BoundaryML/baml · error

baml.fetch_as expects 1 type argument, got

Error message

baml.fetch_as expects 1 type argument, got {} at {:?}

What it means

baml.fetch_as is a builtin function in the BAML interpreter that fetches a URL and parses the response JSON into a specified type. It requires exactly one type argument (the target type) supplied via generics/apply. The compiler throws this error at interpretation time when the call site supplies a number of type arguments other than one.

Solutions

1. Supply exactly one type argument to baml.fetch_as naming the target type, e.g. baml.fetch_as::<MyType>(url)
2. Check the BAML source at the span reported in the error (the meta location) to find the offending call
3. Simplify: if you need multiple shapes, use two separate fetch_as calls, each with one type argument

Example fix

// before
let v = baml.fetch_as(url)
// after
let v = baml.fetch_as::<MyType>(url)

fn validate_fetch_as_type_args(type_args: &[Type]) -> Result<(), String> {
    if type_args.len() != 1 {
        return Err(format!("baml.fetch_as needs exactly 1 type argument, got {}", type_args.len()));
    }
    Ok(())
}

When it happens

Trigger: Calling baml.fetch_as(url) with zero type arguments, or with two or more type arguments (e.g. baml.fetch_as::<A, B>(url)) inside a BAML expression evaluated by evaluate_builtin_function.

Common situations: Forgetting to annotate the target type when writing a fetch expression; copying a fetch_as call from an example with multiple generic params; a refactor that changed the target type but left a stale second type argument.

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

• unknown builtin function
• array assignment index out of bounds
• array assignment on non-list value at
• array assignment requires a non-negative integer index at
• assign to undeclared variable

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/ccc0e897de3caec1. Report an issue: GitHub.

Appendix: source

Thrown at engine/baml-compiler/src/thir/interpret.rs:2743

                    "baml.media.image.from_url expects a string argument at {:?}",
                    meta.0
                ),
            };
            Ok(BamlValueWithMeta::Media(
                baml_types::BamlMedia::url(baml_types::BamlMediaType::Image, url, None),
                meta.clone(),
            ))
        }
        "baml.fetch_as" => {
            if args.len() != 1 {
                bail!(
                    "baml.fetch_as expects 1 argument (url), got {} at {:?}",
                    args.len(),
                    meta.0
                );
            }
            if type_args.len() != 1 {
                bail!(
                    "baml.fetch_as expects 1 type argument, got {} at {:?}",
                    type_args.len(),
                    meta.0
                );
            }

            let url = match &args[0] {
                BamlValueWithMeta::String(s, _) => s.clone(),
                _ => bail!(
                    "baml.fetch_as expects a string URL argument at {:?}",
                    meta.0
                ),
            };

            let target_type = &type_args[0];

            // Make HTTP request
            let response = reqwest::get(&url).await.with_context(|| {

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