BoundaryML/baml · error

endsWith() argument must be a string at {:?}

Error message

endsWith() argument must be a string at {:?}

What it means

The endsWith method's single argument must be a string. The interpreter destructures args[0] as BamlValueWithMeta::String and bails with this message if it is an int, bool, list, or other value type. The library throws it rather than coercing non-string arguments.

Source

Thrown at engine/baml-compiler/src/thir/interpret.rs:2921

                bail!("startsWith() argument must be a string at {:?}", meta.0);
            };
            Ok(BamlValueWithMeta::Bool(
                s.starts_with(prefix.as_str()),
                meta.clone(),
            ))
        }
        "endsWith" => {
            let BamlValueWithMeta::String(s, _) = receiver else {
                bail!(
                    "endsWith() method only available on strings at {:?}",
                    meta.0
                );
            };
            if args.len() != 1 {
                bail!("endsWith() method takes exactly 1 argument at {:?}", meta.0);
            }
            let BamlValueWithMeta::String(suffix, _) = &args[0] else {
                bail!("endsWith() argument must be a string at {:?}", meta.0);
            };
            Ok(BamlValueWithMeta::Bool(
                s.ends_with(suffix.as_str()),
                meta.clone(),
            ))
        }
        "split" => {
            let BamlValueWithMeta::String(s, _) = receiver else {
                bail!("split() method only available on strings at {:?}", meta.0);
            };
            if args.len() != 1 {
                bail!("split() method takes exactly 1 argument at {:?}", meta.0);
            }
            let BamlValueWithMeta::String(delimiter, _) = &args[0] else {
                bail!("split() argument must be a string at {:?}", meta.0);
            };
            let parts: Vec<BamlValueWithMeta<ExprMetadata>> = s
                .split(delimiter.as_str())

View on GitHub (pinned to bd85ce9dee)

Solutions

1. Convert the argument to a string (e.g. use .to_string() on the value or a string literal) before passing it to endsWith.
2. Verify the type of the expression passed as the argument at the failing call site.
3. Add a type guard around the call if the argument can be a non-string at runtime.

Example fix

// before
let ok = s.endsWith(version); // version: int

// after
let ok = s.endsWith(version.to_string());

// ensure the suffix argument is a string before the call
// suffix must be string: literal or converted value

fn is_string(v: BamlValue) -> bool { matches!(v, BamlValue::String(_)) }

match result {
  Err(e) if e.to_string().contains("argument must be a string") => use_default_suffix,
  other => other,
}

When it happens

Trigger: Calling s.endsWith(5), s.endsWith(myIntVar), s.endsWith(null), or passing a list/bool as the suffix in a BAML expression.

Common situations: Numeric literals or extracted numeric values passed as the suffix without conversion; variables from structured extraction that are not strings; typos where an int variable shadows the intended string.

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

• split() argument must be a string at {:?}
• endsWith() method only available on strings at {:?}
• split() method only available on strings at {:?}
• substring() method only available on strings at {:?}
• type `{ty}` can't be passed through auto-CLI; deliver it via

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/3ad2025e2e57c39d. Report an issue: GitHub.