•RP, MgO, EG enhance PA6 thermal stability without affecting Tg or crystalline fraction.•Monomer conversion remains high (97%) with no significant impact from FRs.•RP and MgO with EG achieved synergistic effects due to different mechanisms.•0.5 mm PA6/5%MgO/5%EGES100 coating reduced the composite pHRR by 27% and THR by 37%.•FRs formed a stable charred layer on the composites and increased the residual mass. Continuous fibre-reinforced polyamide 6 (PA6) composites are predominantly prepared by anionic ring-opening polymerisation of low viscosity ε-caprolactam monomer. PA6 melts easily when exposed to flames, which can lead to rapid fire spread due to dripping. For this reason, the use of flame retardants is necessary. We investigated the effects of magnesium oxide (MgO), red phosphorus (RP), and expandable graphite (EG) on the glass transition temperature, crystalline fraction, and thermal stability of PA6. After investigating the flammability properties, we selected the best compositions (MgO and RP combined with EG) and applied them as coatings on carbon fibre-reinforced polyamide 6 composites. A synergistic effect was achieved when RP or MgO were used together with expandable graphite. The 0.5 mm thick coating containing 5% RP and 5% EG reduced the peak heat release rate (pHRR) of the composite by 21% and the total heat release (THR) by 28%, while 5% MgO and 5% EGES100 reduced the pHRR by 27% and the THR by 37%. Display omitted