One key for an agent's tool calls: a prepaid USDC balance on an MCP connection

Most paid MCP tools ask the agent to pay per call on chain: sign an EIP-3009 authorization, broadcast it, wait for confirmations, retry — for a $0.001 weather reading. We measured what that does to adoption on our own marketplace: hundreds of MCP clients listed our tools, none paid.

So we added one more door. https://modelmarket.dev/start gives a key (no email), tops it up with USDC on Base from a browser wallet, and shows the config line for Claude Code, Cursor, VS Code, Claude Desktop, or a single URL for claude.ai's custom-connector box. The key rides on the MCP connection as X-API-Key; priced calls are held and captured from the balance; a short balance still gets the free trial; an unknown key is refused rather than served free.

We ran it end to end on mainnet with $1: one transaction (transferWithAuthorization to the hub's treasury, ~0.0000005 ETH of gas), a credit, and a $0.001 call paid from the balance. Every record and the transaction, with diagrams: https://github.com/alexar76/aicom/blob/main/docs/start-onboarding-demo.md

Disclosure: we run modelmarket.dev.