This is documentation for Rasa Open Source Documentation v2.1.x, which is no longer actively maintained.
For up-to-date documentation, see the latest version (2.2.x).

Version: 2.1.x


FallbackPolicy Objects

class FallbackPolicy(Policy)

Policy which predicts fallback actions.

A fallback can be triggered by a low confidence score on a NLU prediction or by a low confidence score on an action prediction.


| __init__(priority: int = FALLBACK_POLICY_PRIORITY, nlu_threshold: float = DEFAULT_NLU_FALLBACK_THRESHOLD, ambiguity_threshold: float = DEFAULT_NLU_FALLBACK_AMBIGUITY_THRESHOLD, core_threshold: float = DEFAULT_CORE_FALLBACK_THRESHOLD, fallback_action_name: Text = ACTION_DEFAULT_FALLBACK_NAME) -> None

Create a new Fallback policy.


  • core_threshold - if NLU confidence threshold is met, predict fallback action with confidence core_threshold. If this is the highest confidence in the ensemble, the fallback action will be executed.
  • nlu_threshold - minimum threshold for NLU confidence. If intent prediction confidence is lower than this, predict fallback action with confidence 1.0.
  • ambiguity_threshold - threshold for minimum difference between confidences of the top two predictions
  • fallback_action_name - name of the action to execute as a fallback


| train(training_trackers: List[TrackerWithCachedStates], domain: Domain, interpreter: NaturalLanguageInterpreter, **kwargs: Any, ,) -> None

Does nothing. This policy is deterministic.


| nlu_confidence_below_threshold(nlu_data: Dict[Text, Any]) -> Tuple[bool, float]

Check if the highest confidence is lower than nlu_threshold.


| nlu_prediction_ambiguous(nlu_data: Dict[Text, Any]) -> Tuple[bool, Optional[float]]

Check if top 2 confidences are closer than ambiguity_threshold.


| should_nlu_fallback(nlu_data: Dict[Text, Any], last_action_name: Text) -> bool

Check if fallback action should be predicted.

Checks for:

  • predicted NLU confidence is lower than nlu_threshold
  • difference in top 2 NLU confidences lower than ambiguity_threshold
  • last action is action listen


| fallback_scores(domain: Domain, fallback_score: float = 1.0) -> List[float]

Prediction scores used if a fallback is necessary.


| predict_action_probabilities(tracker: DialogueStateTracker, domain: Domain, interpreter: NaturalLanguageInterpreter, **kwargs: Any, ,) -> PolicyPrediction

Predicts a fallback action.

The fallback action is predicted if the NLU confidence is low or no other policy has a high-confidence prediction.