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The promise

LandedFees never returns an estimated duty rate. Every number on every response is traceable to a primary source: a tariff schedule row, a Federal Register notice, a European Commission implementing regulation, a UK Trade Tariff line, or a WCO Harmonized System code family. If the engine cannot find the rate, it returns a data_gap compliance flag instead of a fabricated number.

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Where the sources are exposed

Two places on every response:

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1. data_as_of

An object mapping each data source used in the calculation to the UTC timestamp when that source was last refreshed. Example:
Use this to satisfy audit-trail requirements (“the rate applied on 2026-08-23 was current as of the previous business day”).

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2. Compliance flag citations

Every entry in compliance_flags[] and fta_opportunities[] carries:
  • source: human-readable authority name (e.g. USITC HTS 2026 Rev 8).
  • source_url: deep link to the underlying document.
  • legal_citation: the statute or regulation section (e.g. 19 USC 1466(a), HTS 9903.88.15, Federal Register 2026-08-19).
For rate-only responses (/v1/rates), the same information ships under sources[] and data_as_of.

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The primary-source ladder

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Why WITS is used for the non-US non-EU tail

The US and EU publish tariff schedules in machine-readable form on a daily-or-better cadence. Most other countries do not. For the long-tail destinations, LandedFees pulls MFN and preferential rates from the World Bank’s WITS system, which aggregates WTO IDB and CTS notifications from member countries. WITS is refreshed monthly and is the most authoritative public source for these destinations. When you calculate for a WITS-sourced destination, the response attributes the rate to wits_wto and links back to the WITS record. The rate is still the country’s official notified rate; the routing through WITS is disclosed for transparency.

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Reading a citation

The legal_citation field on a Section 301 flag might read:
Break it down:
  • USTR Section 301 List 4A: the action.
  • HTS 9903.88.15: the specific Chapter 99 subheading that attaches the ad valorem duty.
  • 2019-08-30: the publication date of the underlying Federal Register notice.
Copy this string verbatim into your audit workpapers. It is the citation that CBP and a broker would recognize.

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Data-gap posture

When a rate cannot be resolved, the response includes:
The calc will still return a total_landed_cost value, but the missing layer’s amount is 0 with a note. Do not ship a critical data_gap response into production without broker review.