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Enums are useful for classification tasks and constraining LLM outputs to a known set of values. BAML provides helper functions that serialize enums into prompts as neatly formatted lists.

​
Syntax

enum EnumName {
  Value1
  Value2
  Value3
}

​
Basic Example

enum Priority {
  Low
  Medium
  High
  Critical
}

​
Value Rules

RuleValidInvalid
Must start with letterActive, _Internal1Active, @Special
Alphanumeric + underscoresInProgress, Status_2In-Progress, Status@2
No spacesInProgressIn Progress
No duplicatesEach value uniqueTwo Active values
Non-emptyAt least one valueEmpty enum
enum ValidExample {
  ValueOne
  Value_Two
  VALUE_THREE
  value4
}

// Invalid examples:
// enum Invalid {
//   1Value        // ✗ Starts with number
//   My-Value      // ✗ Contains hyphen
//   My Value      // ✗ Contains space
// }

​
Enum Attributes

​
@@alias

​
@@alias
string
Changes how the entire enum is rendered in prompts while keeping the original name in code.
enum Status {
  Active
  Inactive
  
  @@alias("Account Status")
}
In prompts, this enum appears as “Account Status” instead of “Status”.

​
@@dynamic

​
@@dynamic
Allows modifying enum values at runtime. Useful for loading values from databases or configuration.
enum Category {
  General
  
  @@dynamic
}
At runtime:
from baml_client.type_builder import TypeBuilder

tb = TypeBuilder()
tb.Category.add_value("Special")
tb.Category.remove_value("General")

result = await b.ClassifyItem(
    item="data",
    baml_options={"tb": tb}
)
See Dynamic Types for details.

​
Value Attributes

​
@alias

​
@alias
string
Renames a specific value for the LLM while keeping the original name in code. Also used when parsing LLM output.
enum Status {
  InProgress @alias("in_progress")
  OnHold @alias("on_hold")
  Completed
}
The LLM sees in_progress and on_hold, but your code uses InProgress and OnHold.

​
@description

​
@description
string
Adds context to help the LLM understand when to select this value.
enum Severity {
  Low @description("Minor issues, no immediate action needed")
  Medium @description("Moderate issues, should be addressed soon")
  High @description("Serious issues, requires immediate attention")
  Critical @description("System-breaking issues, urgent response required")
}

​
@skip

​
@skip
Excludes this value from prompts and parsing. Useful for deprecated or internal-only values.
enum Status {
  Active
  Inactive
  Deprecated @skip
  InternalOnly @skip
}
The LLM will only see Active and Inactive.

​
Complete Examples

​
Simple Classification

enum Sentiment {
  Positive @description("Expresses satisfaction, happiness, or approval")
  Negative @description("Expresses dissatisfaction, frustration, or disapproval")
  Neutral @description("No strong emotion, factual or informational")
}

function AnalyzeSentiment(text: string) -> Sentiment {
  client GPT4
  prompt #"
    Analyze the sentiment of this text:
    {{ text }}
    
    {{ ctx.output_format }}
  "#
}

​
Multi-line Descriptions

enum Priority {
  Low @description(#"
    Low priority items:
    - No deadline pressure
    - Can be deferred
    - Nice-to-have features
  "#)
  
  Medium @description(#"
    Medium priority items:
    - Should be completed within the sprint
    - Important but not urgent
    - Standard feature requests
  "#)
  
  High @description(#"
    High priority items:
    - Urgent deadline
    - Blocking other work
    - Critical features
  "#)
  
  Critical @description(#"
    Critical priority items:
    - Production issues
    - Security vulnerabilities
    - System outages
  "#)
}

​
With Aliases

enum TicketStatus {
  Open
  InProgress @alias("in_progress") @description("Currently being worked on")
  NeedsReview @alias("needs_review") @description("Awaiting code review")
  Blocked @description("Cannot proceed due to dependencies")
  Resolved @description("Issue has been fixed")
  Closed
  WontFix @alias("wont_fix") @skip  // Internal use only
  
  @@alias("Ticket Status")
}

​
Issue Categories

enum IssueCategory {
  Bug @description("Software defects or errors")
  Feature @description("New feature requests")
  Enhancement @description("Improvements to existing features")
  Documentation @description("Documentation updates or fixes")
  Question @description("General questions or support requests")
  Security @description("Security vulnerabilities or concerns")
  Performance @description("Performance-related issues")
  
  @@dynamic  // Allow adding categories at runtime
}

​
Account Types

enum AccountType {
  Free @description("Basic features, no payment required")
  Trial @description("All features, 14-day trial period")
  Pro @alias("professional") @description("Full features, monthly subscription")
  Enterprise @description("Custom features, annual contract")
  Internal @skip  // For testing only
  Legacy @skip    // Deprecated, for existing customers
}

​
Usage in Classes

enum Priority {
  Low
  Medium
  High
}

enum Status {
  Open
  Closed
}

class Task {
  title string
  priority Priority
  status Status
  tags TaskTag[]
}

enum TaskTag {
  Urgent
  External
  Internal
  Billable
}

​
Usage in Functions

​
As Return Type

function ClassifyMessage(message: string) -> Category {
  client GPT4
  prompt #"
    Classify this message into one of these categories:
    {{ ctx.output_format }}
    
    Message: {{ message }}
  "#
}

​
As Parameter

function FilterByPriority(tasks: Task[], min_priority: Priority) -> Task[] {
  client GPT4
  prompt #"
    Filter these tasks to only include {{ min_priority }} priority or higher:
    {{ tasks }}
    
    {{ ctx.output_format }}
  "#
}

​
Literal Alternative

For simple cases, you can use literal types instead of enums: 
function ClassifyIssue(description: string) -> "bug" | "feature" | "question" {
  client GPT4
  prompt #"
    Classify this issue:
    {{ description }}
    
    {{ ctx.output_format }}
  "#
}
Enums are preferred when you need:
  • Descriptions for values
  • Aliases
  • Reusability across multiple functions
  • Dynamic runtime values

​
In Prompt Rendering

When used in prompts, enums are automatically formatted: 
enum Status {
  Active @description("Currently active")
  Inactive
}

function Example(data: string) -> Status {
  client GPT4
  prompt #"
    {{ ctx.output_format }}
  "#
}
Generates a prompt like: 
Return your response in this format:
Status (one of):
- Active: Currently active
- Inactive

​
Generated Code

from baml_client.types import Priority

# Use enum values
priority = Priority.High

if priority == Priority.Critical:
    print("Urgent!")

# Access string value
print(priority.value)  # "High"

​
Best Practices

  1. Naming: Use PascalCase for enum names and values
  2. Descriptions: Add @description to clarify when each value should be used
  3. Size: Keep enums focused (typically 3-10 values)
  4. Aliases: Use @alias when the code name differs from the LLM-friendly name
  5. Documentation: Add docstrings (///) to explain the enum’s purpose
  6. Skip: Use @skip for deprecated values rather than removing them
  7. Literals: For 2-3 simple values, consider literals instead of enums

​
Dynamic Enums

For values loaded from databases or external sources: 
from baml_client.type_builder import TypeBuilder

# Load categories from database
categories = db.get_active_categories()

tb = TypeBuilder()
for cat in categories:
    tb.Category.add_value(cat.name)

result = await b.Categorize(
    item="product description",
    baml_options={"tb": tb}
)
See Dynamic Types for complete documentation.

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