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We present quasi-simultaneous radio, (sub-)millimetre, and X-ray observationsof the Galactic black hole X-ray binary GX 339-4, taken during its 2017--2018outburst, where the source remained in the hard X-ray spectral state. Duringthis outburst, GX 339-4 showed no atypical X-ray behaviour that may act as aindicator for an outburst remaining within the hard state. However,quasi-simultaneous radio and X-ray observations showed a flatter than expectedcoupling between the radio and X-ray luminosities (with a best fit relation of$L_{\rm radio} \propto L_{\rm X}^{0.39 \pm 0.06}$), when compared to successfuloutbursts from this system ($L_{\rm radio} \propto L_{\rm X}^{0.62 \pm 0.02}$).While our 2017--2018 outburst data only span a limited radio and X-rayluminosity range ($\sim$1 order of magnitude in both, where more than 2-ordersof magnitude in $L_{\rm X}$ is desired), including data from other hard-onlyoutbursts from GX 339-4 extends the luminosity range to $\sim$1.2 and $\sim$2.8orders of magnitude, respectively, and also results in a flatter correlation(where $L_{\rm radio} \propto L_{\rm X}^{0.46 \pm 0.04}$). This result issuggestive that for GX 339-4 a flatter radio -- X-ray correlation, implying amore inefficient coupling between the jet and accretion flow, could act as anindicator for a hard-only outburst. However, further monitoring of bothsuccessful and hard-only outbursts over larger luminosity ranges with strictlysimultaneous radio and X-ray observations is required from different, singlesources, to explore if this applies generally to the population of black holeX-ray binaries, or even GX 339-4 at higher hard-state luminosities.

Disk-jet coupling changes as a possible indicator for outbursts from GX 339-4 remaining within the X-ray hard state