We have successfully generated and measured the temporal profile of intense single-cycle 2.5 fs pulses, using a fully inline and collinear over-octave implementation of dispersion-scan (d-scan). We used a single-stage hollow-core fiber (HCF) compressor, pumped by sub-25 fs, 500 uJ pulses from a Ti:Sapphire CPA system (CLPU). The HCF was followed by ultra-broadband doubl- angle chirped mirrors (450-1000 nm, UltraFast Innovations GmbH), which are part of the single-cycle d-scan system (Sphere Ultrafast Photonics, SA). The spectral phase of the pulses was successfully retrieved over more than one octave using only SFG signal from 280 to 420 nm, revealing the generation of intense CEP-stable high-quality single-cycle pulses with 2.51 fs (FWHM), very close to the Fourier limit of 2.43 fs, and with a peak power of approximately 40 GW. This extreme source is highly promising for attoscience and high-field laser physics.