Rasa Managing Slots

Managing Slots in Rasa

Workflow

1. Identify what information the assistant needs to remember.
2. Review existing slots in the domain — reuse before creating new ones.
3. Choose the most restrictive slot type that fits the data (see "Choosing a slot type").
4. Define the slot in the domain file with type and mapping (see "Slot mappings").
5. Set an initial_value if the slot needs a default.
6. Add validation if the slot requires format or range checks

(see "Validation strategies").

1. Use the slot in flows via collect or set_slots steps.

File organization

Slots are defined in domain YAML files, typically under domain/ or in a single domain.yml. Before creating new slots, check the existing domain layout — the user's existing convention always takes precedence.

Domain files are commonly split by business process, so each flow's slots, responses, and actions live together (e.g. domain/transfer_money.yml).

domain/
├── transfer_money.yml  # slots + responses + actions for one flow
├── book_appointment.yml
└── shared.yml          # slots reused across multiple flows

Choosing a slot type

Pick the most restrictive type that fits the data. This prevents invalid values and helps the LLM within the Command Generator extract correctly.

| Type | Use when | Example | |---------------|----------|---------| | text | Any string value | names, emails, free-text input | | bool | Binary yes/no | is_authenticated, terms_accepted | | categorical | Value must come from a fixed set (auto-coerces casing) | account types, priority levels | | float | Numeric value with decimals | temperature, amounts, scores | | any | Structured data (dicts, mixed types) | API responses, shopping carts | | list | List of values (custom actions only) | item lists — cannot be filled via collect or set_slots |

slots:
  patient_name:
    type: text

  terms_accepted:
    type: bool

  priority:
    type: categorical
    values:
      - low
      - medium
      - high

  temperature:
    type: float

  search_results:
    type: any

Slot mappings

Mappings define how a slot gets filled. Choose based on what controls the slot value.

A slot can combine from_llm with NLU-based mappings — NLU takes priority when both extract a value. Use allow_nlu_correction: true on from_llm if the LLM within the Command Generator should be able to correct NLU-filled values.

| Mapping | When to use | |---------------|-------------| | from_llm | Default for CALM (assumed if no mapping defined). The LLM within the Command Generator extracts value from user messages at any point in the conversation, not just at the collect step. | | controlled | Slot should only be filled by custom actions, button payloads, or set_slots. NLU and the LLM within the Command Generator cannot fill it. Use run_action_every_turn to keep the slot up to date on every turn. | | from_entity | NLU pipeline extracts a specific entity. Requires NLUCommandAdapter in config. Use conditions with active_flow to disambiguate when multiple slots map to the same entity. | | from_intent | NLU pipeline maps a predicted intent to a value. Requires NLUCommandAdapter in config. |

slots:
  destination_city:               # from_llm — LLM extracts from user messages
    type: text
    mappings:
      - type: from_llm

  is_authenticated:               # controlled — only set by custom actions or set_slots
    type: bool
    mappings:
      - type: controlled

  session_token:                  # controlled with run_action_every_turn
    type: text
    mappings:
      - type: controlled
        run_action_every_turn: action_refresh_session

  sender_name:                    # from_entity — NLU extracts, scoped to a flow
    type: text
    mappings:
      - type: from_entity
        entity: person
        conditions:
          - active_flow: transfer_money

  username:                       # combined — from_llm + from_entity, LLM can correct NLU
    type: text
    mappings:
      - type: from_llm
        allow_nlu_correction: true
      - type: from_entity
        entity: username

Initial values

Set a default value for slots that should not start empty. The slot resets to this value (not null) when a flow ends. Also useful for optional collect steps with ask_before_filling: false — the flow skips the question if the slot already has a value.

slots:
  language:
    type: categorical
    values:
      - en
      - de
      - fr
    initial_value: en

  num_retries:
    type: float
    initial_value: 0

Validation strategies

Three levels of validation, from simplest to most complex:

| Validation need | Where to define | How | |---------------------------------------|-----------------|-----| | Format/range checks (regex, length) | Domain — validation.rejections on the slot | Runs globally whenever the slot is set. refill_utter is optional (defaults to utter_ask_{slot_name}). | | Business rule tied to a specific flow | Flow — rejections on the collect step | Only runs at that collect step. | | External API or database check | Custom action named validate_{slot_name} | Rasa runs it automatically when the slot is collected. |

# Domain-level — global format check
slots:
  phone_number:
    type: text
    mappings:
      - type: from_llm
    validation:
      rejections:
        - if: not (slots.phone_number matches "^\d{10}$")
          utter: utter_invalid_phone
      refill_utter: utter_ask_phone_again

Naming conventions

This section proposes default conventions. If the user's project already follows different conventions, match those instead.

• Use snake_case: patient_name, order_id, is_authenticated
• Use descriptive nouns: name after the data, not the flow — email not collect_email
• Prefix booleans with is_ or has_: is_verified, has_insurance
• Avoid special characters (, ), =, , in slot names — they break response button

payloads

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: RasaHQ
• Source: RasaHQ/rasa-agent-skills
• License: Apache-2.0

Install and usage instructions live in the source repository linked above.