diff --git a/Api/Api.cs b/Api/Api.cs
index f606ca24e830..30d5f0773208 100644
--- a/Api/Api.cs
+++ b/Api/Api.cs
@@ -1043,9 +1043,6 @@ public virtual byte[] DownloadBytes(string address, IEnumerable<KeyValuePair<str
             {
                 client.Value.DefaultRequestHeaders.Clear();
 
-                // Add a user agent header in case the requested URI contains a query.
-                client.Value.DefaultRequestHeaders.TryAddWithoutValidation("user-agent", "QCAlgorithm.Download(): User Agent Header");
-
                 if (headers != null)
                 {
                     foreach (var header in headers)
@@ -1054,6 +1051,13 @@ public virtual byte[] DownloadBytes(string address, IEnumerable<KeyValuePair<str
                     }
                 }
 
+                // Add a user agent header in case the requested URI contains a query, unless the caller already provided one.
+                // DefaultRequestHeaders.Contains matches header names case-insensitively per the HTTP spec.
+                if (!client.Value.DefaultRequestHeaders.Contains("User-Agent"))
+                {
+                    client.Value.DefaultRequestHeaders.TryAddWithoutValidation("User-Agent", "QCAlgorithm.Download(): User Agent Header");
+                }
+
                 if (!userName.IsNullOrEmpty() || !password.IsNullOrEmpty())
                 {
                     var credentials = Convert.ToBase64String(Encoding.ASCII.GetBytes($"{userName}:{password}"));