Key answer
An FX data API needs more than a currency pair and number. Its contract should define rate type, units, timestamps, transaction conditions, permitted use and failure states so downstream analysis can distinguish facts from assumptions.
MFXIntel editorial · Updated · Methodology, not a live quote
Start with a small, unambiguous observation
Define an observation identifier, source identifier, base and quote currencies, rate direction, original precision and observation time. Add collected_at and a validity boundary only with clear meanings. For transaction quotes, include amount, funding method, payout method and fee convention. Do not invent optional information to satisfy a response schema; null and unavailable are meaningful states that consumers must understand.
Specify how clients should handle changes
Document stable pagination, deduplication identifiers, corrections and removed records. A client needs to distinguish a genuinely new observation from an older record fetched again. State whether history is append-only or can be corrected, and preserve revision context where appropriate. Agree how the consumer will discover a schema change before it breaks a chart, reporting job or customer-facing comparison.
Keep public documentation separate from private payloads
API documentation can explain field semantics and use clearly labelled synthetic examples without exposing actual customer records or internal keys. Enforce authentication and dataset entitlements at the backend. Permission to call an API is not automatically permission to republish its response. Publication and commercial redistribution should be reviewed at the dataset and intended-use level before anything reaches a public page.
Test degraded operation, not just the happy path
Acceptance testing should include expired quotes, missing fees, empty pages, timeouts, unsupported currencies and revoked access. Avoid silently substituting a different provider or benchmark on failure. Public caching may be appropriate for editorial documentation, while private responses and actionable quotes require separate policies. MFXIntel describes this integration approach; this page does not advertise a currently available unrestricted public FX API or a promised service-level agreement.
Sources and methodology
MFXIntel — source, permission and observation methodology ↗
MFXIntel — measuring FX data freshness ↗
External sources support definitions and context. Linking does not imply a partnership or permission to redistribute a source dataset.