BoundaryML/baml · error

includes() method only available on strings at

Error message

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

What it means

includes() performs a substring containment check and is defined only for string receivers in the BAML interpreter. If the receiver is not a string, evaluate_method_call bails with this error before the containment check runs.

Solutions

1. Convert the receiver to a string before calling includes(), if string containment was intended.
2. For arrays/lists, use the appropriate list membership expression instead of includes() (BAML's includes is string-only).
3. Add a type check/branch on the receiver before calling includes().
4. Use the error span to identify the receiver expression and correct its type.

Example fix

// before
let found = tags.includes("admin");      // tags is a list
// after
let found = "{{ tags }}".includes("admin");  // or use list membership

if (typeof receiver !== 'string') {
  throw new Error('includes() receiver must be a string; use list membership for arrays');
}

const isStr = (v) => typeof v === 'string';

try {
  const res = await b.Function(args);
} catch (e) {
  if (String(e.message).includes('includes() method only available on strings')) {
    // switch to string coercion or list membership expression
  }
  throw e;
}

When it happens

Trigger: Calling `x.includes(sub)` where `x` is a non-string value — an array, map, int, bool, or null — expecting JavaScript-like behavior on arrays or other containers.

Common situations: Devs familiar with JavaScript's Array.prototype.includes call it on a BAML array/list; checking containment in a map value that isn't a string; a variable inferred as non-string at runtime though the code assumed 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

• startsWith() method only available on strings at
• toUpperCase() method only available on strings at
• trim() method only available on strings at
• array assignment on non-list value at
• bitwise ^= requires integer operands

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/97b67e56379c1ac4. Report an issue: GitHub.

Appendix: source

Thrown at engine/baml-compiler/src/thir/interpret.rs:2873

                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);
            };
            if !args.is_empty() {
                bail!("trim() method takes no arguments at {:?}", meta.0);
            }
            Ok(BamlValueWithMeta::String(
                s.trim().to_string(),
                meta.clone(),
            ))
        }
        "includes" => {
            let BamlValueWithMeta::String(s, _) = receiver else {
                bail!(
                    "includes() method only available on strings at {:?}",
                    meta.0
                );
            };
            if args.len() != 1 {
                bail!("includes() method takes exactly 1 argument at {:?}", meta.0);
            }
            let BamlValueWithMeta::String(search, _) = &args[0] else {
                bail!("includes() argument must be a string at {:?}", meta.0);
            };
            Ok(BamlValueWithMeta::Bool(
                s.contains(search.as_str()),
                meta.clone(),
            ))
        }
        "startsWith" => {
            let BamlValueWithMeta::String(s, _) = receiver else {
                bail!(

View on GitHub (pinned to bd85ce9dee)