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Each of these actions have a default behavior, described in the sections below. In order to overwrite this default behavior, write a [custom action](https://legacy-docs-oss.rasa.com/docs/rasa/custom-actions) whose `name()` method returns the same name as the default action:

```python
class ActionRestart(Action):
    def name(self) -> Text:
        return "action_restart"

async def run(
            self, dispatcher, tracker: Tracker, domain: Dict[Text, Any]
    ) -> List[Dict[Text, Any]]:
        # custom behavior
        return [...]
```

Add this action to the actions section of your domain file so your assistant knows to use the custom definition instead of the default one:

```yaml
actions:
  - action_restart
```

##### caution

After adding this action to your domain file, re-train your model with `rasa train --force`. Otherwise Rasa won't know you've changed anything and may skip re-training your dialogue model.

## action_listen

This action is predicted to signal that the assistant should do nothing and wait for the next user input.

## action_restart

This action resets the whole conversation history, including any slots that were set during it.

It can be triggered by the user in a conversation by sending a "/restart" message, if the [RulePolicy](https://legacy-docs-oss.rasa.com/docs/rasa/rules) is included in the model configuration. If you define an `utter_restart` response in your domain, this will be sent to the user as well.

## action_session_start

This action starts a new conversation session, and is executed in the following situations:

- at the beginning of each new conversation
- after a user was inactive for a period defined by the `session_expiration_time` parameter in the domain's [session configuration](https://legacy-docs-oss.rasa.com/docs/rasa/domain#session-configuration)
- when a user sends a "/session_start" message during a conversation

The action will reset the conversation tracker, but by default will not clear any slots that were set.

### Customization

The default behavior of the session start action is to take all existing slots and to carry them over into the next session. Let's say you do not want to carry over all slots, but only a user's name and their phone number. To do that, you'd override the `action_session_start` with a custom action that might look like this:

```python
from typing import Any, Text, Dict, List
from rasa_sdk import Action, Tracker
from rasa_sdk.events import SlotSet, SessionStarted, ActionExecuted, EventType

class ActionSessionStart(Action):
    def name(self) -> Text:
        return "action_session_start"

@staticmethod
    def fetch_slots(tracker: Tracker) -> List[EventType]:
        """Collect slots that contain the user's name and phone number."""
        slots = []
        for key in ("name", "phone_number"):
            value = tracker.get_slot(key)
            if value is not None:
                slots.append(SlotSet(key=key, value=value))
        return slots

async def run(
            self, dispatcher, tracker: Tracker, domain: Dict[Text, Any]
    ) -> List[Dict[Text, Any]]:
        # the session should begin with a `session_started` event
        events = [SessionStarted()]
        # any slots that should be carried over should come after the
        # `session_started` event
        events.extend(self.fetch_slots(tracker))
        # an `action_listen` should be added at the end as a user message follows
        events.append(ActionExecuted("action_listen"))
        return events
```

If you want to access the metadata which was sent with the user message which triggered the session start, you can access the special slot `session_started_metadata`:

```python
from typing import Any, Text, Dict, List
from rasa_sdk import Action, Tracker
from rasa_sdk.events import SessionStarted, ActionExecuted

class ActionSessionStart(Action):
    def name(self) -> Text:
        return "action_session_start"

async def run(
            self, dispatcher, tracker: Tracker, domain: Dict[Text, Any]
    ) -> List[Dict[Text, Any]]:
        metadata = tracker.get_slot("session_started_metadata")
        # Do something with the metadata
        print(metadata)
        # the session should begin with a `session_started` event and an `action_listen`
        # as a user message follows
        return [SessionStarted(), ActionExecuted("action_listen")]
```

## action_default_fallback

This action undoes the last user-bot interaction and sends the `utter_default` response if it is defined. It is triggered by low action prediction confidence, if you have this [fallback mechanism](https://legacy-docs-oss.rasa.com/docs/rasa/fallback-handoff) enabled.

## action_deactivate_loop

This action deactivates the active loop and resets the requested slot. This is used when [handling unhappy paths in forms](https://legacy-docs-oss.rasa.com/docs/rasa/forms#writing-stories--rules-for-unhappy-form-paths).

##### note

If you wish to reset all slots, we recommend using a custom action that returns the [`AllSlotsReset`](/content/docs/rasa/reference/rasa/shared/core/events#allslotsreset-objects/index.html) event after form deactivation.

