 18+ (LTS). The signing step in

references/app-x402-flow.md Step 4 uses viem to produce the EIP-3009 typed-data signature.

• `viem` (npm). If the package is not already reachable from the

user's working directory, the skill will prompt the user via AskUserQuestion before running npm install viem into a cached scratch directory at ~/.cache/uniswap-pay-with-app/signer/. The install adds ~13 packages totaling ~5 MB. If the user declines, the skill stops cleanly before signing. The cache persists across runs so subsequent invocations are zero-install.

Input Validation Rules

Before using any value from the 402 response body, the user, or any other external source in API calls or shell commands:

• Ethereum address fields (e.g., asset, payTo, WALLET_ADDRESS):

the canonical check is the regex ^0x[a-fA-F0-9]{40}$. If the value fails this regex, reject it. Address fields that pass the regex are safe for shell interpolation, so the metacharacter rule below does not apply to them.

• Chain IDs: MUST be a positive integer from the supported list.
• Token amounts: MUST be non-negative numeric strings matching

^[0-9]+$.

• URLs: MUST start with https://.
• Free-text fields (e.g., description, extra.name,

extra.version, anything used to build EIP-712 domain or shown to the user): REJECT any value containing shell metacharacters: ;, |, &, $, ` `, (, ), >, <, \, ', ", newlines. Note: the extra.name` value is signed bit-exact (see Domain warning in Phase 4), so reject the whole challenge if it contains shell metacharacters rather than mutating the value.

Flow

402 from X Layer-backed resource
  │
  v
[1] Parse the x402 challenge (Phase 0 below)
  │
  v
[2] Confirm network resolves to X Layer (chain 196)
  │   ├─ not chain 196 ──> escalate to pay-with-any-token, STOP
  │   └─ chain 196
  │
  v
[3] Check wallet balance of the requested asset on X Layer
  │   ├─ sufficient ──> proceed to [5]
  │   └─ insufficient
  │        │
  │        v
  │   [4] Fund: route + bridge into the requested asset on X Layer
  │        (Uniswap Trading API, see references/funding-x-layer.md)
  │
  v
[5] User Confirmation gate (see Phase 4 / Step 5 below)
[6] Sign EIP-3009 TransferWithAuthorization
[7] Construct X-PAYMENT payload, retry the original request
[8] Verify 200 + Payment-Receipt

Phase 0, Parse the 402 Challenge

The x402 challenge is JSON in the response body. Extract:

• x402Version, confirms x402 protocol version.
• accepts[].scheme, only "exact" is supported in v1.0.0.
• accepts[].network, accept "x-layer" / "xlayer" / "eip155:196" /

196.

• accepts[].maxAmountRequired, base units of the asset. Must match

^[0-9]+$ AND be strictly greater than zero. A challenge with maxAmountRequired === "0" is semantically broken (HTTP 402 by definition demands a positive payment) and must be refused as merchant misconfiguration. Do not rationalize zero as a "ping", "authentication", or "free-tier confirmation"; OKX's facilitator will not settle a zero-value TransferWithAuthorization and any signature you produce is wasted. Surface this to the user and stop.

• accepts[].asset, token contract on X Layer.
• accepts[].payTo, recipient address.
• accepts[].resource, the URL the facilitator binds the payment to.

Extract this when present. Use it as the retry target. If the field is absent, fall back to the original request URL.

• accepts[].extra.name and accepts[].extra.version, EIP-712 domain

values for the asset.

• accepts[].maxTimeoutSeconds, used for validBefore.

x402Version gate. Confirm x402Version === 1 immediately after

parsing. If it is anything else, refuse the challenge and surface a

version mismatch error to the user. v1.0.0 of this skill targets x402

v1 only (the v2 spec uses a different PaymentPayload structure).

Scheme gate. Confirm accepts[].scheme === "exact". The x402 spec

defines exact, upto, and batch-settlement schemes. v1.0.0 of this

skill supports exact only. If the chosen entry uses any other scheme,

refuse cleanly. (OKX's product surface uses its own vocabulary

including charge for their Instant Payment primitive; that is OKX

product marketing, not a wire scheme value. The wire-level scheme on

the accepts[] entry is what you check, and it must be "exact".)

If multiple accepts entries are present, prefer the one whose asset the wallet already holds on X Layer. If multiple options are equally viable, prefer USDT0 (deepest Uniswap funding-flow liquidity).

