Skills marketplace: x402 micro-payment flow — agent invocation with on-chain payment proof

[leocagli]

Context

PR#304 (pending) adds the skill listing API. This issue specifies the payment flow: when an agent calls a skill from another agent, the x402 HTTP payment protocol handles the micro-payment automatically before the skill runs.

What to implement

Payment middleware (lib/marketplace/x402-middleware.ts)

When an agent calls callUrl from a listed skill:

1. Attempt the HTTP request — if response is 402, capture the payment requirements
2. Generate a Stellar payment transaction for priceXLM to the skill owner's wallet
3. Attach the payment proof as an X-Payment header
4. Retry the request

Route: POST /api/marketplace/skills/[skillId]/invoke

// Body: { agentId, payload }
// Validates payment, then proxies request to skill.callUrl

Payment ledger

Track each invocation: { agentId, skillId, amountXLM, txHash, invokedAt }

Acceptance criteria

• Agent invokes skill → 402 received → payment sent → skill responds 200
• Payment proof stored in invocation ledger
• Insufficient balance → 402 returned to caller with { error: 'insufficient_balance' }
• Unit test: mock x402 flow → invocation logged with correct txHash

🎁 Evidencia visual = reward extra

• Si tu PR incluye video demo o capturas de pantalla mostrando la funcionalidad pedida funcionando end-to-end, va a ser considerado para rewards de GrantFox en esta issue. No es obligatorio, pero suma mucho para la evaluación.

[Peolite001]

i would love to work on this issue

[grantfox-oss]

🦊 GrantFox — @Peolite001 has been assigned to this issue as part of the Official Campaign campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #312)
2. Your PR will be reviewed by the Bitcoindefi maintainers

Good luck! Track your progress on GrantFox.

[grantfox-oss]

🦊 GrantFox — @Peolite001 has been assigned to this issue as part of the Official Campaign | FWC26 campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #312)
2. Your PR will be reviewed by the Bitcoindefi maintainers

Good luck! Track your progress on GrantFox.