diff --git a/codex-rs/rollout-trace/src/reducer/mod.rs b/codex-rs/rollout-trace/src/reducer/mod.rs
index 9411b1df32..5f5f163502 100644
--- a/codex-rs/rollout-trace/src/reducer/mod.rs
+++ b/codex-rs/rollout-trace/src/reducer/mod.rs
@@ -265,7 +265,7 @@ impl TraceReducer {
         let payload_path = self.bundle_dir.join(&payload.path);
         let file = File::open(&payload_path)
             .with_context(|| format!("open payload {}", payload.raw_payload_id))?;
-        serde_json::from_reader(file)
+        serde_json::from_reader(BufReader::new(file))
             .with_context(|| format!("parse payload {}", payload.raw_payload_id))
     }