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Troubleshooting

Symptom → likely cause → fix. Jump to whatever matches your problem.

Ability Won't Activate

Symptom: TryActivateAbility returns false. Nothing happens when the player presses the button.

Check in order:

  1. Is the ability granted? Use showdebug abilitysystem -- the ability must appear in the ability list. If not, call GiveAbility first.

  2. Activation Blocked Tags? The ASC might have a tag that blocks this ability. Check ActivationBlockedTags on the ability vs the ASC's current tags.

  3. Activation Required Tags? The ASC might be missing a required tag.

  4. On cooldown? Check Active Effects for the cooldown GE. If it's still active, the cooldown hasn't expired.

  5. Can't afford cost? Check the attribute value vs the cost GE's modifier. If stamina is 15 and the cost is 20, the ability won't commit.

  6. Wrong Net Execution Policy? ServerOnly abilities won't activate from client input. LocalOnly abilities won't trigger server-side effects.

  7. CanActivateAbility override? If you've overridden CanActivateAbility in C++, add logging to see which check fails.

Effect Doesn't Modify Attribute

Symptom: The Gameplay Effect is applied (it shows up in Active Effects) but the attribute doesn't change.

Check:

  1. Modifier targets the wrong attribute. Double-check the attribute reference in the modifier. Typos in the attribute set class or attribute name are silent failures.

  2. Magnitude is zero. If using SetByCaller, did you actually set the value? The warning "SetByCaller Magnitude was not set" appears in the log. If using a Curve Table, is the row name correct? If using an MMC, does it return non-zero?

  3. Tag requirements on the modifier. Modifiers can have source/target tag requirements. If the tags don't match, the modifier is skipped during evaluation (it's still "active" but contributes 0).

  4. Wrong modifier operation. Multiply Additive with a value of 20 means +2000%. You probably wanted 0.2 for +20%. See Modifier Formula.

  5. Attribute not on the target. If the target's ASC doesn't have the AttributeSet containing that attribute, the modifier silently does nothing.

SetByCaller Is Always 0

Symptom: The effect applies but the magnitude from SetByCaller is 0.

Cause: The tag used in SetSetByCallerMagnitude doesn't match the tag configured on the GE modifier.

Fix:

  1. Check the tag on the GE modifier's magnitude: it should be something like SetByCaller.Damage
  2. Check your code: SpecHandle.Data->SetSetByCallerMagnitude(Tag, Value) -- is the tag identical?
  3. Enable verbose logging -- the system warns when a SetByCaller value is requested but wasn't set
  4. Common mistake: setting the value on the wrong spec handle (e.g., creating a new spec after setting the value)

Cooldown Not Working

Symptom: The ability can be activated repeatedly with no cooldown.

Check:

  1. Cooldown GE assigned? On the ability's Class Defaults, is the CooldownGameplayEffectClass set?
  2. Cooldown GE has correct tag? The cooldown GE must grant a tag that matches the ability's cooldown query. By default, the system checks for any tag granted by the cooldown GE.
  3. Duration set? The cooldown GE must have Duration Policy = Has Duration with a non-zero duration.
  4. CommitAbility called? If you're handling activation manually, you must call CommitAbility (or CommitExecute) to apply the cooldown.

Duration Buff Doesn't Undo When It Expires

Symptom: A Duration GE expires, but the attribute modifier persists.

Check:

  1. Is it actually a Duration GE? Check that Duration Policy = Has Duration, not Instant. Instant effects permanently change the base value -- they don't revert.
  2. Is something reapplying it? Another effect or ability might be reapplying the same modifier.
  3. Stacking misconfiguration. If stacking is set to Remove Single Stack, and there are multiple stacks, only one stack is removed on expiry.
  4. Clamping in PreAttributeChange. If you're clamping in PreAttributeChange, it might set a "floor" that prevents the attribute from reverting to a lower base value.

Tags Don't Block / Aren't Working

Symptom: Tag-based blocking or requirements seem to be ignored.

