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Context. Recently, SDSS J143016.05+230344.4 (J1430+2303) was reported to be asupermassive black hole binary (SMBHB) in the final coalescence phase. It isprobably the first SMBHB coalescence event observable in human history. Radioobservations of J1430+2303 before and after coalescence will provide a uniquediagnosis of the energetics and environment of the SMBHB.  Aims. We explore the radio emission from the galactic nucleus region that isclosely related to the current X-ray and optical activities and helps tounderstand the state of black hole accretion and outflow before coalescence.  Methods. Very long baseline interferometry (VLBI) imaging is the only methodthat offers milli-arcsecond-level high resolution that can exclude thecontamination by diffuse emission on galactic scales. We observed J1430+2303with the European VLBI Network at 1.7 GHz and with the Very Long Baseline Arrayat 1.6 and 4.9 GHz in late February and early March 2022.  Results. A compact component is detected in all three VLBI images. It has abrightness temperature of > 10^8 K, an unresolved morphology with a size < 0.8pc, and a flat radio spectrum. These observational features are inconsistentwith large opening-angle outflows or winds, but indicate that this compactcomponent might be a jet or a corona. Nearly 60% of the emission is resolved byVLBI and may come from remnant lobes of previous radio activities, the outerlayers of a structured jet, or shocks formed by the disc winds in the narrowline region.  Conclusions. Current VLBI images do not yet show signs of radio outbursts.Our observations provide pre-coalescence radio data that are an importantreference for future comparative studies with the post-merger. In particular,further resolving the jet will pave the way for probing the dynamical featuresassociated with inspiralling binary black holes.

VLBI imaging of the pre-coalescence SMBHB candidate SDSS J143016.05+230344.4