Add conditions to branch conversations - Amazon Lex

Add conditions to branch conversations

You can use conditional branching to control the path that your customer takes through the conversation with your bot. You can branch the conversation based on slot values, session attributes, the contents of the input mode and input transcript fields, or a response from Amazon Kendra.

You can define up to four branches. Each branch has a condition that must be satisfied in order for Amazon Lex V2 to follow that branch. If none of the branches has its condition satisfied, a default branch is followed.

When you define a branch, you define the action that Amazon Lex V2 should take if the conditions corresponding to that branch evaluate to true. You can define any of the following actions:

  • A response sent to the user.

  • Slot values to apply to slots.

  • Session attribute values for the current session.

  • The next step in the conversation. For more information, see Creating conversation paths.

You can choose different options for conditional branching.

Each conditional branch has a Boolean expression that must be satisfied for Amazon Lex V2 to follow the branch. There are comparison and Boolean operators, functions, and quantifier operators that you can use for your conditions. For example, the following condition returns true if the {age} slot is less than 24.

{age} < 24

The following condition returns true if the {toppings} multi-value slot contains the word "pineapple".

{toppings} CONTAINS "pineapple"

You can combine multiple comparison operators with a Boolean operator for more complex conditions. For example, the following condition returns true if the {make} slot value is "Honda" and the {model} slot value is "Civic". Use parentheses to set the evaluation order.

({make} = "Honda") AND ({model} = "Civic")

The following topics provide details on the conditional branch operators and functions.


On August 17, 2022, Amazon Lex V2 released a change to the way conversations are managed with the user. This change gives you more control over the path that the user takes through the conversation. For more information, see Understanding conversation flow management. Bots created before August 17, 2022 do not support dialog code hook messages, setting values, configuring next steps, and adding conditions.

Comparison operators

Amazon Lex V2 supports the following comparison operators for conditions:

  • Equals (=)

  • Not equals (!=)

  • Less than (<)

  • Less than or equals (<=)

  • Greater than (>)

  • Greater than or equals (>=)

When using a comparison operator, it uses the following rules.

  • The left-hand side must be a reference. For example, to reference a slot value, you use {slotName}. To reference a session attribute value, you use [attribute]. For input mode and input transcript, you use $.inputMode and $.inputTranscript.

  • The right-hand side must be a constant and the same type as the left hand side.

  • Any expression referencing an attribute which has not been set is treated as invalid, and is not evaluated.

  • When you compare a multi-valued slot, the value used is a comma-separated list of all interpreted values.

Comparisons are based on the slot type of the reference. They are resolved as follows:

  • Strings – strings are compared based on their ASCII representation. The comparison is case-insensitive.

  • Numbers – number-based slots are converted from the string representation to a number and then compared.

  • Date/Time – time-based slots are compared based on the time series. The earlier date or time is considered smaller. For durations, shorter periods are considered smaller.

Boolean operators

Amazon Lex V2 supports Boolean operators to combine comparison operators. They let you create statements similar to the following:

({number} >= 5) AND ({number} <= 10)

You can use the following Boolean operators:

  • AND (&&)

  • OR (||)

  • NOT (!)

Quantifier operators

Quantifier operators evaluate the elements of a sequence and determine if one or more elements satisfy the condition.

  • CONTAINS – determines if the specified value is contained in a multi-valued slot and returns true if it is. For example, {toppings} CONTAINS "pineapple" returns true if the user ordered pineapple on their pizza.


Functions must be prefixed with the string fn.. The argument to the function is a reference to a slot, session attribute, or request attribute. Amazon Lex V2 provides two functions for getting information from the values of slots, sessionAttribute, or requestAttribute.

  • fn.COUNT() – counts the number of values in a multi-valued slot.

    For example, if the slot {toppings} contains the value "pepperoni, pineapple":

    fn.COUNT({toppings}) = 2

  • fn.IS_SET() – value is true if a slot, session attribute, or request attribute is set in the current session.

    Based on the previous example:


  • fn.LENGTH() – value is the length of the value of the session attribute, slot value, or slot attribute which is set in the current session. This function does not support multi-value slots or composite slots.


    If the slot {credit-card-number} contains the value "123456781234":

    fn.LENGTH({credit-card-number}) = 12

Sample conditional expressions

Here are some sample conditional expressions. NOTE: $. represents the entry point to the Amazon Lex JSON response. The value following $. will be parsed within the Amazon Lex response to retrieve the value. Conditional expressions using the JSON path reference to transcriptions block in the Amazon Lex response will only be supported in the same locales which support ASR transcription scores.

Value type Use case Conditional expression
Custom slot pizzaSize slot value is equal to large {pizzaSize} = "large"
Custom slot pizzaSize is equal to large or medium {pizzaSize} = "large" OR {pizzaSize} = "medium"
Custom slot Expressions with () and AND/OR {pizzaType} = "pepperoni" OR {pizzaSize} = "medium" OR {pizzaSize} = "small"
Custom slot (Multi-Valued Slot) Check if one of the topping is Onion {toppings} CONTAINS "Onion"
Custom slot (Multi-Valued Slot) Number of toppings are more than 3 fn.COUNT({topping}) > 2
AMAZON.AlphaNumeric bookingID is ABC123 {bookingID} = "ABC123"
AMAZON.Number age slot value is greater than 30 {age} > 30
AMAZON.Number age slot value is equal to 10 {age} = 10
AMAZON.Date dateOfBirth slot value before 1990 {dateOfBirth} < "1990-10-01"
AMAZON.State destinationState slot value is equal to Washington {destinationState} = "washington"
AMAZON.Country destinationCountry slot value is not United States {destinationCountry} != "united states"
AMAZON.FirstName firstName slot value is John {firstName} = "John"
AMAZON.PhoneNumber phoneNumber slot value is 716767891932 {phoneNumer} = 716767891932
AMAZON.Percentage Check if percentage slot value is greater than or equals 78 {percentage} >= 78
AMAZON.EmailAddress emailAddress slot value is {emailAddress} = ""
AMAZON.LastName lastName slot value is Doe {lastName} = "Doe"
AMAZON.City City slot value is equal to Seattle {city} = "Seattle"
AMAZON.Time Time is after 8 PM {time} > "20:00"
AMAZON.StreetName streetName slot value is Boren Avenue {streetName} = "boren avenue"
AMAZON.Duration travelDuration slot value is less than 2 hours {travelDuration} < P2H
Input mode Input mode is speech $.inputMode = "Speech"
Input transcript Input transcript is equal to "I want a large pizza" $.inputTranscript = "I want a large pizza"
Session attribute check customer_subscription_type attribute [customer_subcription_type] = "yearly"
Request attribute check retry_enabled flag ((retry_enabled)) = "TRUE"
Kendra response Kendra response contains FAQ fn.IS_SET(((x-amz-lex:kendra-search-response-question_answer-question-1)))
Conditional expression with transcriptions Conditional expressions using transcriptions JSON path $.transcriptions[0].transcriptionConfidence < 0.8 AND $.transcriptions[1].transcriptionConfidence > 0.5
Set session attributes Set session attributes using transcriptions JSON path and slot values [sessionAttribute] = "$.transcriptions..." AND [sessionAttribute] = "{<slotName>}"
Set slot values Set slot values using session attributes and transcriptions JSON path {slotName} = [<sessionAttribute>] AND {slotName} = "$.transcriptions..."

slotName refers to the name of a slot in the Amazon Lex bot. If the slot is not resolved (null), or if the slot does not exist, then the assignments are ignored at runtime. sessionAttribute refers to the name of the session attribute that is set by the customer at build time.