City of Gears and Dragons

Deep underground, the city lives on dragon bones and boiler steam. Everyone else treats that as a fact of engineering. Kessa Emberline hears it as a song. Kessa is a "boiler gremlin" in the Warrens, a low-tier mechanic who keeps ancient dragon-relics humming so the rich tiers above can pretend they run on neat, modern cores. She is good with wrenches, better with sarcasm, and absolutely not supposed to hum to the relics when no one is listening.
Theryn Volkan is the youngest Arch-Calculist in Scalehold history, a prodigy who can predict cave-ins and heat surges with terrifying accuracy. His answer to every problem is more order, more equations, more artificial dragonfire cores. His models say the old relics are dying and the city must move on. Then a failing boiler sings off-key, Kessa's instinctive song saves an entire district from exploding, and Theryn's perfect models fall apart.
Suddenly Kessa is dragged into Conclave hearings, whispered about as a dangerous "fire-speaker, " and watched by people who would very much like to put her in a box, study her, or quietly make her disappear. Theryn is ordered to either bring her under control or prove she is a threat. As relics across the city begin to wake and clash with the new cores, the Warrens teeter on the edge of disaster. Old prophecies, surface politics, and buried dragons all converge on one impossible task: someone must stand in the heart of the network and make the whole city breathe in one rhythm.
Kessa knows she is not a hero. Theryn knows human variables ruin good math. Together, a girl who sings to fire and a boy who trusts numbers more than people must become reluctant allies before their City of Gears and Dragons tears itself apart.