[tool call] Fix prev_tool_call_arr management in base_format_detector.py (#11367)

Co-authored-by: Xinyuan Tong <115166877+JustinTong0323@users.noreply.github.com>

python/sglang/srt/function_call/base_format_detector.py

@@ -265,12 +265,6 @@ class BaseFormatDetector(ABC):
                         # Only remove the processed portion, keep unprocessed content
                         self._buffer = current_text[start_idx + end_idx :]
 
-                        if self.current_tool_id < len(self.prev_tool_call_arr):
-                            self.prev_tool_call_arr[self.current_tool_id].clear()
-                        self.current_tool_name_sent = False
-                        self.streamed_args_for_tool[self.current_tool_id] = ""
-                        self.current_tool_id += 1
-
                     # If the tool is still being parsed, send incremental changes
                     elif prev_arguments:
                         prev_args_json = json.dumps(prev_arguments)
@@ -278,6 +272,20 @@ class BaseFormatDetector(ABC):
                             prefix = _find_common_prefix(prev_args_json, cur_args_json)
                             argument_diff = prefix[sent:]
 
+                    # Update prev_tool_call_arr with current state
+                    if self.current_tool_id >= 0:
+                        # Ensure prev_tool_call_arr is large enough
+                        while len(self.prev_tool_call_arr) <= self.current_tool_id:
+                            self.prev_tool_call_arr.append({})
+                        self.prev_tool_call_arr[self.current_tool_id] = (
+                            current_tool_call
+                        )
+
+                    # Advance to next tool if complete
+                    if is_current_complete:
+                        self.current_tool_name_sent = False
+                        self.current_tool_id += 1
+
                     # Send the argument diff if there's something new
                     if argument_diff is not None:
                         # Use the correct tool_index: completing_tool_id for completed tools, current_tool_id for ongoing
@@ -294,17 +302,7 @@ class BaseFormatDetector(ABC):
                                 )
                             ],
                         )
-                        if not is_current_complete:
-                            self.streamed_args_for_tool[
-                                self.current_tool_id
-                            ] += argument_diff
-
-            # Update prev_tool_call_arr with current state
-            if self.current_tool_id >= 0:
-                # Ensure prev_tool_call_arr is large enough
-                while len(self.prev_tool_call_arr) <= self.current_tool_id:
-                    self.prev_tool_call_arr.append({})
-                self.prev_tool_call_arr[self.current_tool_id] = current_tool_call
+                        self.streamed_args_for_tool[tool_index_to_use] += argument_diff
 
             return res