fix(frontend): classify cancelled requests as 499 instead of 500 (#8090)

Signed-off-by: tmontfort <tmontfort@nvidia.com>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lib/llm/src/http/service/anthropic.rs

@@ -289,6 +289,16 @@ async fn anthropic_messages(
                 .metrics_clone()
                 .inc_rejection(&model, super::metrics::Endpoint::AnthropicMessages);
         }
+        // Check for cancelled request (client disconnected before response was sent)
+        if super::metrics::request_was_cancelled(e.as_ref()) {
+            inflight_guard.mark_error(super::metrics::ErrorType::Cancelled);
+            return anthropic_error(
+                StatusCode::from_u16(499).unwrap(),
+                "request_cancelled",
+                &format!("Request cancelled: {}", e),
+            );
+        }
+        inflight_guard.mark_error(super::metrics::ErrorType::Internal);
         anthropic_error(
             StatusCode::INTERNAL_SERVER_ERROR,
             "api_error",

lib/llm/src/http/service/metrics.rs

@@ -36,6 +36,13 @@ pub fn request_was_rejected(err: &(dyn std::error::Error + 'static)) -> bool {
     dynamo_runtime::error::match_error_chain(err, REJECTION, NON_REJECTION)
 }
 
+/// Check whether an error chain indicates the request was cancelled.
+pub fn request_was_cancelled(err: &(dyn std::error::Error + 'static)) -> bool {
+    const CANCELLATION: &[DynamoErrorType] = &[DynamoErrorType::Cancelled];
+    const NON_CANCELLATION: &[DynamoErrorType] = &[];
+    dynamo_runtime::error::match_error_chain(err, CANCELLATION, NON_CANCELLATION)
+}
+
 pub use prometheus::Registry;
 
 use super::RouteDoc;

lib/llm/src/http/service/openai.rs

@@ -94,6 +94,9 @@ pub(crate) struct ErrorMessage {
 fn map_error_code_to_error_type(code: StatusCode) -> String {
     match code.canonical_reason() {
         Some(reason) => reason.to_string(),
+        // 499 is not IANA-registered (nginx convention for client-closed-request),
+        // so canonical_reason() returns None. Use the de facto standard name.
+        None if code.as_u16() == 499 => "Client Closed Request".to_string(),
         None => "UnknownError".to_string(),
     }
 }
@@ -114,6 +117,7 @@ fn classify_error_for_metrics(code: StatusCode, message: &str) -> ErrorType {
         StatusCode::TOO_MANY_REQUESTS => ErrorType::Overload, // 429
         StatusCode::SERVICE_UNAVAILABLE => ErrorType::Overload, // 503
         StatusCode::INTERNAL_SERVER_ERROR => ErrorType::Internal, // 500
+        _ if code.as_u16() == 499 => ErrorType::Cancelled, // 499 Client Closed Request
         _ if code.is_client_error() => ErrorType::Validation, // other 4xx
         _ => ErrorType::Internal,                     // everything else
     }
@@ -220,6 +224,20 @@ impl ErrorMessage {
             );
         }
 
+        // Check for Cancelled anywhere in the error chain → HTTP 499 (Client Closed Request)
+        if super::metrics::request_was_cancelled(err.as_ref()) {
+            let code = StatusCode::from_u16(499).unwrap();
+            tracing::debug!("Request cancelled before response: {err}");
+            return (
+                code,
+                Json(ErrorMessage {
+                    message: err.to_string(),
+                    error_type: map_error_code_to_error_type(code),
+                    code: code.as_u16(),
+                }),
+            );
+        }
+
         // Then check for HttpError
         match err.downcast::<HttpError>() {
             Ok(http_error) => ErrorMessage::from_http_error(http_error),
@@ -2524,6 +2542,38 @@ mod tests {
         );
     }
 
+    #[test]
+    fn test_cancelled_error_response_from_anyhow() {
+        use dynamo_runtime::error::{DynamoError, ErrorType};
+
+        let err: anyhow::Error = DynamoError::builder()
+            .error_type(ErrorType::Cancelled)
+            .message("Context id abc-123 is stopped or killed")
+            .build()
+            .into();
+        let response = ErrorMessage::from_anyhow(err, BACKUP_ERROR_MESSAGE);
+        assert_eq!(
+            response.0.as_u16(),
+            499,
+            "Cancelled errors should return HTTP 499"
+        );
+        assert_eq!(response.1.code, 499);
+        assert_eq!(response.1.error_type, "Client Closed Request");
+        assert!(response.1.message.contains("stopped or killed"));
+    }
+
+    #[test]
+    fn test_cancelled_error_metrics_classification() {
+        // HTTP 499 should be classified as Cancelled for metrics
+        let error_type =
+            classify_error_for_metrics(StatusCode::from_u16(499).unwrap(), "cancelled request");
+        assert_eq!(
+            error_type,
+            ErrorType::Cancelled,
+            "HTTP 499 should map to ErrorType::Cancelled in metrics"
+        );
+    }
+
     #[test]
     fn test_validate_unsupported_fields_accepts_clean_request() {
         let request = make_base_request();

lib/llm/src/migration.rs

@@ -204,10 +204,14 @@ impl RetryManager {
             self.context.link_child(request.context());
             if self.context.is_stopped() || self.context.is_killed() {
                 tracing::debug!("Abort creating new stream after context is stopped or killed");
-                return Err(Error::msg(format!(
+                return Err(DynamoError::builder()
+                    .error_type(ErrorType::Cancelled)
+                    .message(format!(
                         "Context id {} is stopped or killed",
                         self.context.id()
-                )));
+                    ))
+                    .build()
+                    .into());
             }
             response_stream = Some(self.next_generate.generate(request).await);
             if let Some(err) = response_stream.as_ref().unwrap().as_ref().err()
@@ -960,6 +964,15 @@ mod tests {
                     .to_string()
                     .contains(&format!("Context id {} is stopped or killed", context_id))
             );
+            // Verify the error is a typed DynamoError with Cancelled type
+            let dynamo_err = error
+                .downcast_ref::<DynamoError>()
+                .expect("Error should be a DynamoError");
+            assert_eq!(
+                dynamo_err.error_type(),
+                ErrorType::Cancelled,
+                "Stopped/killed context should produce a Cancelled error"
+            );
         }
 
         assert_eq!(metrics.get_migration_new_request_count(TEST_MODEL), 0);