Semantic artefacts (SA) –a broad term to include ontologies, terminologies, taxonomies, thesauri, vocabularies, metadata schemas and semantic standards– are key for the description of data and for making data FAIR. Plus, describing SAs is fundamental to make them FAIR themselves. The Metadata for Ontology Description and Publication (MOD) ontology was developed since 2017 within the RDA VSSIG and adopted by FAIR-IMPACT to provide a reference vocabulary to semantically describe Semantic Artefacts https://github.com/FAIR-IMPACT/MOD). It was designed as an extension of the Data Catalogue Vocabulary (DCAT), the W3C Recommendation to describe datasets and resources that can be catalogued. Within FAIR-IMPACT’s WP4 on ontologies and metadata, T4.2 aims to establish guidelines and community practices with respect to the lifecycle of FAIR semantic artefacts from creation (T4.2.1) to sharing and reuse via catalogues or repositories (T4.2.2). With work on MOD, the task also standardised the mechanisms to describe and serve semantic artefacts (T4.2.3) within EOSC. The task has previously released the milestone M4.3 (Specification of semantic artefact description) that contained the latest specification of the MOD (version 3), updated mappings for MOD properties (https://github.com/FAIR-IMPACT/MOD-mappings) and added automatic profiles to check compliance with MOD. This deliverable describes the latest changes of MOD, mostly done to encompass the description of Semantic Artefact Catalogues in addition to Semantic Artefacts (and their distributions). Then, the deliverable builds on the MOD vocabulary to provide the specification of an Application Programming Interface (API) for interoperability of Semantic Artefact Catalogues in the European Open Science Cloud (EOSC) ecosystem and beyond. This API is formalised by means of the OpenAPI specification and an OpenAPI template that shall guide for the implementation of a REST API. The objects returned by an implementation of the API shall all be standard semantic objects either from a W3C Recommendation (RDFS, OWL and SKOS) or MOD. We are expecting the MOD-API for Semantic Artefact Catalogues (https://github.com/FAIR-IMPACT/MOD-API) to enable interoperability and unified access to their content, enabling seamless querying and use by stakeholders independent of domain. The API will be adopted by FAIR-IMPACT T4.2’s use case SA-catalogues (at least AgroPortal, EcoPortal, EarthPortal, and potentially LOV) and it is publicly available for other catalogues to implement. The implementation of this API will also be the topic of an upcoming FAIR-IMPACT Open Call.