fix(llm): only send gzip header when response is from upstream (#13912)

kong/llm/plugin/base.lua

@@ -93,19 +93,21 @@ function MetaPlugin:rewrite(sub_plugin, conf)
 end
 
 function MetaPlugin:header_filter(sub_plugin, conf)
-  -- we use openai's streaming mode (SSE)
-  if get_global_ctx("stream_mode") then
-    -- we are going to send plaintext event-stream frames for ALL models
-    kong.response.set_header("Content-Type", "text/event-stream")
-    -- TODO: disable gzip for SSE because it needs immediate flush for each chunk
-    -- and seems nginx doesn't support it
-  else
-
-    if get_global_ctx("accept_gzip") then
+  -- for error and exit response, just use plaintext headers
+  if kong.response.get_source() == "service" then
+    -- we use openai's streaming mode (SSE)
+    if get_global_ctx("stream_mode") then
+      -- we are going to send plaintext event-stream frames for ALL models
+      kong.response.set_header("Content-Type", "text/event-stream")
+      -- TODO: disable gzip for SSE because it needs immediate flush for each chunk
+      -- and seems nginx doesn't support it
+
+    elseif get_global_ctx("accept_gzip") then
       kong.response.set_header("Content-Encoding", "gzip")
-    else
-      kong.response.clear_header("Content-Encoding")
     end
+
+  else
+    kong.response.clear_header("Content-Encoding")
   end
 
   run_stage(STAGES.REQ_POST_PROCESSING, sub_plugin, conf)