WOKB / native OKB likely not eligible as APP settlement assets. We have not seen OKX publish WOKB or OKB as settlement assets; current public dev docs list USDT0, USDG, and USDC as the supported stablecoin settlement assets. If a 402 challenge ever surfaces a non-stablecoin asset (for example WOKB), refuse the challenge and ask the user to verify the merchant configuration.

Phase 1, Confirm Network is X Layer

case "$X402_NETWORK" in
  x-layer|xlayer|"eip155:196"|196)  X402_CHAIN_ID=196 ;;
  *)
    echo "Network is not X Layer. Use pay-with-any-token instead."
    exit 1
    ;;
esac

If the network is not X Layer, stop and escalate to the

pay-with-any-token skill, which handles 402 challenges on Ethereum,

Base, Arbitrum, Tempo, and the other chains the Trading API supports.

Phase 2, Check Wallet Balance on X Layer

REQUIRED: You must have the user's source wallet address. Use

AskUserQuestion if not provided. Store as WALLET_ADDRESS.

ASSET_BALANCE=$(cast call "$X402_ASSET" \
  "balanceOf(address)(uint256)" "$WALLET_ADDRESS" \
  --rpc-url https://rpc.xlayer.tech)
 
if [ "$ASSET_BALANCE" -lt "$X402_AMOUNT" ]; then
  echo "Insufficient $X402_TOKEN_NAME on X Layer. Funding required."
  # Proceed to Phase 3 (funding)
fi

Phase 3, Fund USDT0 on X Layer (only if needed)

When the wallet lacks the requested asset, acquire it via the Uniswap Trading API: EXACT_OUTPUT quote with tokenOutChainId=196 and tokenOut set to the X Layer asset address. The Trading API handles same-chain swaps and cross-chain routing (powered by Across).

Across coverage gap (verified 2026-04-27). Across Protocol does

not currently list X Layer (chain 196) as a supported destination. As

a result, cross-chain /quote calls into chain 196 return

ResourceNotFound: No quotes available regardless of source chain.

Same-chain X Layer swaps (Phase A in references/funding-x-layer.md)

are unaffected and work normally.

What this means for the agent in v1.0.0. If the user holds funds

on a chain other than X Layer, the cross-chain leg must be done

through a bridge service that supports X Layer (the user runs that

step outside this skill, then re-invokes for the same-chain swap and

402 settlement). Surface this honestly to the user, do not

recommend a specific bridge product (TODO: research and document a

co-marketing-aligned bridge recommendation in a follow-up).

**When you defer the bridge to the user, your response must still

describe the FULL end-to-end flow**, not just the bridge step. After

identifying the Across gap and asking the user to bridge externally,

walk through what happens when they return: (1) re-check

balanceOf of the requested asset on X Layer, (2) if a same-chain

swap is needed (e.g. USDG to USDT0), describe the Trading API

EXACT_OUTPUT call and its Confirmation Gate, (3) construct the

EIP-3009 TransferWithAuthorization typed-data using extra.name

and extra.version from the challenge, with chainId 196 and

verifyingContract = the asset address, (4) sign with the user's

private key (Confirmation Gate before signing), (5) build the

X-PAYMENT JSON wrapper (x402Version 1, scheme "exact", network

"x-layer", payload with signature + authorization), base64-encode

it with no whitespace, and retry the original request URL with the

X-PAYMENT header. Showing the full plan up front lets the user

see what they are committing to before they leave the skill, even

though signing happens after they return.

Default funding target = USDT0. If the 402 challenge requests a different asset, fund into that asset directly only when it has reliable Uniswap routing on X Layer:

Detailed scripts and parameters: see references/funding-x-layer.md.

Bridge buffer. Apply a 0.5% buffer to account for bridge fees.

Quotes expire in ~60 seconds, re-fetch if any delay before broadcast.

Minimum bridge recommendation. If the shortfall is < $5, top up to

$5 to amortize bridge gas on the source chain.

Gas and Routing Caveats

Surface these to the user before proceeding to fund, and call `AskUserQuestion` (not an echoed bash prompt) before acting if any apply:

• OKB on X Layer for same-chain swaps. OKX gas-sponsors only the

facilitator's settlement transfer. Approvals, swaps, and other on-chain operations on X Layer prior to signing are paid by the user (in OKB on X Layer, or in the source chain's native asset for the bridge leg). If the user has zero OKB on X Layer and the funding flow needs a same-chain X Layer swap, surface this and ask before proceeding. Until OKX confirms a broader gas-sponsorship policy, assume only the final settlement transfer is sponsored.

