Currency Conversion API for AI Agents: ECB Reference Rates, Pay Per Conversion

The Currency Conversion API for AI agents, known as SML FX Convert, offers pricing, reporting, and budgeting capabilities across multiple currencies. Instead of relying on subscription FX feeds, additional API keys, or treating reference rates as tradable quotes, SML FX Convert provides a pay-per-call service. By checking the source upfront in the response itself, the API identifies the European Central Bank's daily reference rates.

This is facilitated through a free GET request to /api/fx/info, which returns the exchanged rates from the ECB statistics.

The pricing model is straightforward: $0.001 USDC is charged for each accepted conversion on Base mainnet. When making a conversion request without payment, such as GET /api/fx/convert?from=USD&to=EUR&amount=100, the API will respond with an HTTP 402 status code and a PAYMENT-REQUIRED header quoting exactly 1,000 atomic USDC, using the network eip155:8453 and the exact scheme.

A pack of 100-conversion credits is available for $0.005 USDC, equating to $0.00005 USDC per successful conversion when fully utilized. The service adopts a fail-closed approach, meaning that if the real rate is unavailable, no settlement occurs, and the caller receives an alternative route without incurring any costs. Invalid codes, malformed amounts, or unavailability of the reference data will not be silently converted into a paid, fabricated conversion.

It's crucial to note that SML FX Convert is designed for calculating ECB reference-rate conversions, not executing bank, card, exchange, or broker quotes. The service does not custody fiat, trade currencies, guarantee spreads, or guarantee that a reference rate is executable anywhere. Its primary purpose is to normalize vendor prices, compare API costs in different currencies, convert budget limits before tool selection, and maintain accurate reporting in a single reference currency.

The API ensures that source provenance is prioritized over pretending a reference rate is a live dealing price. The terms and conditions remain current until a 402 challenge is issued, which signals the need for payment, not settlement. The complete evidence and claim receipts can be found in the linked article on this article's canonical source.