BoundaryML/baml · error · RuntimeCallbackError

Failed to send cred request across WASM bridge

Error message

Failed to send cred request across WASM bridge: {0}

What it means

RuntimeCallbackError::SendError is raised when the tokio mpsc `Sender::send` used to forward an AWS or GCP credential request across the WASM bridge fails. In BAML's WASM runtime, Rust calls into a JS-side credential provider (Smithy AwsCredentialIdentity-compatible) via a channel; a failed send means the receiver side of the bridge is gone, so no credential request can be delivered. The underlying tokio SendError is stringified into the message.

Solutions

1. Verify the JS-side credential provider (e.g. `b.setupJsCallbackProvider(...)` / AWS SDK cred provider chain) is registered and kept alive before making BAML calls that need AWS/GCP creds.
2. Check that the WASM runtime/worker hosting the bridge receiver has not exited or been disposed; reinitialize the runtime if it has.
3. Retry the operation - the bridge may be transiently down during startup; ensure set_js_callback_provider ran before the request.
4. Inspect the stringified tokio error in the message to confirm whether the channel was closed (receiver dropped) versus a full/lagged channel.

Example fix

// before: calling BAML function before the JS cred provider bridge is set up
const result = await b.GenerateSummary(input); // Rust: req_tx.send fails (receiver dropped)

// after: initialize the provider bridge first, then call
await b.setupJsCallbackProvider({ awsCredentialProvider: fromIni({ profile: 'my-profile' }) });
const result = await b.GenerateSummary(input);

// JS side
try {
  const result = await b.GenerateSummary(input);
} catch (e) {
  if (String(e).includes('Failed to send cred request across WASM bridge')) {
    // bridge receiver dropped: re-initialize provider/runtime, then retry once
    await setupCredentialBridge();
    return await b.GenerateSummary(input);
  }
  throw e;
}

When it happens

Trigger: JsCallbackProvider::aws_req (js_callback_provider.rs:101-104) or gcp_req (js_callback_provider.rs:124-127) calls `req_tx.send(...)` and the receiving task on the JS/WASM side has been dropped or the channel is closed, returning `Err(e)` which becomes `RuntimeCallbackError::SendError(e.to_string())`.

Common situations: Running BAML inside a Node/browser WASM build where the JS credential-provider task has terminated, the WASM bridge receiver was dropped during shutdown, or the AWS credential chain (e.g. `fromIni(profile)` / default provider) was torn down before an LLM call needed credentials.

Related errors

• Failed to recv cred response across WASM bridge
• AbortError
• BAML engine is shutting down
• Expr function error
• Future with ID not found

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/ebe3529978fc2275. Report an issue: GitHub.

Appendix: source

Thrown at engine/baml-runtime/src/types/js_callback_provider.rs:26

pub enum JsCallbackResult<T> {
    #[serde(rename = "ok")]
    Ok(T),
    #[serde(rename = "error")]
    Err(JsCallbackError),
}

#[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>;

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