Finance & FX Data

Fetch exchange rates and crypto prices with keyless calls. The mistake this prevents: reaching for exchangerate.host, Fixer, or Alpha Vantage from training memory - all key-walled now - or hand-waving a stale rate from training data into a real calculation.

Ranked APIs

1. Frankfurter (api.frankfurter.dev) - DEFAULT for fiat FX. European Central Bank reference rates, ~30 major currencies, history back to 1999, time series, no key, no rate limit worth worrying about.
2. CoinGecko free tier (api.coingecko.com) - crypto prices, market caps, 24h change for thousands of coins. No key, but a tight limit: roughly 5-15 calls/min from an anonymous IP. Batch ids into one call; on 429, back off 60s.

Procedure

1. Fiat or crypto? Fiat → Frankfurter. Crypto → CoinGecko. Crypto→fiat conversion is one CoinGecko call (vs_currencies=eur), not two hops.
2. Date the rate. ECB reference rates update ~16:00 CET on business days; the response's date field tells you which day you actually got. Weekend/holiday requests return the previous business day - quote that date, don't present it as today's.
3. Use CoinGecko ids, not tickers. The ids param wants bitcoin, ethereum, solana - not BTC/ETH. For an unfamiliar coin, resolve once via /api/v3/search?query={name} → coins[0].id.
4. Never do FX math from memory. If the task multiplies money by a rate, the rate comes from one of these calls, with the date attached.

Ask the user only if missing: currency pair or coin (no default), date/range (default: latest), amount (default: 1).

URL templates + real response shapes

Latest FX rate:

https://api.frankfurter.dev/v1/latest?base=USD&symbols=EUR,GBP,JPY

{"amount":1.0,"base":"USD","date":"<last business day>","rates":{"EUR":0.87352,"GBP":0.74878,"JPY":161.15}}

Convert an amount server-side with &amount=250 - rates comes back pre-multiplied.

Historical / time series:

https://api.frankfurter.dev/v1/2024-01-15?base=USD&symbols=EUR          ← one date
https://api.frankfurter.dev/v1/2024-01-02..2024-01-31?base=USD&symbols=EUR   ← range
https://api.frankfurter.dev/v1/currencies                               ← code → name map

Range shape: {"rates":{"2024-01-02":{"EUR":0.91274},"2024-01-03":{"EUR":0.91583}}} - keyed by date, business days only (a full year returns ~256 daily points; no weekend keys, so don't treat gaps as missing data). Omit the end date (/2024-01-02..) to run to present.

Crypto spot price:

https://api.coingecko.com/api/v3/simple/price?ids=bitcoin,ethereum&vs_currencies=usd&include_24hr_change=true

{"bitcoin":{"usd":63289,"usd_24h_change":1.19},"ethereum":{"usd":1790.89,"usd_24h_change":1.90}}

Unknown ids are silently dropped from the response - a missing key means a wrong id, not a null price.

Crypto market table (rank, cap, supply):

https://api.coingecko.com/api/v3/coins/markets?vs_currency=usd&order=market_cap_desc&per_page=20&page=1

Array of {"id","symbol","name","current_price","market_cap","market_cap_rank","price_change_percentage_24h","circulating_supply","ath"} - one call replaces twenty /simple/price calls; prefer it for any list.

Deliverable

The rate or price with its date/timestamp and source named, plus the exact URL used. Any conversion shows the arithmetic: 250 USD × 0.87352 (ECB, <rate date>) = 218.38 EUR.

Do NOT use - reach for X instead

• exchangerate.host - every request now requires an access_key; verified failing keyless. Frankfurter replaces it.
• Fixer, Alpha Vantage, Finnhub, Polygon, OpenExchangeRates - key-walled; do not send a user to sign up when Frankfurter/CoinGecko cover the need.
• Stock/equity quotes - nothing keyless survives verification. Say that plainly and name the paid options rather than fabricating a price.
• GDP, inflation, unemployment, trade series - government-open-data.
• Financial advice - this skill fetches numbers; buy/sell/allocation judgments are out of scope. Present data, not recommendations.

Quality bar

• Every number carries its date and source; no rate or price originates from training memory.
• Frankfurter's ~30-currency ceiling respected - exotic pairs (e.g. NGN, VND) are declared unsupported, not approximated.
• CoinGecko calls are batched; a 429 produces a backoff, not a retry loop.

Sourced and liveness-verified from the public-apis project (MIT).