Summer rainfall has increased over the northwestern edge of the Indian Summer Monsoon (ISM) region in recent years, leading to more frequent flood events and profound socio-economic impacts. However, climate model evaluations for this vulnerable marginal zone remain scarce, limiting confidence in climate simulations and future projections. This underscores the need for a rigorous assessment. Here, we evaluate 35 CMIP6 models and find that the multi-model ensemble mean captures only 23.8% of the observed rainfall trend. Meanwhile, 27 out of 35 models successfully reproduce the basin-wide Indian Ocean warming trend, which is the primary driver of the regional rainfall enhancement. Using the bulk formula, we attribute this failure to the overestimation of near-surface specific humidity. This excessive atmospheric moisture directly limits latent heat release from the warm ocean, reducing the sea level pressure response to ocean warming. Consequently, the models overestimate the sea level pressure over the tropical Indian Ocean. Accordingly, the cross‑equatorial pressure gradient weakens, diminishing southwesterly moisture transport to the marginal area and leading to rainfall underestimation. As a result, despite capturing Indian Ocean warming, only 5 out of 35 models can reproduce half of the observed changes in ISM rainfall.