EDDYLINE

Olympic-style whitewater slalom. 392 metres of class III–IV canyon river, eighteen gates hanging over the water on wires. Green gates you take heading downstream. Red gates must be taken heading upstream — and they hang over the eddies behind boulders. Your score is your running time plus 2 seconds for every pole you touch and 50 for every gate you miss. Lowest total wins; gold is 138 seconds or under, and it needs all four red gates.

How to play

• W / ↑ — paddle forward
• A D / ← → — sweep strokes, your only steering
• S / ↓ — back-paddle (half power, but it stops you)
• Space — brace; also rolls you back up when you capsize

Press H or ? at any time for the full controls; P pauses. Touch controls appear on phones.

The twist

The river is a real flow field, and the boat reads it twice — once at the bow, once at the stern. Behind every boulder the water genuinely flows upstream: about −1 m/s in the eddy core, against +4.6 m/s in the tongue three metres away. You top out at 3.5 m/s through the water, so the red gates are physically unreachable head-on. The only way in is the real kayaker's move — cross the eddyline with speed and angle, let the bow bite the upstream water while the stern is still in the tongue, and the boat whips around on its own. Nothing scripts that turn; it falls out of the drag equations. So do ferry angles, peel-outs, and the sickening moment a hole grabs you sideways. Brace late and briefly. Get it wrong and you swim, and a swim ends the run.

How it was made

Conceived and written this run by Claude Fable 5; the game itself — the analytic flow field, the two-point hull sampling that makes eddy turns emerge, the four-moment roll model, the synthesized river roar — was built and playtest-tuned in a single session by Claude Opus 5 under Claude Code. The medal cutoffs were measured, not guessed: a headless autopilot was run through the exact shipping physics core (free drift 102 s, green-gates-only sprint 67 s, each eddy catch timed), putting a clean expert descent near 119 s. One self-contained HTML file, three.js vendored beside it, all audio from WebAudio oscillators — no assets, no CDN.