[router] fix error response in pd_router (#9505)

Co-authored-by: bruce.xu <bruce.xu@gmicloud.ai>

sgl-router/src/routers/pd_router.rs

@@ -28,7 +28,7 @@ use axum::{
 use futures_util::StreamExt;
 use reqwest::Client;
 use serde::Serialize;
-use serde_json::Value;
+use serde_json::{json, Value};
 use std::collections::HashMap;
 use std::sync::{Arc, RwLock};
 use std::time::{Duration, Instant};
@@ -808,6 +808,57 @@ impl PDRouter {
         .await
     }
 
+    async fn handle_decode_error_response(
+        &self,
+        res: reqwest::Response,
+        context: &PDRequestContext,
+        prefill: &dyn Worker,
+        decode: &dyn Worker,
+    ) -> Response {
+        let status = res.status();
+
+        if context.is_stream {
+            // Handle streaming error response
+            let response_headers = header_utils::preserve_response_headers(res.headers());
+            let error_payload = match res.bytes().await {
+                Ok(error_body) => {
+                    if let Ok(error_json) = serde_json::from_slice::<Value>(&error_body) {
+                        json!({ "message": error_json, "status": status.as_u16() })
+                    } else {
+                        json!({ "message": String::from_utf8_lossy(&error_body).to_string(), "status": status.as_u16() })
+                    }
+                }
+                Err(e) => {
+                    json!({ "message": format!("Decode server error: {}", e), "status": status.as_u16() })
+                }
+            };
+
+            let sse_data = format!(
+                "data: {{'error': {}}}",
+                serde_json::to_string(&error_payload).unwrap_or_default()
+            );
+            let error_stream = tokio_stream::once(Ok(axum::body::Bytes::from(sse_data)));
+
+            let decode_url = decode.url().to_string();
+            self.create_streaming_response(
+                error_stream,
+                status,
+                None,
+                context.return_logprob,
+                Some(decode_url),
+                Some(response_headers),
+                prefill,
+                decode,
+            )
+        } else {
+            // Handle non-streaming error response
+            match res.bytes().await {
+                Ok(error_body) => (status, error_body).into_response(),
+                Err(e) => (status, format!("Decode server error: {}", e)).into_response(),
+            }
+        }
+    }
+
     // Internal method that performs the actual dual dispatch (without retry logic)
     async fn execute_dual_dispatch_internal(
         &self,
@@ -881,16 +932,9 @@ impl PDRouter {
                             status
                         );
 
-                        // Return the error response from decode server
-                        match res.bytes().await {
-                            Ok(error_body) => {
-                                return (status, error_body).into_response();
-                            }
-                            Err(e) => {
-                                return (status, format!("Decode server error: {}", e))
-                                    .into_response();
-                            }
-                        }
+                        return self
+                            .handle_decode_error_response(res, &context, prefill, decode)
+                            .await;
                     }
 
                     // Process prefill response for logprobs
@@ -1034,13 +1078,8 @@ impl PDRouter {
                             status
                         );
 
-                        // Return the error response from decode server
-                        match res.bytes().await {
-                            Ok(error_body) => (status, error_body).into_response(),
-                            Err(e) => {
-                                (status, format!("Decode server error: {}", e)).into_response()
-                            }
-                        }
+                        self.handle_decode_error_response(res, &context, prefill, decode)
+                            .await
                     } else if context.is_stream {
                         // Streaming response without logprobs - direct passthrough
                         let decode_url = decode.url().to_string();