STAT3 and neoplasm: In summary, HMP1G nanoparticles downregulated tumor‐derived IDO1 via the AhR/STAT3/IL axis, ameliorating KYN/TRP metabolic dysregulation, effectively overcoming the metabolic immune suppression in the TME, reversing the immune tolerance of cold tumors, and making tumor cells sensitive to 1‐MT‐mediated ICB.