This study examines changes in the isotopic composition of Indian summer monsoon precipitation during the Last Glacial Maximum (LGM) relative to the pre-industrial period, using an isotope-enabled general circulation model with vapor source tagging. The authors found that the LGM simulation shows 15% less monsoon rainfall, mainly due to thermodynamic drying from lower atmospheric moisture and enhanced subsidence over India. With the water vapor source tagging method, they further found that while primary moisture sources remain the same, their contributions weaken, producing δ¹⁸O enrichment. This enrichment stems mainly from reduced input of isotopically depleted vapor rather than the local amount effect. The results suggest that δ¹⁸O in Indian monsoon records reflects large-scale circulation changes rather than local precipitation intensity. The paper is very well organized and clearly described. I would like to recommend an acceptation for publication after some minor improvements.

1. For Fig. 4, it would be better to also mark the research domain as in Fig. 3.
2. One of the main points of this study is to explain the reason of the reduced rainfall during the LGM compared to the pre-industrial period. While the manuscript has only 8 figures in the main text, I suggest to change one Figure (Fig. S8 or S9) from the supplements into the main text to show the corresponding changes in temperature and circulation.

This manuscript uses isotope-enabled CESM with water-vapor source tagging to compare Indian Summer Monsoon precipitation and δ18O between the Last Glacial Maximum and pre-industrial climates. The authors find monsoon drying at LGM and enrichment of precip​itation d18O, and convincingly show that the enrichment cannot be explained by the local amount effect but instead arises primarily from reduced relative contributions from distant, isotopically depleted sources and weaker upstream rainout along transport pathways. The modeling framework and decomposition are solid, and the results are clearly presented. I recommend minor revision.

Minor comments:

1. It is interesting to see that some monsoon indices show the Indian monsoon is strengthened in the LGM, while other indices show the opposite. Since the paper is run with prescribed SST/sea ice, do similar results appear in the coupled run of iCESM? 

2. It is also interesting to see that the changes in Pacific moisture contribution are very important in regulating the positive precip d18O anomaly in India. Why is that? Is it because less moisture comes from the Pacific due to the strengthened westerlies in the subtropics? The authors can add a discussion for this. Also, how to understand that precipitation is less in India, but the condensation effect is negative in Figure 8c? 

3. After the authors draw conclusions that Indian precip d18O in the LGM cannot be explained by the amount effect but by moisture source changes, how should we interpret speleothem/marine sediment d18O changes in the Indian monsoon region in the LGM?

Equation (1): Some letters in the subscripts of P are not subscribed.

Line 251: P J et al. 2020: Please write the complete last name

Equation (3): Please write the equation in the equation mode.

Lines 327-337: Though the averaged precip d18O has a large bias, it seems that precip d18O in most places of India is close to the observations.

Figure 2c: “Source contribution” is kind of misleading here. I thought it was the contribution due to moisture source changes (like you defined in Line 293). Please consider changing it to other words.

Lines 439-442: What is the region of the “weakening of westerley”? It is not clear to me. The strengthening of the westerly is obvious.

Line 467: missing right parentheses

Line 485: It seems that the vertical advection term can also explain some drying of western India.

Line 623: Speleothem -> speleothem