Reading source and market_session in your pipeline

Every call to /v1/latest, /v1/convert, or /v1/range returns a source field and a market_session field alongside the rate data. Read both before you act on a number. source tells you the quality class of the data; market_session tells you the state of the FX trading day when that data was captured.

What each field contains

source classifies where the rate came from. ecb_daily means the European Central Bank reference fix, published once per business day around 16:00 CET. fred_daily means the Federal Reserve daily series. live means aggregated spot data that updates approximately every 60 seconds through the trading week.

market_session describes the state of the interbank trading week. It is open during the trading week, weekend from the Friday 17:00 New York close until Sunday 17:00 New York, or interbank_closed for a known non-weekend closure. It does not name Sydney, Tokyo, London, or New York sessions.

Reading both fields together

Neither field alone gives you the full picture. A rate with source: live and market_session: weekend means the interbank market is closed and the live field is carrying its last intraday consensus from the trading week, not a fresh weekend quote. A rate with source: ecb_daily and market_session: weekend means you have a reference fix that has not moved since Friday.

The example below reads both fields and logs a freshness decision. It calls /v1/latest/USD and checks whether the rate is from the live source before using it in a downstream calculation:

import requests

API = "https://api.exchangerate.dev/v1"
KEY = "exr_live_..."

resp = requests.get(
    f"{API}/latest/USD",
    headers={"Authorization": f"Bearer {KEY}"},
    timeout=10,
)
resp.raise_for_status()
data = resp.json()

source = data["source"]           # e.g. "ecb_daily"
session = data["market_session"]  # e.g. "open"
updated = data["data_updated_at"] # e.g. "2026-06-19T15:05:12Z"

if source == "live":
    print(f"Live rate, session: {session}, updated: {updated}")
    eur = data["rates"]["EUR"]
    # proceed with calculation
elif source == "ecb_daily":
    print(f"Reference fix, updated: {updated}, session: {session}")
    # decide whether to proceed or wait
else:
    print(f"FRED daily series, updated: {updated}")

Enforcing live-only data in your client

The API does not accept a source=live query filter. If your pipeline requires live spot, request only the symbols you need and reject any response whose per-currency sources value is not live. This keeps the freshness policy in your application and makes a daily-reference fallback explicit.

resp = requests.get(
    f"{API}/latest/USD",
    headers={"Authorization": f"Bearer {KEY}"},
    params={"symbols": "EUR"},
    timeout=10,
)
resp.raise_for_status()
data = resp.json()

if data["sources"]["EUR"] != "live":
    raise RuntimeError(
        f"Live EUR unavailable; received {data['sources']['EUR']}"
    )

eur = data["rates"]["EUR"]
print(eur, data["data_updated_at"], data["market_session"])

For a multi-symbol request, use the sources map rather than the top-level source: the top-level value reports the least-fresh class anywhere in the response. A single daily-reference currency can therefore make a mixed basket report ecb_daily even when the other symbols are live.

Weekend behavior by source

On weekends, the source field separates two situations. source: ecb_daily or source: fred_daily with market_session: weekend means the reference institution has not published since Friday, and the rate reflects Friday's fix. For source: live with market_session: weekend, the interbank market is closed — the last intraday consensus from the trading week is carried.

The notice field

Every response also carries a notice field. For live data it reads: "Indicative rates, not for settlement." For reference-rate data it adds attribution to the ECB. The notice is machine-readable so you can surface it to your own users if you display rates in a UI.