We present the combined JWST/NIRSpec/G395H and NIRISS/SOSS transmission spectrum of a warm mini-Neptune, TOI-1130 b (3.66 R⊕, 19.8 M⊕, and Teq ∼ 825 K). It is part of a rare and unique multiplanet system, TOI-1130, which hosts an inner mini-Neptune and an outer hot Jupiter locked in a 2:1 mean motion resonance. From the transmission spectrum of TOI-1130 b we detect multiple molecules—H2O (7.5σ), CO2 (3.3σ), and SO2 (3.6σ), as well as a tentative detection of CH4 (∼2σ). We find a strong optical slope in the NIRISS/SOSS spectrum, which is consistent with TESS and CHEOPS transit depth measurements. From equilibrium chemistry retrievals we measure the atmospheric metallicity ( logZ/Z⊙=1.8−0.3+0.4 ) and C/O ratio (<0.75 at 3σ level confidence) and constrain the atmospheric mean molecular weight, μ = 5.5 −0.8+1.3 amu. These constraints are consistent with self-consistent forward model grids. We detect no significant He I 1.083 μm absorption signal and find a mass-loss rate upper limit of 1011 g s−1. The volatile-rich high mean molecular weight atmosphere of TOI-1130 b along with the "pebble-filtering" effect of the outer hot Jupiter supports the ex situ formation scenario beyond the water ice line and subsequent migration, coherent with its present orbital architecture. A volatile-rich formation scenario could also potentially explain the location of TOI-1130 b at the edge of the "radius cliff." This result hints that the mini-Neptune population may not have a homogeneous formation history; rather, volatile-rich ex situ formation also contributes to its population. *This study uses CHEOPS data observed as part of the Guaranteed Time Observation (GTO) programme CH_PR149003.