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Euca is deterministic by construction. The simulation runs on a single canonical seeded RNG — no ambient rand, no wall-clock in the tick, no per-system randomness: every stochastic decision in a tick draws from that one stream (a CI test enforces it — no hidden RNG). Give the engine the same seed and the same inputs and you get the same run back, every time, within a given build and platform.
One RNG, engine-wide. The engine knows the dice — so a run is a pure function of (initial state, seed, inputs).

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Why it matters

Determinism is the property the rest of the engine is built on:

Replay

Re-run the same world from the same seed and inputs to reproduce a result exactly — no flakiness, no “works on my machine.”

Forks that mean something

A fork only isolates a counterfactual if the fork and the original would otherwise evolve identically. Determinism is what makes the comparison valid.

A real answer key

Because the next step is a fixed distribution rather than noise, Euca can report it exactly — the basis of world-model evaluation.

Reproducible datasets

Trajectories recorded from the engine carry a content digest, so a dataset can be verified bit-for-bit against the run that produced it.

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How reproducibility is checked

The engine can flatten the entire world into a canonical, ordered representation — entities sorted by id, components sorted by name, fields in declaration order — and hash it into a stable digest. Two runs that diverge by a single field produce different digests, which is how the engine’s own test suite and the evaluation track verify that a replay is exact. Physics is part of this guarantee: the solver is bit-deterministic, so contacts and integration reproduce exactly across runs on the same platform.
Determinism is guaranteed for a given build and platform. The digest is a within-platform reproducibility oracle, not a claim of identical floating-point results across different CPU architectures.

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Replaying through the API

State is data, so a run can be captured and restored as data. Two complementary tools:
  • Snapshots capture a labeled summary of the world at a tick, and GET /snapshot/diff compares two of them — see Forks & counterfactuals.
  • Scenarios serialize the whole world — entities, templates, rules, and assertions — as a declarative document. GET /scenario exports the current world and POST /scenario rebuilds it. A scenario plus a seed is a reproducible starting point you can save, edit, and re-apply.
# Export the current world as a declarative scenario document.
curl -s http://localhost:3917/scenario > world.json

# Later — or on another machine — rebuild that exact world and continue.
curl -s -X POST http://localhost:3917/scenario \
  -H 'content-type: application/json' --data @world.json