BoundaryML/baml · error

baml.fetch_as: expected url to be a string, got {}

Error message

baml.fetch_as: expected url to be a string, got {}

What it means

When `baml.fetch_as` receives a baml.HttpRequest class, it reads the `url` field and expects it to be a string. The field was missing or held a non-string value, so the HTTP request cannot be built. (The message prints the whole request object, not the offending field.)

Source

Thrown at engine/baml-runtime/src/async_vm_runtime.rs:606

                                    &parse_as_type,
                                    &baml_types::EvaluationContext::default(),
                                    baml_types::StreamingMode::NonStreaming,
                                )
                                .unwrap();

                                async move {
                                    let response = 'res: {
                                        let client = reqwest::Client::new();

                                        let req = match &url_or_request {
                                            BamlValue::String(url) => {
                                                client.get(url)
                                            }

                                            // we type checked earlier, should be http request type
                                            BamlValue::Class(name, fields) => {
                                                let Some(BamlValue::String(url)) = fields.get("url") else {
                                                    break 'res Err(anyhow!(
                                                        "baml.fetch_as: expected url to be a string, got {}",
                                                        url_or_request
                                                    ));
                                                };

                                                let Some(BamlValue::Enum(_, method)) = fields.get("method") else {
                                                    break 'res Err(anyhow!(
                                                        "baml.fetch_as: expected method to be a valid HTTP method, got {}",
                                                        url_or_request
                                                    ));
                                                };

                                                let mut req = match method.as_str() {
                                                    "Get" => client.get(url),
                                                    "Post" => client.post(url),
                                                    "Put" => client.put(url),
                                                    "Patch" => client.patch(url),
                                                    "Delete" => client.delete(url),

View on GitHub (pinned to bd85ce9dee)

Solutions

1. Add a `url` string field to the baml.HttpRequest constructor
2. Ensure the url variable is a string (coerce/interpolate it) before building the request
3. Fix field-name typos (`url` exactly, not `uri` or `endpoint`)

Example fix

// before (BAML)
let req = baml.HttpRequest{method: "Get", url: null}
// after
let req = baml.HttpRequest{method: "Get", url: "https://api.example.com/data"}

function validateHttpRequest(req) {
  if (!req || typeof req !== 'object') throw new Error('HttpRequest required');
  if (typeof req.url !== 'string' || req.url.length === 0) throw new Error('HttpRequest.url must be a non-empty string, got: ' + JSON.stringify(req.url));
}

function hasStringUrl(req) {
  return typeof req === 'object' && req !== null && typeof req.url === 'string' && req.url.length > 0;
}

try {
  const data = await runtime.callFunction(fnName, args);
} catch (e) {
  if (String(e).includes('expected url to be a string')) {
    // inspect the baml.HttpRequest construction site and fix the url field
  } else throw e;
}

When it happens

Trigger: Constructing baml.HttpRequest without a `url` field, or with a url that is not a string (e.g. an int, null, or a nested object).

Common situations: Typos like `uri:` instead of `url:`, building the request dynamically where the url variable is null/undefined at runtime, or deserializing a request from JSON that lacks the url field.

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

• baml.fetch_as: expected method to be a valid HTTP method, go
• baml.fetch_as: expected header value to be a string, got {}
• type `{ty}` can't be passed through auto-CLI; deliver it via
• baml.json.deserialize failed: {e:?}
• baml.json.serialize returned non-string value: {other:?}

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/c6944e0111a5786a. Report an issue: GitHub.