Cancer-associated fibroblasts (CAFs) support and drive cancer development, including the growth and metastasis of tumours. CAFs can differentiate from mesenchymal stem cells (MSCs), a process shown to be influenced by the stiffness of the extracellular matrix (ECM) in the tumour microenvironment. However, by analysing single-cell RNAseq data, in this study (Saxena et al., 2024), Shamik Sen and colleagues identify a subpopulation of MSC-derived CAFs from triple-negative breast cancer (TNBC) tumours that express markers for both MSCs and CAFs (termed MSC-CAFs). The authors investigate the effect of ECM stiffness on MSC-to-CAF differentiation in this context by growing MSCs on hydrogels of differing stiffness and treating them with conditioned medium from TNBC cells. Here, they find that MSCs grown on 2 kPa hydrogels, which mimic the stiffness of the pre-metastatic niche, show the highest efficiency of MSC-to-CAF differentiation and chemotaxis in response to TNBC-conditioned medium. Using TNBC spheroids with or without MSC-CAFs, they show that MSC-CAFs from 2 kPa hydrogels maximally enhance the proliferation, invasiveness and shear resistance of TNBC cells compared to MSC-CAFs grown on stiffer hydrogels. Taken together, these findings suggest that homing of MSCs to pre-metastatic tumours, their differentiation into CAFs, and their concomitant pro-tumorigenic and pro-metastatic effects are dependent on ECM stiffness in the tumour microenvironment.