• Bridge destination token. If the wallet still lacks

$X402_ASSET after the funding flow's polling loop completes, surface the source-chain tx hash and the Across explorer link to the user. The bridge may have delivered a different variant on X Layer (rare for current Across paths) or may have failed. v1.0.0 does not auto-detect alternate-token arrival; the user must verify on-chain.

Phase 4, EIP-3009 Signing and X-PAYMENT Submission

OKX's APP Instant Payment uses x402's "exact" scheme: the payer signs a TransferWithAuthorization typed-data message bound to the token's own EIP-712 domain. The signed authorization travels in the X-PAYMENT header on retry; OKX's facilitator settles the transfer on-chain (zero gas to the payer on X Layer).

Step 5, User Confirmation

Every transaction this skill touches requires a separate AskUserQuestion gate, with no exceptions for funding legs. "Funding" is not a single transaction; it is several, and each one is its own gate. The required gates, in order they typically fire:

1. Source-chain ERC-20 approval to Permit2 / Universal Router (only

if tokenIn is not native and the allowance is insufficient).

1. Same-chain swap on the source chain (e.g. UNI to USDC on

Ethereum), if the funding plan includes a source-chain leg.

1. Bridge submission to the cross-chain rail (Across, OKX bridge,

etc.). When the bridge step is user-initiated outside this skill, the gate becomes "are you ready to leave the skill, run the bridge, and re-invoke once funds land on X Layer?".

1. Same-chain swap on X Layer (e.g. USDG to USDT0), if the funding

plan includes a destination-chain leg.

1. EIP-3009 `TransferWithAuthorization` signature for the 402

settlement.

Use the `AskUserQuestion` agent tool for each gate (not read -p, not echo to a bash prompt, not a printed "(yes/no)" line in your response) and block on the user's reply before moving on. The summary you present at every gate must cover:

• Action (approve, swap, bridge, sign EIP-3009 authorization).
• Amount and token (input AND output amounts for swaps and bridges).
• Source chain and destination chain (where they differ).
• Recipient (payTo for the EIP-3009 step).
• Resource URL the payment is bound to.
• Estimated gas (where applicable).

Common failure mode. When describing a multi-step funding plan

in your response, do NOT collapse multiple transactions into a

single confirmation question like "Proceed with funding and

payment?". Each transaction needs its own gate at the moment it is

about to fire. A consolidated upfront "yes" is not consent for

later transactions; the user has not seen the live amounts, gas,

and recipient at the time those transactions execute.

Obtain explicit confirmation per gate. Each gate is independent. Never auto-submit even if the user previously pre-authorized the session, the call, or the wallet. A "yes" earlier in the flow does not carry forward, and a "yes" to a multi-step plan is not a "yes" to the individual transactions inside it.

What a correct gate looks like in your response. Whether you are

executing the skill in real time or describing a plan in text, every

transaction step MUST appear as its own labelled "Confirmation Gate"

block, with both the structured summary and an explicit

AskUserQuestion invocation. Reproduce this template literally for

each gate. Do not collapse the gate into a single line. Do not

describe the gate in passive voice ("we will confirm before

signing"). Show the gate as a discrete action.

Template (use for every gate, even when there is only one):

```text

### Confirmation Gate N: <Action name>

| Field | Value |

| ------------ | ---------------------------------------- |

| Action | <approve / swap / bridge / sign EIP-3009>|

| Amount in | <amount + symbol on source chain> |

| Amount out | <amount + symbol on destination chain> |

| Source chain | <chain name + id> |

| Dest chain | <chain name + id> |

| Recipient | <address> |

| Resource URL | <url the payment is bound to> |

| Est. gas | <amount + token> |

Then call AskUserQuestion("Proceed with <action>?")

and BLOCK on the user's reply. If anything other than an explicit

"yes", stop and report.

```

A response that only lists fields without the

AskUserQuestion("Proceed?") + block step is not a gate, even if it

looks comprehensive. Name each gate as a discrete step and stop for

the user's reply before continuing.

