diff --git a/src/gateway/routes.rs b/src/gateway/routes.rs
index 1207165..c26f4e3 100644
--- a/src/gateway/routes.rs
+++ b/src/gateway/routes.rs
@@ -2288,6 +2288,20 @@ mod tests {
         assert_eq!(body.get("code"), Some(&json!("INVALID_INPUT")));
     }
 
+    #[tokio::test]
+    async fn publish_first_line_invalid_json_returns_400() {
+        let router = build_router(publish_registry(), unused_provider());
+        let req = raw_request("POST", "/publish", b"not-json-at-all\n".to_vec());
+        let (status, body) = send(router, req).await;
+        assert_eq!(status, StatusCode::BAD_REQUEST);
+        assert_eq!(body.get("code"), Some(&json!("INVALID_INPUT")));
+        let message = body.get("message").and_then(Value::as_str).unwrap_or("");
+        assert!(
+            message.contains("not valid JSON"),
+            "the 400 must name the JSON parse failure: {message}"
+        );
+    }
+
     #[tokio::test]
     async fn publish_first_line_missing_chunk_returns_400_invalid_input() {
         let router = build_router(publish_registry(), unused_provider());