fix(chat): disable buffering for sse stream responses

app/(chat)/api/chat/route.ts

@@ -223,6 +223,17 @@ function writeAssistantTextChunk(
   return messageId;
 }
 
+function createSseResponse(stream: ReadableStream) {
+  return new Response(stream.pipeThrough(new JsonToSseTransformStream()), {
+    headers: {
+      "Content-Type": "text/event-stream; charset=utf-8",
+      "Cache-Control": "no-cache, no-transform",
+      Connection: "keep-alive",
+      "X-Accel-Buffering": "no",
+    },
+  });
+}
+
 function startStreamKeepalive(writer: StreamWriter, label: string, intervalMs = 15000) {
   const timer = setInterval(() => {
     safeWriteStreamChunk(
@@ -1417,7 +1428,7 @@ export async function POST(request: Request) {
           });
         },
       });
-      return new Response(duplicateStream.pipeThrough(new JsonToSseTransformStream()));
+      return createSseResponse(duplicateStream);
     }
     inFlightChatRequests.set(requestDedupKey, now);
     acquiredInFlightLock = true;
@@ -2104,7 +2115,7 @@ export async function POST(request: Request) {
           });
         },
       });
-      return new Response(duplicateStream.pipeThrough(new JsonToSseTransformStream()));
+      return createSseResponse(duplicateStream);
     }
 
     const streamId = generateUUID();
@@ -2173,7 +2184,7 @@ export async function POST(request: Request) {
         },
       });
 
-      return new Response(clarificationStream.pipeThrough(new JsonToSseTransformStream()));
+      return createSseResponse(clarificationStream);
     }
 
     let finalMergedUsage: AppUsage | undefined;
@@ -2672,7 +2683,7 @@ export async function POST(request: Request) {
         },
       });
       
-      return new Response(geminiStream.pipeThrough(new JsonToSseTransformStream()));
+      return createSseResponse(geminiStream);
     }
     // ========== Gemini 处理结束 ==========
 
@@ -3010,7 +3021,7 @@ export async function POST(request: Request) {
         },
       });
       
-      return new Response(alibabaStream.pipeThrough(new JsonToSseTransformStream()));
+      return createSseResponse(alibabaStream);
     }
     // ========== 阿里云图片生成处理结束 ==========
 
@@ -3342,7 +3353,7 @@ export async function POST(request: Request) {
         },
       });
       
-      return new Response(volcanoStream.pipeThrough(new JsonToSseTransformStream()));
+      return createSseResponse(volcanoStream);
     }
     // ========== 火山引擎视觉处理结束 ==========
 
@@ -3488,7 +3499,7 @@ export async function POST(request: Request) {
         },
       });
       
-      return new Response(qianwenVideoStream.pipeThrough(new JsonToSseTransformStream()));
+      return createSseResponse(qianwenVideoStream);
     }
     // ========== 千问/FAL 视频处理结束 ==========
 
@@ -4267,7 +4278,7 @@ export async function POST(request: Request) {
     //   );
     // }
 
-    return new Response(stream.pipeThrough(new JsonToSseTransformStream()));
+    return createSseResponse(stream);
   } catch (error) {
     const vercelId = request.headers.get("x-vercel-id");