BoundaryML/baml · error · ApiError

Transport error: {0}

Error message

Transport error: {0}

What it means

This is the Display message of ApiError::Transport in the BAML trace publisher. It wraps a reqwest::Error, meaning the HTTP request to the BAML cloud tracing API (used when publishing trace events or uploading BAML source) failed at the transport layer before a response was received: DNS resolution failure, TCP/TLS connect failure, or connection reset. The library throws it because the collector endpoint could not be reached at all.

Source

Thrown at engine/baml-runtime/src/tracingv2/publisher/publisher.rs:192

                    body: body_str,
                });
            }

            // D) happy path: 2xx → attempt to parse into T
            serde_json::from_slice::<TEndpoint::Response<'resp>>(&bytes)
                .map_err(ApiError::Deserialize)
        };

        match timeout(timeout_duration, fut).await {
            Ok(res) => res,
            Err(_) => Err(ApiError::Timeout(timeout_duration)),
        }
    }
}

#[derive(thiserror::Error, Debug)]
pub enum ApiError {
    #[error("Transport error: {0}")]
    Transport(reqwest::Error),
    #[error("HTTP error: {status} {body}")]
    Http {
        status: reqwest::StatusCode,
        body: String,
    },
    #[error("Failed to deserialize response: {0}")]
    Deserialize(serde_json::Error),
    #[error("Request timed out after {0:?}")]
    Timeout(Duration),
}

impl TypeLookup for RuntimeAST {
    fn type_lookup(&self, name: &str) -> Option<Arc<baml_rpc::BamlTypeId>> {
        self.ast.type_lookup(name)
    }

    fn function_lookup(&self, name: &str) -> Option<Arc<baml_rpc::ast::tops::BamlFunctionId>> {

View on GitHub (pinned to bd85ce9dee)

Solutions

1. Verify network connectivity to the collector host (curl the URL from the same machine).
2. Check BAML_URL / endpoint env vars for typos or stale hosts.
3. Fix DNS/proxy settings (HTTPS_PROXY, corporate CA certs) in the runtime environment.
4. Enable retries/backoff for publishing, or disable trace export if telemetry is not required.

Example fix

// before
baml_client.tracing.disable()
// after (guarded, non-fatal publish)
match publisher.publish(event) {
    Err(ApiError::Transport(e)) => log::warn!("trace transport failed: {e}"),
    _ => {}
}

// before enabling trace export
curl -sS -o /dev/null -w "%{http_code}" "$BAML_URL/health" || echo "collector unreachable"

fn is_transport_err(e: &ApiError) -> bool { matches!(e, ApiError::Transport(_)) }

match publisher.publish(ev) {
    Err(ApiError::Transport(e)) => { log::warn!("transport: {e}"); retry_with_backoff(); }
    Err(e) => log::error!("publish failed: {e}"),
    Ok(_) => {}
}

When it happens

Trigger: The reqwest client fails to send the request or receive any response: DNS failure for the collector host, network unreachable, TLS handshake failure, proxy misconfigured, or connection dropped mid-request while sending trace events to the BAML API.

Common situations: Running BAML in an offline/air-gapped CI environment, corporate firewall or proxy blocking the collector host, BAML_URL/BAML_SECRET pointing at a wrong or unreachable host, VPN disconnected, or DNS misconfiguration in containers/K8s.

Related errors

• baml.fetch_as: HTTP request failed: HTTP {} Body: {} at {:?}
• Failed to fetch media: {e:?}
• Failed to fetch media bytes: {e:?}
• Failed to fetch media: {} {}, {}
• Failed to fetch: {url}

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/4162f9b8ad3b970d. Report an issue: GitHub.