Frankfurter (ECB reference rates)#

finance-query = { version = "...", features = ["frankfurter"] }

Frankfurter publishes the European Central Bank's daily reference exchange rates over a keyless JSON API — no registration, no API key.

It is the only keyless FOREX route in the library. Polygon, FMP, and Alpha Vantage all require a key, which meant providers.forex() did nothing out of the box; Frankfurter closes the same gap for forex that Yahoo covers for quotes, CoinGecko for crypto, and EDGAR for filings.

Setup#

use finance_query::{Capability, Provider, Providers};

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let providers = Providers::builder()
        .route(Capability::FOREX, [Provider::Frankfurter])
        .build()
        .await?;

    let quote = providers.forex("USD", "EUR").quote().await?;
    println!("USD/EUR {:?} ({:+.2?}%)", quote.price, quote.change_percent);
    Ok(())
}

What ECB Reference Rates Are#

That shapes the response:

If you need intraday rates or a real bid/ask, route FOREX to a keyed provider first and leave Frankfurter as the fallback:

use finance_query::{Capability, Provider, Providers};

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let providers = Providers::builder()
        .route(Capability::FOREX, [Provider::Polygon, Provider::Frankfurter])
        .build()
        .await?;
    Ok(())
}

Coverage#

Roughly 30 currencies — the set the ECB publishes against the euro, with any pair among them derivable. Notably not included: most emerging-market currencies, and cryptocurrencies. Requesting a currency the ECB does not publish returns FinanceError::SymbolNotFound.

Passing the same currency twice (forex("USD", "USD")) is answered locally with a rate of 1.0; Frankfurter itself rejects that pair with HTTP 422.

Historical Rates#

ForexPair::chart() and history() route through Capability::CHART, which Frankfurter does not serve — those still resolve on the Yahoo default route (or whichever provider you route CHART to). Frankfurter contributes the current-rate path only.

Rate Limits#

Frankfurter is free and unmetered but community-run. The client paces itself at 5 requests/second, and a 429 surfaces as FinanceError::RateLimited.

Each quote costs exactly one request: the adapter asks for a short date range rather than the latest endpoint, so the current rate and the previous close needed for the day's change arrive together.

Next Steps#

• Forex Domain — the ForexPair handle
• Providers Overview — routing and fallback