What causes "startsWith() method takes exactly 1 argument at {:?}" in BoundaryML/baml?

Pass exactly one string argument: `s.startsWith(prefix)`.

startsWith() method takes exactly 1 argument at

Error message

startsWith() method takes exactly 1 argument at {:?}

What it means

THIR interpreter runtime error: `startsWith()` was invoked with an argument count other than exactly one. The method takes a single string prefix argument; zero or multiple arguments fail the arity check at evaluation time.

Solutions

Pass exactly one string argument: `s.startsWith(prefix)`.
If a positional check is needed, slice the string first (e.g. via substring-style operations) or move the logic to the host language.
Verify the intended method and its arity against BAML's string method set.

Example fix

// before
let ok = path.startsWith("/api", 4);
// after
let ok = path.startsWith("/api");

Defensive patterns

Strategy: validation

Validate before calling

const call = src.match(/\.startsWith\s*\(([^)]*)\)/);
if (call && call[1].split(',').filter(a => a.trim()).length !== 1) {
  throw new Error('startsWith() takes exactly 1 argument: ' + call[0]);
}

Try / catch

try {
  const res = await b.Function(args);
} catch (e) {
  if (String(e.message).includes('startsWith() method takes exactly 1 argument')) {
    // fix the arity at the reported span and re-run
  }
  throw e;
}

Prevention

Drop JS-style position arguments — BAML's startsWith() takes one argument only.
For offset prefix checks, slice the string first.
Lint BAML sources for startsWith() calls with zero or multiple arguments.

When it happens

Trigger: Calling `s.startsWith()` with no argument, or with multiple arguments — BAML's startsWith has no position/offset parameter unlike JavaScript's String.prototype.startsWith.

Common situations: Porting JavaScript code using `s.startsWith(prefix, position)` and keeping the second argument; omitting the argument when translating from a different prefix-check API.

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

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/f681a0f4409a7449. Report an issue: GitHub.

Appendix: source

Thrown at engine/baml-compiler/src/thir/interpret.rs:2897

                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!(
                    "startsWith() method only available on strings at {:?}",
                    meta.0
                );
            };
            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
                );

View on GitHub (pinned to bd85ce9dee)