DeepSeek V4.1 Flash built an Interstellar-style black hole in Three.js, wrote 109 tests, and caught its own physics bugs. Under $1

I gave V4.1 Flash one prompt in AgenC Core at max effort: build a real-time black hole simulator in Three.js with gravitational lensing, Doppler beaming, a probe you can drop in, and no external assets. 47 minutes and 328 model calls later: - a raymarched Schwarzschild black hole where the far side of the accretion disk arcs over and under the shadow, one side blue-white from Doppler beaming and the other dim amber - a starfield that lenses around it, with a photon ring and an Einstein ring - a probe that stretches, reddens and freezes at the horizon - a fly-in camera with time dilation on the HUD, plus a cinematic recording mode - 144 fps at 1440x813 on my M4 Pro, with automatic quality scaling The part I didn't expect: it ported the shader's light integrator to plain JS so the physics could be unit tested. The shadow edge matches the analytic formula at 2, 3, 6 and 15 Schwarzschild radii, and deflection matches Einstein's two-term result within 1%. Writing those tests is how it caught three of its own bugs: rays launched in local instead of Schwarzschild coordinates, which made the shadow far too big up close disk turbulence that looked like wood grain the probe spawning off screen It fixed all three and pinned them with tests: 109 tests across 8 files, and the production build passes. It also wrote a README section called "Approximations, stated plainly": light bending is Schwarzschild with no frame dragging, the disk has no vertical structure, and the fly-in stops at 2.5 rs because below that the captured light cone is bigger than any normal field of view, so the view is legitimately black. Cost was under a dollar, with a 97% cache hit rate. AgenC Core is open source: github.com/tetsuo-ai/agenc-core