BoundaryML/baml · error

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

Error message

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

What it means

The BAML expression interpreter evaluates the "endsWith" string method at runtime. Before calling ends_with on the receiver it pattern-matches it as a String value; if the receiver is any other type (int, list, map, null, etc.) it bails with this message. The library throws it because endsWith is only defined for string receivers, and static typing did not catch the mismatch.

Source

Thrown at engine/baml-compiler/src/thir/interpret.rs:2912

                );
            };
            if args.len() != 1 {
                bail!(
                    "startsWith() method takes exactly 1 argument at {:?}",
                    meta.0
                );
            }
            let BamlValueWithMeta::String(prefix, _) = &args[0] else {
                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);

View on GitHub (pinned to bd85ce9dee)

Solutions

1. Inspect the receiver value at the failing expression and confirm its actual type (print it or check the type annotation in the BAML file).
2. Convert the receiver to a string explicitly before calling endsWith, or fix the upstream expression so it yields a string.
3. If the receiver may legitimately be non-string, guard the call with a type check (e.g. `x is string` / match on the type) before invoking endsWith.
4. Check whether type annotations on the variable are wrong so the compiler can catch this statically instead of at runtime.

Example fix

// before (x is an int)
let flag = x.endsWith("5");

// after
let flag = x.to_string().endsWith("5");

// in BAML expression code, before the call
if (x is string) {
  return x.endsWith("suffix");
}

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

// interpreter-level: handle the bail
match evaluate_method_call(...) {
  Ok(v) => v,
  Err(e) if e.to_string().contains("only available on strings") => fallback_value,
  Err(e) => return Err(e),
}

When it happens

Trigger: Calling <non-string>.endsWith(...) in a BAML expression, e.g. `let x = 5; x.endsWith("5")` or `myList.endsWith("a")`, where the receiver's value is not BamlValueWithMeta::String when evaluate_method_call runs.

Common situations: Variables assumed to be strings but actually produced by a prompt/extraction block as a number or list; refactors that change a variable's type without updating downstream method calls; dynamic values from maps or function results that are null or untyped at runtime.

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() 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/e9fe1041825dae0c. Report an issue: GitHub.