currency-rates

Currency Rates Skill

Currency conversion is a daily agent chore with a clean keyless answer: Frankfurter serves European Central Bank reference rates (current and historical) over bare HTTPS, and open.er-api.com covers the long tail of currencies Frankfurter doesn't track. This skill knows both, always quotes the rate and its date, and never confuses a reference rate with the rate any bank will actually give.

What This Skill Produces

• The conversion — the amount, converted, stated first
• The rate and its date — reference rates update on banking days; the date is part of the answer
• The command — exact curl, rerunnable and scriptable
• The caveat — reference rates ≠ what a card, bank, or exchange desk will charge; the spread is real

Required Inputs

Ask for these if not provided:

• From, to, and amount — ISO codes resolved from natural language ("dollars" → USD unless context says AUD/CAD/SGD — ask when genuinely ambiguous)
• When — today (default) or a historical date (Frankfurter serves history back to 1999)
• The purpose, if it matters — budgeting tolerance vs. invoice-precision changes how hard to caveat the spread

Framework: The Two Sources

1. Frankfurter — primary (ECB reference rates): latest: curl -s "https://api.frankfurter.dev/v1/latest?base=USD&symbols=EUR,INR" · convert directly: ...?amount=500&base=USD&symbols=EUR · historical: https://api.frankfurter.dev/v1/2024-03-15?base=USD&symbols=EUR · a date range for trends: .../v1/2026-01-01..2026-07-01?base=USD&symbols=EUR. ~30 major currencies, updated ~16:00 CET on ECB working days.
2. open.er-api.com — fallback and long tail: curl -s "https://open.er-api.com/v6/latest/USD" → 160+ currencies in one response. Use when Frankfurter lacks the currency (many African, Asian, Latin American currencies) or is down. Quote its time_last_update_utc field.
3. Date honesty: both responses carry the rate's date — quote it verbatim. A Saturday question gets Friday's rate, and the answer says so.
4. The spread sentence: consumer conversions happen 1–4% worse than reference (card networks near the low end, airport desks at the high end). For "how much will I get" questions, give the reference number and the realistic band.
5. Arithmetic discipline: for cross rates neither API serves directly, convert through the base and show the two hops; round to sensible precision (whole yen, cents for EUR/USD) rather than echoing eight decimals of false precision.

Output Format

Conversion: [amount] [FROM] → [TO]

[Converted amount] at [rate], ECB/reference rate dated [date].

[If relevant: "expect roughly X–Y after typical consumer spread"]

[Historical/trend questions: the series or the two dates compared]

Source: [Frankfurter (ECB) / open.er-api.com] · rerun: [exact curl]

Reference rates for information — not a trading quote; actual bank/card rates include a spread.

Quality Checks

• [ ] The rate's date is quoted, not implied
• [ ] Ambiguous currency words ("dollars", "francs") resolved or asked about
• [ ] "How much will I get" questions include the spread band
• [ ] The fallback engaged silently when needed, and the answer names its source
• [ ] Precision is sensible for the currency pair

Anti-Patterns

• [ ] Do not answer from memory — rates move; no network means hand over the command
• [ ] Do not present reference rates as attainable consumer rates
• [ ] Do not silently serve a stale weekend rate as "today's" — date it
• [ ] Do not echo full API precision — eight decimals on a lunch bill is theater
• [ ] Do not give currency-trading advice — conversion is arithmetic; timing the market is not this skill