BamlValidationError

Thrown when BAML fails to parse or validate LLM output against the expected schema.

Type Definition

class BamlValidationError extends Error {
  type: 'BamlValidationError'
  message: string
  prompt: string
  raw_output: string
  detailed_message: string
}

Properties

Property	Type	Description
type	'BamlValidationError'	Literal type identifier
message	string	Specific validation failure description
prompt	string	Original prompt sent to LLM
raw_output	string	Raw LLM output that failed validation
detailed_message	string	Complete history of failed attempts (for fallback/retry)

Detection

TypeScript

import { BamlValidationError } from '@boundaryml/baml'

try {
  const result = await b.ExtractUser(text)
} catch (error) {
  if (error instanceof BamlValidationError) {
    console.log('Validation failed:', error.message)
    console.log('Raw output:', error.raw_output)
    console.log('Prompt used:', error.prompt)
  }
}

Python

from baml_py import BamlValidationError

try:
    result = await b.ExtractUser(text)
except BamlValidationError as e:
    print(f"Validation failed: {e}")
    print(f"Raw output: {e.raw_output}")
    print(f"Prompt used: {e.prompt}")

Common Causes

• LLM output doesn't match expected JSON structure
• Missing required fields in output
• Type mismatches (e.g., string instead of number)
• Enum values not in allowed list
• Array/object structure doesn't match schema

Debugging Tips

1. Check raw_output to see what the LLM actually returned
2. Review prompt to ensure output format instructions are clear
3. Use detailed_message when debugging fallback chains

See Also

• Error Overview
• BamlClientFinishReasonError
• BamlAbortError