What causes "baml.fetch_as expects a string URL argument at {:?}" in BoundaryML/baml?

Ensure the first argument is a string literal or an expression evaluating to a string

baml.fetch_as expects a string URL argument at {:?}

Error message

baml.fetch_as expects a string URL argument at {:?}

What it means

After checking arity, baml.fetch_as validates that its single argument is a string URL. The interpreter throws this when the first argument evaluates to any non-string BamlValue (int, object, list, etc.).

Source

Thrown at engine/baml-compiler/src/thir/interpret.rs:2752

        "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(|| {
                format!(
                    "baml.fetch_as: failed to fetch URL '{}' at {:?}",
                    url, meta.0
                )
            })?;

            let status = response.status();
            if !status.is_success() {
                let body = response

View on GitHub (pinned to bd85ce9dee)

Solutions

Ensure the first argument is a string literal or an expression evaluating to a string
Coerce or extract the URL, e.g. pass obj.url instead of obj
Inspect the value at the reported span and add an explicit string construction

Example fix

// before
baml.fetch_as::<MyType>(config.endpoint_port)
// after
baml.fetch_as::<MyType>("https://example.com/endpoint")

Defensive patterns

Strategy: type-guard

Validate before calling

fn ensure_string_url(args: &[BamlValueWithMeta<ExprMetadata>]) -> Result<String, String> {
    match args.first() {
        Some(BamlValueWithMeta::String(s, _)) => Ok(s.clone()),
        _ => Err("baml.fetch_as first argument must be a string URL".into()),
    }
}

Type guard

fn is_string(v: &BamlValueWithMeta<ExprMetadata>) -> bool {
    matches!(v, BamlValueWithMeta::String(_, _))
}

Prevention

Keep URLs in explicitly typed string variables
Interpolate dynamic parts into a string literal instead of passing raw values
Add pre-evaluation checks that the URL argument is a string

When it happens

Trigger: Calling baml.fetch_as::<T>(x) where x evaluates to a non-string value, e.g. an int, a field of a map, or the result of a non-string expression.

Common situations: Passing a URL stored in a non-string variable; forgetting to interpolate into a string; passing a parsed JSON object instead of the URL string field within it.

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

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/1b5ffe57fed729f9. Report an issue: GitHub.