Bug Reporting for Games With Client-Side Prediction

Client-side prediction is one of the most effective tricks in multiplayer game development, and also one of the hardest to debug. The client runs the local player's simulation immediately, then waits for the server to confirm or correct it. When prediction works, movement feels instant. When it breaks, the player sees a snap-back, a rubber-band, or a jitter that is gone before they can describe it. The root cause is almost always a mismatch between the predicted state and the authoritative state at a specific tick, and if your bug report does not capture both, you are debugging blind. This post covers what to record and how.

Why mispredictions are so hard to reproduce

A misprediction is a divergence between two simulations that ran on two machines with different information. The client predicted the local player at tick 4120 using the inputs it had; the server resolved the same tick later, perhaps with a collision the client never saw. By the time the correction arrives and the player notices a snap, the inputs that caused it have already scrolled out of memory. Reproducing it means recreating the exact input sequence, the exact server state, and the exact network timing, which almost never happens by hand.

This is why a vague report like the character teleported backwards is useless on its own. The teleport is the visible symptom of a reconciliation that rolled the player back to an authoritative position. Without the predicted state, the authoritative state, and the tick number where they diverged, you cannot tell whether the bug is in your prediction code, your input handling, or a genuine server-side collision the client mispredicted. The fix depends entirely on which of those it is, so the report has to disambiguate them.

Capture the predicted and authoritative state together

The single most useful artifact is a side-by-side snapshot at the divergent tick: predicted position, velocity, and the input frame the client applied, next to the authoritative position the server returned and the correction delta between them. A delta of a few centimeters is normal smoothing; a delta of several meters is a real bug. Include the tick number on both sides so you can confirm they refer to the same simulation step rather than two unrelated frames that happened to be logged close together.

Just as important is the short ring buffer of inputs leading up to the divergence. Prediction errors compound, so the tick where the player saw the snap is rarely the tick where prediction first went wrong. A buffer of the last thirty to sixty input frames, with their sequence numbers and the predicted result of each, lets you replay the divergence locally and watch exactly where the two simulations parted ways. That replay is the difference between guessing and knowing.

Reconciliation and the resimulation path

When the authoritative state arrives, the client rewinds to that confirmed tick and resimulates every unacknowledged input forward to the present. Bugs hide in this resimulation: a non-deterministic physics step, an input applied twice, or a tick that was dropped during the rewind. If the resimulation produces a slightly different result each time, the player gets continuous micro-corrections that read as jitter even when no single correction is large. Capturing the pre-rewind and post-rewind state makes this visible.

Record how many inputs were replayed during reconciliation and whether the final resimulated position matched the previous prediction. A healthy reconciliation converges: the resimulated present should land close to where the player already was. A reconciliation that consistently lands somewhere new points at a determinism problem in your simulation. This metric is cheap to log and turns a fuzzy feel bug into a number you can watch trend across builds and across players on different hardware.

Timing, latency, and the network context

Prediction quality is a function of latency and jitter, so every report should carry the round-trip time, the jitter estimate, and the current input delay or buffer depth at the moment of the bug. A snap-back that only happens at 180 milliseconds of latency is a tuning issue, not a logic bug, and you cannot tell the two apart without the number. Packet loss matters too: a dropped input or a lost server correction forces a larger catch-up that often looks identical to a prediction error.

Attach the server tick the client believed it was synchronized to alongside the client's local tick. Clock drift between the two is a common, quiet cause of prediction problems, especially after the game has been running for a while or after the device sleeps and wakes. If the client thinks it is two ticks ahead but is actually five, every prediction is built on a stale baseline. Logging both ticks at report time exposes that drift immediately.

Setting it up with Bugnet

Bugnet gives you an in-game report button that captures game state at the moment a player hits it, which is exactly the hook you want for a prediction bug. Wire the button to serialize your prediction context: the predicted and authoritative position at the current tick, the correction delta, the round-trip time, and the last sixty input frames from your ring buffer. Add player attributes for region and connection type. When a player rubber-bands, one tap ships you a snapshot that already contains both sides of the divergence instead of a screenshot of a blur.

Because reconciliation bugs tend to recur, Bugnet's occurrence grouping folds duplicate reports into a single issue with a count, so you see that two hundred players hit the same snap rather than two hundred separate tickets. Use custom fields for tick number, correction magnitude, and latency bucket, then filter to find whether the problem clusters at high latency or on a specific platform. One dashboard turns a stream of feel complaints into a ranked, reproducible list of divergences.

Build a prediction-debugging culture

The teams that ship smooth prediction treat divergence as a first-class signal rather than a cosmetic annoyance. Add a developer overlay that shows live correction deltas, and make a large delta during playtesting something testers are trained to flag immediately with a report. The earlier you catch a growing divergence, the cheaper the fix, because prediction errors that ship to players turn into hard-to-reproduce field reports instead of an obvious red number on someone's screen during QA.

Over time, the captured deltas and reconciliation counts become a regression suite. Watch the median correction magnitude per build and treat a sudden increase as a bug even before a player complains, because it means your simulation drifted. Pair that with replays reconstructed from the input buffers and you can fix prediction issues with confidence instead of nudging smoothing constants and hoping. Good telemetry turns the dark art of prediction into ordinary, debuggable engineering.

A prediction bug is two simulations disagreeing. Capture both at the same tick, plus the inputs that led there, and the snap-back stops being a mystery.