BoundaryML/baml · error

unknown builtin function

Error message

unknown builtin function '{}' at {:?}

What it means

The BAML interpreter dispatches builtin function calls by name. When evaluate_builtin_function encounters a function name it does not recognize (not one of the implemented builtins such as baml.fetch_as), it throws this error with the name and source span.

Solutions

1. Check the spelling of the function name against the documented builtin list
2. Verify your BAML engine version supports the builtin; upgrade if needed
3. Implement the logic as a regular BAML function or expression if no builtin exists

Example fix

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

const KNOWN_BUILTINS: [&str; 1] = ["baml.fetch_as"];
fn is_known_builtin(name: &str) -> bool {
    KNOWN_BUILTINS.contains(&name)
}

When it happens

Trigger: Calling a function that does not exist as a builtin, e.g. a typo like baml.fetch_as2 or baml.fetc_as, or calling a function only available in a newer BAML version on an older interpreter.

Common situations: Typo in a builtin name; copying code using a builtin from newer docs than the installed engine version; assuming a helper exists when it must be defined in BAML or supplied another way.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

• baml.fetch_as expects 1 type argument, got
• 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/2fe5d906b3a0ebb2. Report an issue: GitHub.

Appendix: source

Thrown at engine/baml-compiler/src/thir/interpret.rs:2793

                    "baml.fetch_as: HTTP request failed: HTTP {}\nBody: {} at {:?}",
                    status,
                    body,
                    meta.0
                );
            }

            let body = response.text().await.with_context(|| {
                format!(
                    "baml.fetch_as: failed to read response body at {:?}",
                    meta.0
                )
            })?;

            // Parse the JSON body into the target type
            let parsed_value = parse_json_to_baml_value(&body, target_type, meta)?;
            Ok(parsed_value)
        }
        _ => bail!("unknown builtin function '{}' at {:?}", fn_name, meta.0),
    }
}

fn evaluate_method_call(
    receiver: &BamlValueWithMeta<ExprMetadata>,
    method_name: &str,
    args: &[BamlValueWithMeta<ExprMetadata>],
    meta: &ExprMetadata,
) -> Result<BamlValueWithMeta<ExprMetadata>> {
    match method_name {
        "length" => {
            // Array/List/String/Map length method
            match receiver {
                BamlValueWithMeta::List(items, _) => {
                    if !args.is_empty() {
                        bail!("length() method takes no arguments at {:?}", meta.0);
                    }
                    Ok(BamlValueWithMeta::Int(items.len() as i64, meta.clone()))

View on GitHub (pinned to bd85ce9dee)