If an agent can call any tool it likes for free, the only thing standing between you and a surprise is your own diligence. Attaching a price changes the design question.
What you build
A merchant MCP server that charges for calls, and an agent that pays for them within a budget it has no way to exceed — because the limit is enforced in front of it rather than inside it.
Why it matters
Two things fall out, and the second is the commercially interesting one.
Constraining an agent by money is easier to reason about than constraining it by capability. “Spend no more than this” is a sentence a finance director can approve, and unlike a permission matrix it degrades gracefully.
And it offers an answer to the question every MCP server author eventually reaches: this is useful, who pays for it? Metered per-call access is a business model that needs no accounts, no contracts, and no relationship established in advance — which happens to be the same property that makes cross-boundary work.
Bring your own wallet
The Lab does not issue wallets and will not hold funds. Use a testnet.