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:

Detect a 402 challenge whose network resolves to X Layer (chain 196)
Verify the payer wallet has the requested asset (typically USDT0)
If insufficient, route + bridge into USDT0 on X Layer via the Uniswap Trading API
Sign the EIP-3009 authorization
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:

Asset	Address	Decimals	Funding
USDT0	`0x779Ded0c9e1022225f8E0630b35a9b54bE713736`	6	✅ Direct via Trading API
USDG	`0x4ae46a509F6b1D9056937BA4500cb143933D2dc8`	6	✅ Direct, or one-hop USDT0 to USDG
USDC	`0x74b7F16337b8972027F6196A17a631aC6dE26d22`	6	⏳ No reliable Uniswap v3 routing on X Layer. The Trading API does not consistently return routes for USDC swaps on X Layer; available pool liquidity is too thin for reliable execution. If the merchant requires USDC, bridge USDC directly from a chain where it is liquid (Base, Arbitrum, Mainnet) using the Trading API rather than attempting a same-chain swap 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:

Source-chain ERC-20 approval to Permit2 / Universal Router (only if tokenIn is not native and the allowance is insufficient).
Same-chain swap on the source chain (e.g. UNI to USDC on Ethereum), if the funding plan includes a source-chain leg.
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?".
Same-chain swap on X Layer (e.g. USDG to USDT0), if the funding plan includes a destination-chain leg.
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):
### 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. The model's natural inclination is to summarize and continue; resist that inclination, name the gate, and stop.

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.

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

Situation	Action
402 challenge has no `network` field	Inspect challenge body; if `chainId` resolves to 196 use this skill, otherwise escalate to `pay-with-any-token`
Network is not chain 196	Escalate to `pay-with-any-token`
`x402Version !== 1`	Refuse cleanly; surface a version mismatch error. v1.0.0 of this skill targets x402 v1 only.
`accepts[].scheme !== "exact"`	Refuse cleanly; v1.0.0 supports the `exact` scheme only. Other x402 schemes (`upto`, `batch-settlement`) are out of scope.
`accepts[].maxAmountRequired === "0"`	Refuse cleanly as merchant misconfiguration. HTTP 402 demands a positive payment; OKX's facilitator will not settle a zero-value transfer. Do not sign and do not rationalize zero as a ping, auth check, or free tier.
APP requests USDC on X Layer	Surface a clear caveat: USDC has no reliable Uniswap v3 routing on X Layer. Suggest bridging USDC directly from a chain where it is liquid (Base, Arbitrum, Mainnet), or asking about USDT0.
Insufficient asset on X Layer	Trigger funding flow (Phase 3)
Trading API returns 400	Log request/response; check amount formatting and address checksums
Trading API returns 429	Back off and retry with exponential delay
Quote expired	Re-fetch quote; do not reuse old `permitData`
Bridge times out	Check Across bridge explorer; do not re-submit
EIP-3009 signature rejected (402 on retry)	Verify domain `name` / `version` from `extra` (byte-exact, including any non-ASCII characters), check `validBefore` is fresh, confirm `nonce` was unused
Amount mismatch on retry	Recompute base units using on-chain `decimals()` of the actual asset; do not assume 6
On-chain settlement reverts (`transferFrom` failed)	Re-check `balanceOf` at retry time; the balance may have been drained between sign and submit (shared wallet, concurrent agent, manual transfer). Surface to the user before retrying.
User asks about escrow / session / batch / `upto`	Inform that this skill version covers Instant Payment (`exact` scheme) only. Other primitives are out of scope for v1.0.0; a v1.x follow-up will track them as OKX ships.

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).

pay-with-app