What causes a non-deterministic simulation?

Replays re-run recorded inputs and require the simulation to produce identical results, so floating-point differences, variable timestep, uncontrolled RNG, and iteration order make the replay diverge.

How do I catch this when it only happens for some players?

Capture it automatically from the player's device with the full stack trace, the device and OS, the build, and a breadcrumb trail of what they did. That turns a vague complaint into a specific, reproducible issue you can fix without owning the hardware it happens on.

How to Make a Game Simulation Deterministic for Replays

Quick answer: Use a fixed timestep, control RNG with a seed, ensure deterministic iteration and math, and record inputs rather than state so replays reproduce exactly.

Replays that drift are a determinism problem. Controlling the simulation's inputs and math fixes it. Here is how.

How to fix it

1. Use a fixed timestep and seeded RNG

Run the simulation at a fixed step and drive all randomness from a recorded seed, so the same inputs produce the same results every replay. Variable timestep or uncontrolled RNG diverges immediately.

2. Make math and iteration deterministic

Avoid platform-dependent floating point where it matters (or use fixed point), and iterate collections in a stable order. Nondeterministic ordering or math makes two runs of the same inputs differ.

3. Record inputs, not state

Store the player inputs per tick and replay them through the deterministic simulation, rather than recording positions. Input-based replays are tiny and exact — if the simulation is deterministic, they reproduce perfectly.

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 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.

Reproduce it once with full context and the fix writes itself. The hunt is the expensive part.