How to Fix Unity Burst/Job Nondeterminism Breaking a Sim Replay

A Unity colony sim whose replay drifts from the original run has nondeterminism in its parallel jobs. Thread order and float reductions differ between runs. Structure the jobs so results are order-independent and apply them deterministically. Here is how.

How to fix it

1. Make parallel writes order-independent

In IJobParallelFor, write only to the element owned by each index and avoid cross-element accumulation, so the result does not depend on which thread finished first.

2. Gather then apply on one thread

Collect proposed changes in parallel, then apply them in a deterministic single-threaded pass (sorted by entity id), so conflicting writes resolve the same way every run.

3. Seed and order all sim RNG

Drive simulation randomness from a seeded generator advanced in a fixed entity order inside the tick, never from per-thread or wall-clock sources, so the same seed replays identically.

Catching the ones you can't reproduce

The hardest version of this to fix is the one you can't reproduce — it only happens on a player's hardware, OS, driver, or save state, under conditions that simply aren't present on your machine. A report that says “it crashed” or “it froze” gives you nothing to act on, so the bug survives release after release while quietly costing you players.

Automatic error capture closes that gap. Each failure arrives with its full stack trace, the device and OS, the build number, and a breadcrumb trail of what the player did right before it broke, so even a failure you have never seen becomes a specific, reproducible issue. Fold identical failures into one signature ranked by how many players each hits, and your worklist sorts itself worst-first instead of arriving as a stream of vague complaints.

This is where a tool like Bugnet earns its place. Its SDK captures every Unity error automatically with the full stack trace plus device, OS, memory, build, and game-state context, folds duplicates into one grouped issue with an occurrence count, and ties each to the build it first appeared on — so you fix the problem that hurts the most players first and confirm it is gone when its signature disappears from the next release.