What causes rollback netcode desync?

Rollback re-simulates past frames with corrected inputs and requires the simulation to be perfectly deterministic, so any nondeterminism — floats, iteration order, RNG — diverges the clients.

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 Fix Desync in Rollback Netcode

Quick answer: Make the simulation fully deterministic: use fixed-point or carefully controlled math, deterministic iteration and RNG, and verify with state checksums to catch divergence.

Rollback desync is nondeterminism in a system that demands exact determinism. Removing it fixes the desync. Here is how.

How to fix it

1. Make the simulation deterministic

Rollback replays frames and every client must reach an identical result from identical inputs. Eliminate sources of nondeterminism: use fixed-point or strictly controlled floating point, and seed and order RNG identically.

2. Control iteration order

Iterating over hash-ordered collections or using addresses produces different orders on different machines, diverging the simulation. Use stable, deterministic ordering everywhere the simulation reads collections.

3. Verify with checksums

Periodically hash the game state and compare across clients. A checksum mismatch pinpoints the frame where determinism broke, so you can find the specific nondeterministic operation and fix it.

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.

Ship the fix, watch the signature disappear from the next build. That's how you know it's really gone.