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Version: Master/Unreleased

Fallback and Human Handoff

This is a guide on how to handle various failures of your assistant.

Even if you design your bot perfectly, users will inevitably say things to your assistant that you did not anticipate. In these cases, your assistant will fail, and it's important you ensure it does so gracefully.

Handling Out-of-scope Messages

To avoid user frustration, you can handle questions you know your users may ask, but for which you haven't implemented a user goal yet.

1. Creating an Out-of-scope Intent

You will need to define an out_of_scope intent in your NLU training data and add any known out-of-scope requests as training examples, for example:

- intent: out_of_scope
examples: |
- I want to order food
- What is 2 + 2?
- Who's the US President?

As with every intent, you should source the majority of your examples from real conversations.

2. Defining the response message

You'll need to define an out-of-scope response in the domain file. Using the utterance utter_out_of_scope as the default response, that would look like:

- text: Sorry, I can't handle that request.

3. Creating an Out-of-Scope Rule

Finally, you will need to write a rule for what should happen for in out-of-scope request:

- rule: out-of-scope
- intent: out_of_scope
- action: utter_out_of_scope

Handling Specific Out-of-scope Messages

If you observe your users asking for certain things that you'll want to turn into a user goal in future, you can handle these as separate intents, to let the user know you've understood their message, but don't have a solution quite yet. For example, if the user asks “I want to apply for a job at Rasa”, we can then reply with “I understand you're looking for a job, but I'm afraid I can't handle that skill yet.”

Similar to the out_of_scope intent example, you'll need to create a new intent with training examples, define the response message, and create a rule.


Although Rasa will generalize to unseen messages, some messages might receive a low classification confidence. Using Fallbacks will help ensure that these low confidence messages are handled gracefully, giving your assistant the option to either respond with a default message or attempt to disambiguate the user input.

NLU Fallback

To handle incoming messages with low NLU confidence, use the FallbackClassifier. Using this configuration, the intent nlu_fallback will be predicted when all other intent predictions fall below the configured confidence threshold. You can then write a rule for what the bot should do when nlu_fallback is predicted.

1. Updating the configuration

To use the FallbackClassifier, add it to your NLU pipeline:

# other components
- name: FallbackClassifier
threshold: 0.7

2. Defining the response message

Define the message the bot should send when a message is classified with low confidence by adding a response:

- text: I'm sorry, I didn't quite understand that. Could you rephrase?

3. Creating an NLU fallback rule

The following Rule will ask the user to rephrase when they send a message that is classified with low confidence:

- rule: Ask the user to rephrase whenever they send a message with low NLU confidence
- intent: nlu_fallback
- action: utter_please_rephrase

Handling Low Action Confidence

As users might send unexpected messages, it is possible that their behavior will lead them down unknown conversation paths. Rasa's machine learning policies such as the TED Policy are optimized to handle these unknown paths.

To handle cases when the machine learning policies can't predict the next action with high confidence, you can configure the Rule Policy to predict a default action if no Policy has a next action prediction with confidence above a configurable threshold.

You can configure the action that is run in case low of action confidence as well as the corresponding confidence threshold using the following steps:

1. Updating the configuration

You will need to add the RulePolicy to your policies in config.yml. By default, the rule policy comes with the settings below:

- name: RulePolicy
# Confidence threshold for the `core_fallback_action_name` to apply.
# The action will apply if no other action was predicted with
# a confidence >= core_fallback_threshold
core_fallback_threshold: 0.4
core_fallback_action_name: "action_default_fallback"
enable_fallback_prediction: True

2. Defining the default response message

To define what your bot will say when action confidence is below the threshold, define a response utter_default:

- text: Sorry I didn't get that. Can you rephrase?

When an action confidence is below the threshold, Rasa will run the action action_default_fallback. This will send the response utter_default and revert back to the state of the conversation before the user message that caused the fallback, so it will not influence the prediction of future actions.

