Under Chapter I, at the same time as establishing the search report the search examiner must establish the written opinion of the ISA (WO‑ISA) to be sent to the applicant together with the search report. The WO‑ISA gives a preliminary and non-binding opinion on whether the claimed invention appears to be novel, to involve an inventive step and to be industrially applicable. When appropriate, an opinion will also be given on added subject-matter, unity, insufficient disclosure and clarity or support issues, as well as formal defects.[Rule 43bis; GL/ISPE 17; ]

The findings of the written opinion must be consistent with the document categories assigned in the search report and must also be consistent with any other issues raised in the search report, such as lack of unity of invention or limitation of the search.

If there are no defects in the application, the WO‑ISA will state the reasons why the application is considered to fulfil the requirements of novelty, inventive step and industrial applicability.

The written opinion (and any informal comments filed by the applicant) will be made available to the public by the IB at the same time as the international publication.[Art. 21(3); GL/ISPE 2.17; ]

If the application subsequently enters the EP phase, the applicant is obliged to reply to any negative WO‑ISA or IPRP/IPER. The WO‑ISA is thus comparable to the ESOP in the European procedure.