Check:

  1. Typo in tag name. Tags are case-sensitive and hierarchy-sensitive. CrowdControl.Stun is not the same as Crowdcontrol.Stun or CC.Stun.
  2. Tag not registered. If the tag doesn't exist in the project's tag table, it's invalid. Check Project Settings > Gameplay Tags.
  3. Tag query direction. Are you checking for the tag on the right actor? ActivationBlockedTags checks the owning ASC's tags. TargetTagRequirements on a GE checks the target's tags.
  4. Tag count mismatch. If two effects grant the same tag, removing one effect doesn't remove the tag -- it decrements the count. The tag is only fully removed when the count reaches 0.
  5. Hierarchy mismatch. Checking for CrowdControl matches CrowdControl.Stun (child matches parent). But checking for CrowdControl.Stun does NOT match CrowdControl (parent doesn't match child).

Cue Doesn't Play

Symptom: The effect applies, the attribute changes, but no VFX/SFX.

Check:

  1. Cue tag matches? The tag on the GE's GameplayCue component must match the tag on the cue notify asset. Exactly.
  2. Cue in scan path? The cue asset's folder must be in GameplayCueNotifyPaths. Check Project Settings > Gameplay Abilities.
  3. Cue loaded? Check the log for "Missing gameplay cue handler". If the cue isn't loaded, it won't play.
  4. Suppressed? If ShouldSuppressGameplayCues returns true for the target actor, all cues are silently dropped.
  5. Local vs replicated? A replicated cue on a simulated proxy won't play if the proxy doesn't have the ASC configured for cue handling.
  6. Effect is Instant? Instant effects trigger Execute cues. Duration effects trigger Add/Remove cues. Make sure the cue handles the right event type.

Compile Errors

"cannot open include file: *.generated.h"

Cause: The .generated.h file is auto-generated by Unreal Header Tool (UHT). It doesn't exist until you compile.

Fix: Build the project. If the error persists, check for syntax errors in the header file above the GENERATED_BODY() macro -- UHT will fail silently and not generate the file.

"unresolved external symbol"

Cause: You're using a GAS class but haven't linked the module.

Fix: Add "GameplayAbilities", "GameplayTags", and "GameplayTasks" to your module's Build.cs:

PublicDependencyModuleNames.AddRange(new string[]
{
    "GameplayAbilities",
    "GameplayTags",
    "GameplayTasks"
});

"ATTRIBUTE_ACCESSORS undeclared identifier"

Cause: Missing include.

Fix: Include "AttributeSet.h" in your attribute set header. The ATTRIBUTE_ACCESSORS macro is defined there.

AttributeSet Is Null

Symptom: GetSet<UMyAttributeSet>() returns null. Attribute modifications crash.

Check:

  1. Is the AttributeSet a subobject of the ASC's owner? AttributeSets must be created as subobjects (usually CreateDefaultSubobject in the character/player state constructor) of the actor that owns the ASC, or registered manually.
  2. Is the ASC initialized? InitAbilityActorInfo must be called before accessing attribute sets.
  3. Wrong ASC? If the ASC is on the PlayerState but you're checking the Pawn's ASC, you'll get null. Make sure IAbilitySystemInterface::GetAbilitySystemComponent() returns the right ASC.
  4. Timing issue? In multiplayer, the ASC and its attribute sets might not be ready immediately on BeginPlay. Use OnAbilitySystemInitialized or a similar delegate.

Prediction Feels Wrong

Symptom: Ability activates but there's a visible delay, rubber-banding, or double-triggering of effects.

Check:

  1. Net Execution Policy: Must be LocalPredicted for predicted abilities
  2. Replication Mode: Mixed or Full for predicted abilities (not Minimal without extra work)
  3. Effects outside prediction window: Any effect applied after ActivateAbility returns (timers, delays) won't be predicted
  4. ExecCalc prediction: Execution Calculations are server-only. The client can't predict the calculated value.
  5. Double cues: If you see a cue play twice (once predicted, once from server), the prediction key matching might be failing. Check that the GE has a valid prediction key.