diff --git a/src/Plugins/BotSharp.Plugin.DeepSeekAI/Providers/Chat/ChatCompletionProvider.cs b/src/Plugins/BotSharp.Plugin.DeepSeekAI/Providers/Chat/ChatCompletionProvider.cs
index 4f05ee8c..dbaeb82d 100644
--- a/src/Plugins/BotSharp.Plugin.DeepSeekAI/Providers/Chat/ChatCompletionProvider.cs
+++ b/src/Plugins/BotSharp.Plugin.DeepSeekAI/Providers/Chat/ChatCompletionProvider.cs
@@ -284,20 +284,28 @@ public class ChatCompletionProvider : IChatCompletion
 
             if (choice.FinishReason == ChatFinishReason.ToolCalls || choice.FinishReason == ChatFinishReason.FunctionCall)
             {
-                // The model may emit several tool calls in one response. The OpenAI SDK streams each
-                // call's arguments as interleaved fragments per call index, so we must accumulate them
-                // PER CALL (grouped by index), not concatenate every fragment of every call together
-                // (that would corrupt arguments, e.g. get_node({"node_id": "10"}{"maxDepth": 2})).
-                var grouped = toolCalls
-                    .Where(x => !string.IsNullOrEmpty(x.FunctionName))
+                // The model may emit several tool calls in one response. The OpenAI SDK streams
+                // arguments as interleaved fragments PER CALL INDEX: the first fragment of a call
+                // carries the name/id and an empty arguments payload, later fragments carry only
+                // the argument text (FunctionName is null on them). We must accumulate fragments
+                // grouped by Index and must NOT pre-filter on FunctionName (that would drop the
+                // argument-carrying fragments and yield empty FunctionArgs).
+                //
+                // Old bug: string.Join over ALL fragments of ALL calls produced corrupted
+                // arguments (get_node({"node_id": "10"}{"maxDepth": 2})), which made every
+                // tool call fail with e.g. "node_id is required".
+                var groups = toolCalls
                     .GroupBy(x => x.Index)
                     .OrderBy(g => g.Key)
                     .ToList();
 
-                var functionName = grouped.Count > 0 ? grouped[0].First().FunctionName : null;
-                var toolCallId = grouped.Count > 0 ? grouped[0].First().ToolCallId : null;
-                var functionArgument = grouped.Count > 0
-                    ? string.Concat(grouped[0].Where(x => x.FunctionArgumentsUpdate != null).Select(x => x.FunctionArgumentsUpdate.ToString()))
+                var namedGroup = groups.FirstOrDefault(g => g.Any(x => !string.IsNullOrEmpty(x.FunctionName)));
+                var namedUpdate = namedGroup?.FirstOrDefault(x => !string.IsNullOrEmpty(x.FunctionName));
+
+                var functionName = namedUpdate?.FunctionName;
+                var toolCallId = namedUpdate?.ToolCallId;
+                var functionArgument = namedGroup != null
+                    ? string.Concat(namedGroup.Where(x => x.FunctionArgumentsUpdate != null).Select(x => x.FunctionArgumentsUpdate.ToString()))
                     : string.Empty;
 
 #if DEBUG