Handling Prediction Errors and Reconciliation

In this lesson, you will learn how to identify and resolve visual or state discrepancies when the server rejects a client’s predicted Gameplay Ability action.

When a client predicts an ability, it executes the logic locally before the server has confirmed it. If the server’s simulation disagrees (e.g., the player was stunned, or a cooldown wasn't actually ready), it will reject the action. When this happens, the client must perform reconciliation: snapping its local state back to the truth provided by the server.

Detecting and Resolving Discrepancies

The most common symptom of a prediction error is "rubber-banding" or a "state pop," where the character moves or animates as if an action occurred, then suddenly snaps back.

To debug these issues, first enable the console command AbilitySystem.Debug.Predict 1. This allows you to see the prediction key status in the top-left of your viewport. If you notice a "Prediction Key Mismatch" or a "Reject" warning in your logs, the server has invalidated your local prediction.

Handling Reconciliation Logic

Since GameplayAbilities are designed to handle most of the heavy lifting, the system automatically tries to roll back changes when a rejection occurs. However, if your ability modifies custom non-GAS properties, you must ensure those properties are also replicated and reverted.

If you find that an ability is consistently rejected, inspect your CanActivateAbility logic. A classic pitfall is performing a check on the client that isn't perfectly mirrored on the server:

bool UMyAbility::CanActivateAbility(...) const {
    // Both Client and Server must evaluate this identically!
    if (!HasAuthority() && !OwnerCharacter->IsReadyToFire()) {
        return false; 
    }
    return Super::CanActivateAbility(...);
}

If the client allows activation because it thinks it’s "Ready to Fire," but the server thinks otherwise, the server will reject the prediction, leading to a visual stutter. The goal of reconciliation is to ensure your client-side checks are a "strict subset" of the server’s validation logic. If it might be rejected by the server, do not predict it locally.

Key Takeaway: Reconciliation occurs when client-side state is forced to match the server; prevent these errors by ensuring your ability activation criteria are deterministic and perfectly mirrored between both environments.