Judgment of the Court (First Chamber) of 11 December 2025.#A v GKV-Spitzenverband.#Request for a preliminary ruling from the Landessozialgericht für das Saarland.#Reference for a preliminary ruling – Agreement between the European Community and its Member States, of the one part, and the Swiss Confederation, of the other, on the free movement of persons – Migrant workers – Social security – Applicable legislation – Regulation (EC) No 883/2004 – Article 11 – Article 13(1) – Regulation (EC) No 987/2009 – Article 14(8) – Worker pursuing an activity as an employed person in the territory of several States, including a Member State, the Swiss Confederation and third countries – Concept of ‘substantial part of the activity’ – Taking account of the activity pursued in third countries.#Case C-743/23.