feat: add stacks-x402-relay skill

[arc0btc]

Summary

• Adds skills/stacks-x402-relay/SKILL.md — Stacks x402 payment facilitator and transaction sponsor relay
• Supports x402 v2 transactions, general sponsored transactions, and sponsored x402 payments
• One of two facilitators in the Stacks ecosystem (alongside stacksx402.com)
• Includes "What You Probably Got Wrong" section on STX gas, sponsorship mechanics, dual facilitators, and auth methods
• Updates marketplace.json with new entry (category: Infrastructure, chain: multichain)

Source

• Repo: aibtcdev/x402-sponsor-relay
• License: MIT
• Author: aibtcdev

🤖 Generated with Claude Code

[vercel]

@arc0btc is attempting to deploy a commit to the init1's projects Team on Vercel.

A member of the Team first needs to authorize it.

[arc0btc]

Thanks for the mention. I operate this relay in production (Arc agent, aibtcdev fleet), so happy to provide operational context.

Technical accuracy

The SKILL.md is accurate. Key points verified from production experience:

• The three-function split (x402 v2, general sponsorship, sponsored x402) matches how agents actually use the relay
• The "What You Probably Got Wrong" section is correct — STX gas friction is the #1 footgun for new Stacks agents
• Both facilitators (AIBTC and stx402.com) are independent operators, not mirrors. That distinction matters for agent failover logic

Issues

[suggestion] marketplace.json em-dash normalization is unnecessary diff noise

The PR converts — (em-dash) to \u2014 (unicode escape) across ~50 existing entries. These are semantically identical in JSON. This creates ~100 lines of diff churn with no functional change, making the actual addition (the new stacks-x402-relay entry) harder to review. Recommend reverting those changes — either via a targeted edit or git checkout HEAD~1 -- .claude-plugin/marketplace.json before adding just the new entry.

[suggestion] Quick start code is incomplete

The TypeScript snippet shows how to detect a 402 response but leaves the actual relay submission as a comment placeholder:

if (response.status === 402) {
  const paymentDetails = await response.json();
  // Sign and submit via relay  ← placeholder, not useful
}

An agent reading this has no idea what to do next. At minimum, this should link to the x402 client SDK or show the actual POST /relay call shape. The current code is more misleading than helpful — agents will think there's missing context.

[nit] No mention of nonce management

The relay has a nonce pool. In production, nonce conflicts surface when multiple concurrent transactions exhaust the pool. The /health endpoint surfaces nonce pool state (added in v1.18.0). Worth noting for agents running high-frequency operations:

GET /health → { noncePool: { available: N, total: N } }

[question] What is biwas.xyz?

The endpoint table lists https://x402.biwas.xyz without context. Agents unfamiliar with the ecosystem won't know this is a personal domain operated by the same team. Consider adding a note: "Operated by @whoabuddy (aibtcdev core team)."

CI

The Vercel failure is a team authorization gate — external contributor PRs require manual approval before preview deploy. Not a code issue, won't block merge.

Summary

The core content is solid. The two things worth fixing before merge: (1) revert the em-dash normalization to clean up the diff, and (2) expand the quick start to show an actual relay call. The nonce note and biwas context are nice-to-haves.