3. Customizing the default action (optional)

action_default_fallback is a default action in Rasa Open Source that sends the utter_default response to the user. You can create your own custom action to use as a fallback (see Custom Actions for more info on custom actions). The following snippet is an implementation of a custom action which does the same as action_default_fallback but dispatches a different template utter_fallback_template:
from typing import Any, Text, Dict, List
from rasa_sdk import Action, Tracker
from import UserUtteranceReverted
from rasa_sdk.executor import CollectingDispatcher
class ActionDefaultFallback(Action):
"""Executes the fallback action and goes back to the previous state
of the dialogue"""
def name(self) -> Text:
async def run(
dispatcher: CollectingDispatcher,
tracker: Tracker,
domain: Dict[Text, Any],
) -> List[Dict[Text, Any]]:
# Revert user message which led to fallback.
return [UserUtteranceReverted()]

Two-Stage Fallback

To give the bot a chance to figure out what the user wants, you will usually want it to attempt to disambiguate the user's message by asking clarifying questions. The Two-Stage Fallback is made to handle low NLU confidence in multiple stages using the following sequence:

  1. A user message is classified with low confidence
    • The user is asked to confirm the intent
  2. The user confirms or denies the intent
    • If they confirm, the conversation continues as if the intent was classified with high confidence from the beginning. No further fallback steps are taken.
    • If they deny, the user is asked to rephrase their message.
  3. The user rephrases their intent
    • If the message is classified with high confidence, the conversation continues as if the user had this intent from the beginning.
    • If the rephrased user message still has low confidence, the user is asked to confirm the intent.
  4. The user confirms or denies the rephrased intent
    • If they confirm, the conversation continues as if the user had this intent from the beginning.
    • If they deny, an ultimate fallback action is triggered (e.g. a handoff to a human).

The Two-Stage-Fallback can be enabled using the following steps:

1. Updating the configuration

Add FallbackClassifier to your pipeline and the RulePolicy to your policy configuration:

# other components
- name: FallbackClassifier
threshold: 0.7
# other policies
- RulePolicy

2. Defining the fallback responses

To define how your bot asks the user to rephrase their message, define the response utter_ask_rephrase:

- text: I'm sorry, I didn't quite understand that. Could you rephrase?

Rasa provides default implementations for asking which intent the user meant and for asking the user to rephrase. To customize the behavior of these actions, see the documentation on default actions.

3. Defining a Two-Stage Fallback rule

Add the following Rule to your training data. This rule will make sure that the Two-Stage-Fallback will be activated whenever a message is received with low classification confidence:

- rule: Implementation of the Two-Stage-Fallback
- intent: nlu_fallback
- action: action_two_stage_fallback
- active_loop: two_stage_fallback

Human Handoff

As part of your fallback action, you may want the bot to hand over to a human agent e.g. as the final action in Two-Stage-Fallback, or when the user explicitly asks for a human. A straightforward way to achieve human handoff is to configure your messaging or voice channel to switch which host it listens to based on a specific bot or user message.

For example, as the final action of Two-Stage-Fallback, the bot could ask the user, "Would you like to be transferred to a human assistant?" and if they say yes, the bot sends a message with a specific payload like e.g. "handoff_to_human" to the channel. When the channel sees this message, it stops listening to the Rasa server, and sends a message to the human channel with the transcript of the chat conversation up to that point.

The implementation for handing off to a human from the front end will depend on which channel you're using. You can see an example implementation using an adaption of the chatroom channel in the Financial Demo and Helpdesk-Assistant starterpacks.


To let your assistant gracefully handle failures, you should handle known out-of-scope messages and add a form of fallback behavior. If you want to add human handoff, you can add it in addition or as a final step in your fallback set up. Here's a summary of changes you need to make for each method:

For out-of-scope intents:

  • Add training examples for each out-of-scope intent to your NLU data
  • Define the out-of-scope response or action
  • Define rules for each out-of-scope intent
  • Add the RulePolicy to config.yml

For single stage NLU fallback:

  • Add FallbackClassifier to your pipeline in config.yml
  • Define the fallback response or action
  • Define a rule for the nlu_fallback intent
  • Add the RulePolicy to config.yml

For handling low core confidence:

  • Configure the RulePolicy for core fallback in config.yml
  • Optionally customize the fallback action you configure
  • Define an utter_default response

For Two-Stage Fallback:

  • Add FallbackClassifier to your pipeline in config.yml
  • Define a rule for the nlu_fallback intent that triggers the action_two_stage_fallback action
  • Define an out-of-scope intent in your domain
  • Add RulePolicy to config.yml

For handing off to a human:

  • Configure your front end to switch hosts
  • Write a custom action (which could be your fallback action) to send the handoff payload
  • Add a rule for triggering handoff (if not part of fallback)
  • Add RulePolicy to config.yml