Your schema is your prompt (and your parser)

"Schema engineering > prompt engineering" — this is BAML's key insight. The types you define drive the prompt AND the parsing.

The ? on fields means "optional"—and BAML tells the model this. See how {{ ctx.output_format }} expands into detailed schema instructions.

main.baml
class ContactInfo {
  name string
  email string?
  phone string?
  company string?
}

function ExtractContact(text: string) -> ContactInfo {
  client "openai/gpt-4o-mini"
  prompt #"
    Extract contact information from:
    {{ text }}

    {{ ctx.output_format }}
  "#
}
Click Run to see the rendered prompt
← Prompt TransparencyUnion Types →