EPO Espacenet & Open Patent Services
Espacenet is the European Patent Office's public patent search service, giving free access to over 140M patent documents from around the world. Alongside it, Open Patent Services (OPS) provides programmatic access and text analysis tools that have made getting at patent full texts considerably more straightforward.
This is a strong resource for reading and searching patents, but the access model matters for RAG. The EPO's terms allow free use for most purposes, yet they do not grant a blanket right to reuse the data in a commercial product. This is a case of "open access, not open licence": you can read the documents for free, but building a product on top of them is restricted.
If you plan to commercialise anything derived from Espacenet data, check the EPO's terms carefully or seek a specific arrangement before you rely on it.
Related sources
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Harvard USPTO Patent Dataset (HUPD)
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WIPO PATENTSCOPE
A search service for international patent collections, maintained by the World Intellectual Property Organization (WIPO). It lets you search across patent applications filed under international treaties alongside national collections from many countries.