The MER DR1-Foundation data processing build from the Q1 version of the PF described in detail in the Euclid Data Release Q1 - From images to multiwavelength catalogues: the Euclid MERge Processing Function article. Improvements introduced for DR1 processing are described in Euclid Data Release 1 (DR1). The MERge processing. The MER processing has been extensively tested on the COSMOS field in Euclid Data Release 1 (DR1). Multi-band photometry validation on COSMOS extragalactic sources.