Precise small-strain detection is critical for various applications, yet most existing technologies lack conformability, environmental robustness, high performance, and fabrication simplicity. Here, we introduce a material-independent design strategy to prepare ultrasensitive stretchable sensors that leverage two cracking mechanisms: cut-through cracks abruptly rupture dominant conductive pathways, while mesh-like cracks drive the progressive degradation of auxiliary conductive pathways, collectively leading to a rapid and stable resistance increase under small strains. The resulting sensor possesses an ultrahigh gauge factor (~430), an ultralow detection limit (0.0003365% strain), and high durability (> 5,000 tensile cycles). Through conformal, in situ, and real-time structural monitoring with a high signal-to-noise ratio (> 50 dB), the sensor reliably captured a small compressive strain signal for deep-sea equipment under extreme hydrostatic pressure (50 MPa). Our design offers a general, scalable route for ultrasensitive sensing in aerospace, ocean engineering, and precision manufacturing.