A 2^。 C climate-warming scenario is expected to further exacerbate average crop losses by 3%–13%, yet few heat-tolerant staple-crop varieties are available toward meeting future food demands. Here, we develop high-efficiency prime-editing tools to precisely knockin a 10-bp heat-shock element (HSE) into promoters of cell-wall-invertase genes (CWINs) in elite rice and tomato cultivars. HSE insertion endows CWINs with heat-responsive upregulation in both controlled and field environments to enhance carbon partitioning to grain and fruits, resulting in per-plot yield increases of 25% in rice cultivar Zhonghua11 and 33% in tomato cultivar Ailsa Craig over heat-stressed controls, without fruit quality penalties. Up to 41% of heat-induced grain losses were rescued in rice cultivar Zhonghua11. Beyond a prime-editing system for tweaking gene expression by efficiently delivering bespoke changes into crop genomes, we demonstrate broad and robust utility for targeted knockin of cis-regulatory elements to optimize source-sink relations and boost crop climate resilience.