Feature request: surface a first-class pay-per-call (HTTP 402 / x402) step when an agent tool answers 402 #3550

Feature request: surface a first-class pay-per-call (HTTP 402 / x402) step when an agent tool answers 402

Context. Inkeep Agents builds AI assistants and multi-agent workflows that call tools. When an agent steps outside the sandbox to call a metered third-party tool or API, today that usually means a billed API key or a subscription. There is no runtime-level way to settle a per-call bill when a priced endpoint answers.

Proposal. Add an optional hook on the agent tool-call path (the agents-api / agent runtime): when a tool answers 402 Payment Required with a machine-readable payment challenge (an x402 accepts[] envelope — the emerging pay-per-call standard for agent APIs, with no API key, no account and no card), surface that as a first-class settlement step a builder can wire up: read the price, approve a spend cap, pay, retry, and record the charge. The runtime then handles pay-per-call APIs natively.

Why it fits.

• Metered tool APIs are the common case for agent workflows (search, scraping, enrichment, compute), and per-call charges often sit below the card floor — per-call settlement is the natural fit.
• The tool-call / MCP integration points already exist, so this hook sits beside existing tool handling; no tool changes behavior.
• Optional and additive: flows that use billed keys today keep working unchanged.

How to verify. I can provide a reference implementation: a tiny x402 client that reads the 402 accepts[] list, pays, and retries, tested against a live pay-per-call endpoint that returns real 402s with an x402 v2 envelope on nano:mainnet (keyless — its x402 discovery file is live at https://extract.paypercall.dev/.well-known/x402). Ready as a drop-in adapter on the tool-call path.

Happy to draft the reference client and a minimal hook if this direction is welcome.