## action_two_stage_fallback

This is a fallback loop that can be used to handle low NLU confidence. Read more about [handling low NLU confidence](https://legacy-docs-oss.rasa.com/docs/rasa/fallback-handoff#nlu-fallback).

## action_default_ask_affirmation

This action is used by the `action_two_stage_fallback` loop. It asks the user to confirm the intent of their message. This action can be customized to be more personalized to your specific use case.

## action_default_ask_rephrase

This action is used by the `action_two_stage_fallback` loop if the user denies the intent `action_default_ask_affirmation` displays. It asks the user to rephrase their message.

## action_back

This action undoes the last user-bot interaction. It can be triggered by the user by sending a "/back" message to the assistant if the [RulePolicy](https://legacy-docs-oss.rasa.com/docs/rasa/policies#rule-policy) is configured.

## Form Action

By default Rasa uses `FormAction` for processing any [form logic](https://legacy-docs-oss.rasa.com/docs/rasa/forms). You can override this default action with a custom action by adding a custom action with the form's name to the domain. Overriding the default action for forms should **only** be used during the process of migrating from Rasa 1.0 to 2.0.

## action_unlikely_intent

Rasa triggers `action_unlikely_intent` via [`UnexpecTEDIntentPolicy`](https://legacy-docs-oss.rasa.com/docs/rasa/policies#unexpected-intent-policy). You can control how often this action is predicted by tuning the [`tolerance`](https://legacy-docs-oss.rasa.com/docs/rasa/policies#unexpected-intent-policy) parameter of `UnexpecTEDIntentPolicy`.

### Customization

You can customize your assistant's behaviour to configure what should happen once `action_unlikely_intent` is triggered. For example, as a follow up you can trigger a hand-off to a human agent with a rule:

```yaml
- rule: trigger human handoff with action_unlikely_intent
  steps:
    - action: action_unlikely_intent
    - action: ask_human_handoff
    - intent: affirm
    - action: trigger_human_handoff
```

Alternatively, you can also override its behaviour as a [`custom action`](https://legacy-docs-oss.rasa.com/docs/rasa/custom-actions) by adding `action_unlikely_intent` to the list of actions in the domain and implementing the custom behaviour:

```python
class ActionUnlikelyIntent(Action):
    def name(self) -> Text:
        return "action_unlikely_intent"

async def run(
            self, dispatcher, tracker: Tracker, domain: Dict[Text, Any],
    ) -> List[Dict[Text, Any]]:
        # Implement custom logic here
        return []
```

##### note

Since `action_unlikely_intent` can be triggered at any conversation step during inference, all policies which are trained on only story data, for example - `TEDPolicy`, `UnexpecTEDIntentPolicy`, `MemoizationPolicy` ignore its presence in the tracker when making a prediction. However, `RulePolicy` takes its presence into account so that [conversation behaviour is customizable](https://legacy-docs-oss.rasa.com/docs/rasa/default-actions#customization-1).

##### note

`action_unlikely_intent` cannot be included in the training stories. It can **only** be added to rules.

## action_extract_slots

This action runs after each user turn, before the next assistant action prediction and execution. `action_extract_slots` loops through the [slot mappings](https://legacy-docs-oss.rasa.com/docs/rasa/domain#slot-mappings) of each domain slot in order to set or update slots throughout the conversation with information extracted from the latest user message.

If `action_extract_slots` finds a [custom slot mapping](https://legacy-docs-oss.rasa.com/docs/rasa/domain#custom-slot-mappings), it will check first if a custom action was defined in the mapping via the `action` key and then run it.

After applying all the slot mappings, `action_extract_slots` will run the custom validation action `action_validate_slot_mappings` if it is present in the domain actions. Otherwise it will immediately return the already extracted slots.

Note that custom actions used by slot mappings or slot mapping validation should only return events of type `SlotSet` or `BotUttered`. Events of any other type are not permitted and will be ignored when updating the tracker.

The default action `action_extract_slots` replaces the slot extraction previously executed by `FormAction`. If you wish to set a slot based on information extracted from intents that trigger forms, you must explicitly specify a mapping that does not contain the `conditions` key. A slot mapping with `conditions` applies only once the specified form is active. `action_extract_slots` runs directly after each user message, and thus before the activation of the form. Therefore a mapping that should apply to user messages that trigger a form must not specify `conditions`, or the form will re-ask for the slot once it is activated.