You can either treat TSS and CDS as two separate integrations, or as one pipeline with two exit points. The second approach ages better.
Understand the roles
CDS is HMRC's Customs Declaration Service. It processes UK import and export declarations. TSS is a free portal specifically for Northern Ireland movements that formats and submits data into CDS.
If you build customs software, CDS is where you certify. TSS is where many Northern Ireland movements pass through on their way to CDS.
Data model first
Design your internal declaration record to match CDS field structure. If your record is CDS-shaped, feeding it into TSS is straightforward, and feeding it into a direct CDS submission is a matter of connector configuration.
This TSS vs CDS explainer is a useful primer for engineers coming into the space for the first time.
Auth and identity
Plan for GB EORI, XI EORI, Government Gateway credentials, UTR and NI number handling. Different traders on the same platform will have different identifiers, and Northern Ireland movements often need both a GB and XI EORI.
Error surfacing
Rejections need to reach the operator with enough context to fix the source data — not just "TSS rejected" or "CDS rejected". Map error codes to human-readable prompts.
Volume considerations
Above a certain declaration volume, portal-based TSS filing becomes a bottleneck. Route those flows through direct CDS instead.
See a real integration between customs software, TSS and CDS. Watch a demo with iCustoms.
