The most extreme construction site in the early universe is already running hot beyond its budget. SPT2349-56, a protocluster at redshift 4.3 caught with at least fourteen starbursting galaxies packed into a region barely 130 kiloparsecs across, was expected to be a system still assembling its intracluster medium; instead, a 10.4 sigma thermal Sunyaev-Zeldovich detection (Zhou et al. 2025, Nature) reveals hot gas with a thermal energy of (11.8 +/- 1.2) x 10^60 erg, sitting 6.4 sigma above the TNG-Cluster simulation expectation for halos of its mass, five times above the universal mass-Compton-Y scaling, and an order of magnitude beyond what gravitational collapse of its 9 x 10^12 solar mass halo can supply. The repair within the standard model requires AGN feedback to have injected the difference, and the arithmetic turns self-refuting: the required thermal coupling efficiency comes out at 120 +/- 20 percent of the available AGN energy, more heat deposited than the engine produces.
SPT2349-56 leads a class: massive, evolved, tightly bound protoclusters at redshifts 2.5 through 4 and beyond (the Spiderweb's ICM detection, distant red-core systems) keep arriving earlier, denser, and thermally fuller than hierarchical assembly schedules, compounding the early massive-galaxy and early-quenching timeline problems into the cluster regime. Each system's gas should take additional gigayears of infall and shock heating to assemble; the measurements find it present, hot, and bound at cosmic times when the model is still gathering the ingredients.
The standing is a young, sharp anomaly with a flagship measurement: a single 6.4 sigma thermal excess whose conventional repair violates energy conservation, and an observational program (ALMA SZ follow-up of additional z above 3 protoclusters) explicitly designed to determine whether SPT2349-56 is an outlier or the rule.