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Transitional millisecond pulsars (tMSPs) provide a unique laboratory to study the interaction between an accretion disc and a rapidly rotating neutron star magnetosphere. A key open question is whether, during the X-ray high mode, the accretion disc penetrates the magnetosphere and extends close to the neutron star surface. This issue has important implications for the accretion geometry, pulsar emission mechanisms, and the possibility of continuous gravitational wave emission. In this presentation, I will discuss results from our investigation of the disc-magnetosphere configuration in the prototypical tMSP PSR J1023+0038 using detailed X-ray spectral analysis. We isolate the X-ray high-mode intervals and analyse observations from multiple telescopes, including XMM-Newton (EPIC-PN, MOS1, MOS2), NuSTAR, NICER, and Chandra X-ray Observatory. Combining the longest available exposures, we obtain about 202 ks of high-mode data from a total exposure of ~364 ks and perform a self-consistent spectral modelling. Our analysis constrains the inner disc radius to 16.8 ± 3.8 km with at least 3𝜎 significance, indicating that the accretion disc extends very close to the neutron star. Consistent constraints are also obtained from independent NICER observations and from joint XMM-Newton + NuSTAR data. Furthermore, first time, we reported a Fe emission line at 6.45 keV from a tMSP in the Chandra spectrum with 99% significance, implying an upper limit on the inner disc radius of ~21 𝑅𝑔. These results strongly suggest that, during the high mode, the accretion disc in PSR J1023+0038 penetrates the magnetosphere and extends within the corotation radius, supporting models of accretion-powered pulsations and a possible source for continuous gravitational wave emission.