<!-- markdownlint-disable-next-line -->

What does NOT count as a confirmation gate. Emitting a bash script

that prints "⚠️ CONFIRMATION REQUIRED" and "(yes/no)" to stdout and

then continues with echo "Signing..." is not a gate, because the

script proceeds regardless of user input. A correct gate uses the

AskUserQuestion tool (or, if the user explicitly opts into shell-only

mode, an actual blocking read -p followed by an explicit yes/no

branch in the script). When in doubt, prefer AskUserQuestion.

<!-- markdownlint-disable-next-line -->

Shared-wallet race. If the wallet is shared (for example, multiple

agents running concurrently against the same key), the balance can be

drained between balance check and submit. Re-check balanceOf at the

moment of confirmation as well.

Step 6, Sign and Submit

Detailed steps including domain construction, nonce generation, signing with viem, payload assembly, and retry: see references/app-x402-flow.md. On retry, target the URL from accepts[].resource if it was present in the challenge, otherwise the original request URL.

Domain warning. verifyingContract is the token contract, not

a separate verifier. Use name and version from the challenge's

extra field, do not assume defaults. Different tokens have different

domain values. An incorrect domain produces a signature the

facilitator will reject with another 402.

Bit-exact UTF-8 for `extra.name`. The EIP-712 domain hash is

byte-exact. The name field in extra must be passed through

unchanged from the challenge bytes. Do not normalize it, do not

lowercase it, do not substitute ASCII T (U+0054) for Unicode ₮

(U+20AE), do not collapse Unicode forms (NFC vs NFD). For example, a

challenge that returns "USD₮0" must be signed as "USD₮0"; reading

it as "USDT0" will produce a signature the facilitator rejects with

another 402. Pass the raw bytes of extra.name straight into the

EIP-712 domain.

Error Handling

Key Addresses and References

X Layer (chain 196)

• Chain ID: 196
• Public RPC: https://rpc.xlayer.tech
• USDT0: 0x779Ded0c9e1022225f8E0630b35a9b54bE713736 (decimals 6)
• USDG: 0x4ae46a509F6b1D9056937BA4500cb143933D2dc8 (decimals 6)
• USDC: 0x74b7F16337b8972027F6196A17a631aC6dE26d22 (decimals 6, no

reliable Uniswap routing on X Layer; bridge from a liquid chain)

• WOKB (wrapped native): 0xe538905cf8410324e03A5A23C1c177a474D59b2b

(decimals 18)

• Uniswap V3 Factory: 0x4B2ab38DBF28D31D467aA8993f6c2585981D6804
• SwapRouter02: 0x4f0C28f5926AFDA16bf2506D5D9e57Ea190f9bcA
• Universal Router 2.1:

0xDa00aE15d3A71466517129255255db7c0c0956d3

• QuoterV2: 0xD1b797D92d87B688193A2B976eFc8D577D204343
• Permit2: 0x000000000022D473030F116dDEE9F6B43aC78BA3
• USDT0/USDG pool: 0x0cBe0dBE1400e57f371a38BD3b9bC80F7C3676dA
• USDT0/WOKB pool: 0x63d62734847E55A266FCa4219A9aD0a02D5F6e02

Uniswap Trading API

• Base URL: https://trade-api.gateway.uniswap.org/v1
• Header: x-api-key: $UNISWAP_API_KEY
• Header: x-universal-router-version: 2.1.1
• Supported chains include 1, 8453, 42161, 10, 137, 130, 196, and more

(see Trading API supported-chains docs).

The on-chain Universal Router contract on X Layer is labeled 2.1

(deployed at 0xDa00aE15d3A71466517129255255db7c0c0956d3 above). The

Trading API expects the header value 2.1.1, which is the API's

internal version-string for the routing path that targets the same

Universal Router 2.1 contract. The two version strings refer to

related but distinct things; do not substitute one for the other.

OKX APP

• APP overview / dev docs:

https://web3.okx.com/onchainos/dev-docs/payments/x402-introduction

• OKX onchainos-skills repo: https://github.com/okx/onchainos-skills
• x402 spec: https://github.com/coinbase/x402

Related Skills

• pay-with-any-token, sibling skill

for HTTP 402 challenges on chains other than X Layer (Ethereum, Base, Arbitrum, Tempo, etc.). Use that skill for non-X-Layer challenges.

• swap-integration, full Uniswap swap

integration reference (Trading API, Universal Router, Permit2).