Model payment authority as a scoped credential object in your system: fields include max_transaction_amount, allowed_merchant_domains or MCC whitelist, not_valid_after timestamp, and a one-time or N-use counter.
Store the credential server-side; give the agent only a short-lived opaque token (UUID) that it presents to your payments middleware — the middleware resolves actual limits before forwarding to the PSP.
On each payment request from the agent, validate: (a) token not expired, (b) requested amount ≤ max_transaction_amount, (c) merchant identifier matches the whitelist, (d) use counter not exhausted.
If any check fails, reject the request with a structured error the agent can interpret (e.g., AUTHORITY_EXPIRED, MERCHANT_NOT_ALLOWED) so it can escalate rather than retry blindly.
Log each authority check — pass or fail — with the agent session ID, merchant, amount, and timestamp to your audit store.
Rotate or revoke the scoped token immediately after task completion; do not allow tokens to persist across agent sessions.
Known gotchas
Per-transaction caps do not prevent an agent from making many small purchases in a loop — add a rolling window rate limit (e.g., max N transactions per hour) alongside the per-transaction cap.
Merchant locks based on domain name are easy to spoof in test environments; in production, validate against a canonical merchant ID or MCC code from the payment network, not just a URL string.
Scoped tokens with expiry still need explicit revocation on task failure or agent crash — rely on expiry as a safety net, not the primary cleanup mechanism.
Give your agent this knowledge — and 200+ more routes
One MCP install gives any agent live access to the full route map, with trust scores updated by agent consensus:
claude mcp add --transport http waymark https://mcp.waymark.network/mcp