BoundaryML/baml · error

split() method only available on strings at {:?}

Error message

split() method only available on strings at {:?}

What it means

The "split" string method is only defined for string receivers. When evaluate_method_call dispatches "split" it first pattern-matches the receiver as BamlValueWithMeta::String; any other receiver type (int, list, map, null, ...) triggers this bail. It exists because split(delimiter) has no meaning on non-string values in BAML's interpreter.

Source

Thrown at engine/baml-compiler/src/thir/interpret.rs:2930

                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())
                .map(|part| BamlValueWithMeta::String(part.to_string(), meta.clone()))
                .collect();
            Ok(BamlValueWithMeta::List(parts, meta.clone()))
        }
        "substring" => {
            let BamlValueWithMeta::String(s, _) = receiver else {
                bail!(
                    "substring() method only available on strings at {:?}",
                    meta.0

View on GitHub (pinned to bd85ce9dee)

Solutions

1. Check the receiver's actual type at the failing expression and make it a string first (explicit conversion or upstream type annotation fix).
2. If the value is already a list, drop the split call entirely — no delimiter-based splitting is needed.
3. Guard with a type check before calling split when the receiver type is dynamic.

Example fix

// before (items is already a list)
let parts = items.split(",");

// after
let parts = items;

// guard the receiver before split
if (v is string) {
  return v.split(",");
} else if (v is string[]) {
  return v;
}

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

match result {
  Err(e) if e.to_string().contains("split() method only available on strings") => default_split_result,
  other => other,
}

When it happens

Trigger: Calling <non-string>.split(",") in a BAML expression, e.g. splitting a number or a list, or splitting a value that is null at runtime.

Common situations: Values returned from LLM extraction typed as numbers/lists but assumed to be strings; splitting the result of an expression that actually returns a list already; null values from optional fields being split.

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

• endsWith() method only available on strings at {:?}
• substring() method only available on strings at {:?}
• baml.json.serialize returned non-string value: {other:?}
• endsWith() argument must be a string at {:?}
• split() argument must be a string at {:?}

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/e98d134c76f7e33a. Report an issue: GitHub.