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The present-day envelope of gaseous planets is a relic of how these giantplanets originated and evolved. Measuring their elemental composition thereforepresents a powerful opportunity to answer long-standing questions regardingplanet formation. Obtaining precise observational constraints on the elementalinventory of giant exoplanets has, however, remained challenging due to thelimited simultaneous wavelength coverage of current space-based instruments.Here, we present thermal emission observations of the non-transiting hotJupiter $\tau$ Boo b using the new wide wavelength coverage(0.95$-$2.50$\,\mu$m) and high spectral resolution ($R=70\,000$) SPIRouspectrograph. By combining a total of 20 hours of SPIRou data obtained overfive nights in a full atmospheric retrieval framework designed forhigh-resolution data, we constrain the abundances of all the major oxygen- andcarbon-bearing molecules and recover a non-inverted temperature structure usinga new free-shape, nonparametric TP profile retrieval approach. We find a volumemixing ratio of log(CO)$\,\,=-2.46_{-0.29}^{+0.25}$ and a highly depleted waterabundance of less than $0.0072$ times the value expected for a solarcomposition envelope. Combined with upper limits on the abundances of CH$_4$,CO$_2$, HCN, TiO, and C$_2$H$_2$, this results in a gas-phase C/H ratio of5.85$_{-2.82}^{+4.44}\times\,$solar, consistent with the value of Jupiter, andan envelope C/O ratio robustly greater than 0.60, even when taking into accountthe oxygen that may be sequestered out of the gas-phase. Combined, the inferredsuper-solar C/H, O/H, and C/O ratios on $\tau$ Boo b support a formationscenario beyond the water snowline in a disk enriched in CO due to pebbledrift.

Where is the Water? Jupiter-like C/H ratio but strong H$_2$O depletion found on $\tau$ Bo\"otis b using SPIRou