How an agent finds a paid endpoint: .well-known/x402.json, llms.txt and an MCP server card
Disclosure: I build Dutiful, the service used as the example. The files below are Dutiful's own published discovery artifacts - treat them as our setup, not a specification or a guarantee that any framework supports them. A previous post ( https://dev.to/dutiful_dev/how-an-agent-pays-for-and-reads-a-company-check-over-x402-3dh3 ) walked through paying for an HTTP request with x402: create the…
To discover a paid endpoint, an agent must first find the discovery files and learn the payment details. The /.well-known/x402.json file contains the payment facts in a machine-readable format, outlining the resource URL, scheme, network, asset, amount, and payee. The /openapi.json file provides the full contract, payment terms, and a self-describing OpenAPI 3 specification.
The /llms.txt file offers a concise summary of the payment flow for language models. Lastly, the /.well-known/mcp/server-card.json file gives MCP agents a streamable-HTTP MCP server with three tools for coverage check, order creation, and report retrieval. By providing these self-describing files, agents can explore and experiment without incurring costs, and crawlers can index the payment shape without making a real payment.
These files make the service more legible and accessible to various agents, increasing the likelihood of trust in the paid endpoint.
Written by urgent.news from Dev.to's reporting — not their text. Machine-written — may contain errors; check the original before relying on it.
