<p>Magnetic resonance imaging (MRI) remains a cornerstone of non-invasive, high-resolution tumor diagnostics, with tens of millions of scans performed annually worldwide. However, its diagnostic precision is often limited by contrast agents that fail to respond to key pathological cues such as pH, redox state, or biomolecular gradients. Widely used gadolinium-based complexes and magnetic nanomaterials raise significant concerns regarding biocompatibility and biosafety, including nephrotoxicity, prolonged retention, and inadequate selectivity within the tumor microenvironment (TME), ultimately limiting contrast enhancement and undermining diagnostic reliability. Here, we report the design of a new class of gadolinium-free, magnetically switchable coordination cages as tumor-activated MRI contrast agents. The self-assembled heterobimetallic cages undergo rapid and complete oxidation of Fe²⁺ to Fe³⁺ under acidic and hydrogen peroxide-rich conditions, leading to the formation of dual-paramagnetic structures with a substantial improvement in longitudinal relaxivity. <i>In vitro</i> and <i>in vivo</i> MRI studies reveal that these cages selectively enhance <i>T</i>₁-weighted contrast in tumor tissues while maintaining low signal in normal cells and healthy organs, reflecting both high biocompatibility and biosafety. This differential imaging response enables highly precise, tumor-activated MRI. Our approach provides a general design strategy for constructing biochemically responsive MRI contrast agents, offering new opportunities for integrated diagnosis and therapy.</p>