Contagion

Contagion

Six governors, one epidemic, nine roads. Six LLM-piloted governors each run one region of a six-node road network for twenty weeks. Every week each governor sets three dials — lockdown, testing and one border gate per road — talks to the table, and may wire aid to any other region. The infection then crosses the roads whether or not the road was closed, the economy pays for the dials and for the sickness, and the dead are counted.

score(seat) = your region's accumulated GDP − 2 × your cumulative deaths

Higher is better and it can be negative: an uncontrolled epidemic loses more to the death penalty than the region ever earned. Your neighbour's looseness is your problem.

Live at softmax.com/contagion.

The map

Six regions of 1,000,000 people each — Harborlea, Kestrel Flats, Riverbend, Ash Hollow, Wintermoor, Saltmarch — on the 6-cycle plus its three long diagonals. Nine roads; every region has exactly two main roads and one back road, so every governor's structural situation is byte-for-byte identical and no seat is stuck anywhere. What differs is where the outbreak started (seeded) and what the other five governors do.

Seat → region is a fresh seeded permutation every episode. Governors know regions, never players.

The rules, in one screen

Every week, simultaneously, each governor submits:

Then the week resolves, in this exact order: dials latch (a road's effective gate is the tighter of its two ends) → talk queues → aid settles → the infection crosses every road → the economy pays → the dead are counted → the reports go out.

Every road leaks. A road sealed at both ends still passes 12% of its traffic. Closure buys time, never safety — and a road you shut costs the region at the far end money too.

You never see the truth. Not even your own: you see reported cases, which are your detection rate times reality. At testing 0 that is 15% of the truth. Every region's testing level is public, so any governor can de-bias any other region's number — that inference is intended play, not a leak.

All arithmetic is integer parts-per-million with truncating division. That is load-bearing: the wasm replay viewer re-runs the same Nim rules in your browser and its re-derivation is checked field-for-field against the recorded weeks.

A policy is just a prompt

Every decision is made by Claude acting on a per-seat policy prompt. Field your own by reusing the published player runnable and setting PLAYER_PROMPT:

coworld upload-policy coworld-contagion:latest \
  --name my-contagion --run /bin/contagion-player \
  --secret-env PLAYER_PROMPT="<your strategy>"

All six seats' requests go out as one parallel batch per week, so an episode is twenty round trips, not one hundred and twenty. A reply that does not parse is retried once with a hint, bounded by what is left of the week's budget, and then falls back to the scripted sentinel move — an episode always completes.

Two scripted baselines ship in the same image, selected with PLAYER_SCRIPTED:

• sentinel — the threshold dial policy: de-bias your own reported cases, step lockdown and testing at fixed prevalence thresholds, and gate each road against its neighbour's de-biased estimate. Also the universal fallback move.
• laggard — the leaky neighbour: testing always 0 (so it under-reports by 6.7× and never isolates), gates always open, and lockdown 3 for exactly three weeks once its own blind estimate finally crosses 4%. Late, blind and expensive for everyone downwind.

With no LLM credentials at all every seat plays sentinel and the episode still completes — that is the offline-certification path and it is load-bearing.

Watching it

The replay is a static wasm bundle: the browser re-derives every frame from the replay bytes and contacts nothing but S3 for the .replay file. The outbreak is a red stain washing across each region's tile; lockdown is 0–4 wooden shutters slamming down; testing is a lantern whose glow is literally how far into that region you can see; aid arcs as gold packets; and each road carries a crawling red seep whose thickness is the imported case count — a shut road still seeping is the single most important picture in the game. Under the map, six curves of true infections against the hospital-capacity line, with each region's reported curve dotted underneath.

Repo layout

src/contagion/{types,sim,llm,server}.nim   the rules, the LLM batch, the mummy server
src/contagion.nim  src/contagion_player.nim  the two entrypoints in one image
client/                                    renderer.js + chrome.css + the three pages
replay-viewer/                             the wasm entry, its link flags and the static shell
tools/build_replay_viewer.sh               the `coworld build` hook (mode 100755)
tools/ci/                                  docker_smoke.sh, viewer_smoke.mjs, policies.json
tests/                                     sim, baselines, replay bytes, packaging invariants
scripts/art/                               the recipes for the committed art

Development

The rules are pure and have no IO, so the tests run with nothing but Nim:

nimby use 2.2.4 && nimby --global sync nimby.lock
nim r --path:src tests/test_sim.nim

CI runs every tests/*.nim twice (debug and -d:release), builds the production image and plays a real six-seat episode in raw Docker, then builds the wasm bundle and opens it in headless chromium against the replay that episode produced.

MIT licensed. Art: the governor portraits derive from the MIT-licensed coworld-ctf sprites (scripts/art/recolor_sprite.py); the map plate and props are generated by scripts/art/make_props.py.