I built an MCP server where AI agents pay for their own compute in USDC

Agents can browse docs and call APIs, but they can't do checkout. No credit card form, no OAuth dance, no "verify your email". So when I shipped an AI generation API, the interesting question was: how does an agent pay?

What I built

An MCP server (https://livepairai.com/mcp, streamable HTTP) exposing ~70 image/video models behind one interface. An agent connects, lists models with prices, calls generate, and gets back either a result or an HTTP 402 with payment requirements.

The payment rail is x402 — USDC on Base. The flow:

POST /v1/agent/generate {model, prompt}
← 402 Payment Required

x-payment-requirements: {amount: "400000", currency: USDC, chain: base, payTo} → agent signs USDC transfer, retries with X-PAYMENT header ← 200 + result URL (hosted 48h, auto-deleted)

One HTTP round-trip pattern. No API key, no account, no session — the payment signature is the credential.

What actually happens inside

• Model catalog with live per-call pricing: GET /v1/agent/models (5s 720p video ≈ $0.40, images ≈ $0.02–0.05)
• Quote freshness was the annoying part — prices are per-result, so the 402 response carries a short-lived quote the agent must settle before expiry
• For humans/API keys there's a chat-completions-compatible surface instead, so integrators who want accounting get Authorization: Bearer lp_… and skip the wallet dance entirely

Failure modes I hit

• Agents that retry the original request without attaching payment — had to make the 402 body self-describing (amount, recipient, expiry, example header) so a generic agent could act on it without me hardcoding the client
• Wallet-funded agents settling twice on timeout — made fulfillment idempotent on the payment reference
• The privacy constraint shaped the storage layer: renders live 48h max, never indexed, share links are a separate opt-in path gated by model

Try it

• MCP endpoint: https://livepairai.com/mcp
• Agent docs: https://livepairai.com/SKILL.md
• Manifest: https://livepairai.com/v1/agent/manifest.json

If your agent has a USDC wallet, it can already use this. Curious how others are handling