BoundaryML/baml · error

toLowerCase() method takes no arguments at {:?}

Error message

toLowerCase() method takes no arguments at {:?}

What it means

THIR interpreter runtime error: `toLowerCase()` was invoked with arguments. The builtin string method takes none; any arguments at the call site are a mistake and abort evaluation with the expression location attached.

Source

Thrown at engine/baml-compiler/src/thir/interpret.rs:2843

                    }
                    Ok(BamlValueWithMeta::Int(map.len() as i64, meta.clone()))
                }
                _ => bail!(
                    "length() method not available on type {:?} at {:?}",
                    receiver,
                    meta.0
                ),
            }
        }
        "toLowerCase" => {
            let BamlValueWithMeta::String(s, _) = receiver else {
                bail!(
                    "toLowerCase() method only available on strings at {:?}",
                    meta.0
                );
            };
            if !args.is_empty() {
                bail!("toLowerCase() method takes no arguments at {:?}", meta.0);
            }
            Ok(BamlValueWithMeta::String(s.to_lowercase(), meta.clone()))
        }
        "toUpperCase" => {
            let BamlValueWithMeta::String(s, _) = receiver else {
                bail!(
                    "toUpperCase() method only available on strings at {:?}",
                    meta.0
                );
            };
            if !args.is_empty() {
                bail!("toUpperCase() method takes no arguments at {:?}", meta.0);
            }
            Ok(BamlValueWithMeta::String(s.to_uppercase(), meta.clone()))
        }
        "trim" => {
            let BamlValueWithMeta::String(s, _) = receiver else {
                bail!("trim() method only available on strings at {:?}", meta.0);

View on GitHub (pinned to bd85ce9dee)

Solutions

1. Call toLowerCase() with empty parentheses
2. Remove any locale or extra arguments; the BAML builtin performs an unconditional lowercase

Example fix

// before
let lower = name.toLowerCase("en-US")
// after
let lower = name.toLowerCase()

if !args.is_empty() {
    return Err("toLowerCase() takes no arguments".into());
}

fn is_string(v: &BamlValueWithMeta<ExprMetadata>) -> bool {
    matches!(v, BamlValueWithMeta::String(_, _))
}

When it happens

Trigger: Evaluating s.toLowerCase(arg) with a non-empty argument list, regardless of argument type.

Common situations: Porting locale-aware lowercase calls (e.g. toLowerCase(locale)) from other languages; accidentally passing extra arguments in generated code.

Understand the failure class

Background: "missing required argument" and "the following required arguments were not provided": what required-argument errors mean and how to fix them — this error's family across 20 libraries.

Related errors

• length() method takes no arguments at {:?}
• toLowerCase() method only available on strings at {:?}
• env.get expects exactly one argument
• baml.media.image.from_url expects 1 argument, got {} at {:?}
• baml.fetch_as expects 1 argument (url), got {} at {:?}

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/2895251578cf5dd2. Report an issue: GitHub.