Fetch live exchange rates from a currency API (e.g., Open Exchange Rates, Frankfurter, or ECB data feed) for the traveler's home currency against all currencies in their itinerary destinations; refresh rates at least once daily.
Store each booking and expense in both the transaction currency and the home currency equivalent at the time of booking; record the exchange rate applied so the conversion can be audited.
Implement a budget envelope per trip or per destination; as new bookings and expenses are added, compute the running total in home currency and compare against the envelope.
For future expenses (e.g., estimated daily spend), apply the latest exchange rate with a clearly labeled 'estimated at current rates' caveat rather than treating them as confirmed amounts.
Alert the traveler when projected total spend exceeds their budget, factoring in known future bookings plus an estimated daily spend per remaining travel day.
On post-trip reconciliation, compare actual charged amounts (from bank/card feeds) against estimated converted amounts to surface exchange-rate variance.
Known gotchas
Credit card foreign transaction fees (typically 1-3%) and dynamic currency conversion at point of sale add to the effective exchange cost; surface these as estimated additional costs in your budget model.
Exchange rates at the time of a card charge may differ from when the booking was made — especially for hotel incidentals charged at checkout; always show estimates as estimates.
Some currencies have restricted exchange markets (capital controls); in these cases live API rates may not reflect the actual rate available to the traveler — add a disclaimer for affected destinations.
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