BoundaryML/baml · error · RuntimeCallbackError

Type error in JS callback: {0}

Error message

Type error in JS callback: {0}

What it means

RuntimeCallbackError::JsCallbackTypeError is raised when the value returned from the JS credential callback cannot be correctly shaped/typed when crossing the WASM bridge - i.e. the JS side returned something that does not deserialize into the expected credential result envelope (JsCallbackResult<T>). It is a data-shape mismatch between the JS provider's return value and the Rust expected type, not a thrown JS exception.

Source

Thrown at engine/baml-runtime/src/types/js_callback_provider.rs:32

#[derive(Debug, serde::Deserialize, Eq, PartialEq)]
/// Deserialization helper for js_callback_bridge; declared here to enable deserialization unit testing.
pub struct JsCallbackError {
    pub name: String,
    pub message: String,
}

#[derive(Debug, Error, Clone)]
/// For baml-src-reader and aws-cred-provider, provide a statically defined type which is Send + Sync
/// anyhow::Error is not Send + Sync, so it's convoluted to use it in this callback context
pub enum RuntimeCallbackError {
    #[error("Failed to send cred request across WASM bridge: {0}")]
    SendError(String),

    #[error("Failed to recv cred response across WASM bridge: {0}")]
    RecvError(String),

    #[error("Type error in JS callback: {0}")]
    JsCallbackTypeError(String),

    #[error("JS callback error: {name}: {message}")]
    JsCallbackRuntimeError { name: String, message: String },

    #[error("BAML internal error - credential provider bridges not initialized")]
    NoCredProviderBridge,
}

static_assertions::assert_impl_all!(RuntimeCallbackError: Send, Sync);

pub type RuntimeCallbackResult<T> = Result<T, RuntimeCallbackError>;

static JS_CALLBACK_PROVIDER_SINGLETON: OnceLock<JsCallbackProvider> = OnceLock::new();

pub fn get_js_callback_provider() -> Result<&'static JsCallbackProvider, RuntimeCallbackError> {
    JS_CALLBACK_PROVIDER_SINGLETON
        .get()

View on GitHub (pinned to bd85ce9dee)

Solutions

1. Make the JS credential callback return an object matching AwsCredentialIdentity (`accessKeyId`, `secretAccessKey`, optional `sessionToken`) or the documented ok/error envelope.
2. Log the raw value returned by your JS provider and compare it field-by-field with the expected camelCase schema.
3. Check that your provider resolves with the credential object rather than returning undefined (e.g. an async function without a return).
4. If wrapping the Smithy provider, verify the SDK version's AwsCredentialIdentity shape matches what BAML's bridge expects.

Example fix

// before: wrong field names in JS provider return
async () => ({ accessKey: 'AKIA...', secret: '...' });

// after: match the expected AwsCredentialIdentity shape
async () => ({ accessKeyId: 'AKIA...', secretAccessKey: '...', sessionToken: undefined });

// validate the JS provider's return shape before registering it
function isValidAwsCred(c) {
  return c && typeof c.accessKeyId === 'string' && typeof c.secretAccessKey === 'string';
}
const provider = async (args) => {
  const cred = await myProvider(args);
  if (!isValidAwsCred(cred)) throw new Error('Provider returned invalid credential shape');
  return cred;
};

function isAwsCredentialIdentity(v) {
  return (
    typeof v === 'object' && v !== null &&
    typeof v.accessKeyId === 'string' &&
    typeof v.secretAccessKey === 'string' &&
    (v.sessionToken === undefined || typeof v.sessionToken === 'string')
  );
}

When it happens

Trigger: The JS/WASM bridge deserializes the callback response via JsCallbackResult<T>/serde (js_callback_provider.rs:8-20) and the returned object fails type checking - e.g. missing `ok`/`error` envelope keys, wrong field names, or wrong primitive types - producing a type error surfaced as `JsCallbackTypeError(String)`.

Common situations: A custom JS AWS credential provider returns credentials with misspelled camelCase fields (e.g. `accessKey` instead of `accessKeyId`), returns a raw credential object instead of the expected envelope, or returns undefined/null when the provider fails silently.

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

• JS callback error: {name}: {message}
• baml.json.serialize returned non-string value: {other:?}
• Failed to resolve {}
• JavaScript error: {0:?}
• BAML internal error - credential provider bridges not initia

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/440b60c9ebeb2218. Report an issue: GitHub.