Observed broadband SED of M87 contemporaneous with the EHT campaign in April 2018, with the exception of GMVA (carried on February 2018) and HST (carried on July 2018) observations (see Table C.1). The fluxes were measured by various instruments, and they are highlighted with different colours and markers. Filled markers represent flux point estimates, while empty markers indicates ULs. In the X-ray band, the two sets of points represent the core flux from the models with a power law (dark purple) and broken power law (light purple; see Table C.12). Grey points represent the observed broadband SED of the 2017 EHT campaign (EHT MWL Science Working Group 2021). MAGIC observations were performed before the γ-ray flare, VERITAS observations only partially overlap the flare, while all H.E.S.S. observations have been taken during the VHE γ-ray flaring episode (see Table C.1). For the radio/millimetre VLBI and interferometric data, the upper limits on emission size (radius) for several representative frequencies, that are obtained by following the procedures described in M87 MWL2017, are labelled to clarify the constraints used in Sect. 4.