diff --git a/tensorrt_llm/serve/openai_disagg_service.py b/tensorrt_llm/serve/openai_disagg_service.py
index a3a916f1ffd9..1437908a4b1c 100644
--- a/tensorrt_llm/serve/openai_disagg_service.py
+++ b/tensorrt_llm/serve/openai_disagg_service.py
@@ -233,6 +233,11 @@ def _rewrite_usage_sse_event_from_ctx(
         event: bytes,
         ctx_response: Optional[UCompletionResponse],
     ) -> bytes:
+        # Only the final chunk has a top-level `usage` to rewrite, but this ran
+        # json.loads on every relayed chunk. A usage field can't exist unless the
+        # bytes contain `"usage"`, so skip the parse when they don't.
+        if b'"usage"' not in event:
+            return event
         separator_match = self._sse_separator_index(event)
         separator = separator_match[1] if separator_match else b""
         event_body = event[: -len(separator)] if separator else event