[Bugfix] Fix missing first token in tool calls during reasoning-to-tool transition (#30671)

Signed-off-by: mondaylord <[email protected]>

Author: mondaylord

vllm/entrypoints/openai/serving_chat.py

@@ -964,21 +964,9 @@ async def chat_completion_stream_generator(
                         assert reasoning_end_arr is not None
                         output_token_ids = as_list(output.token_ids)
                         if not reasoning_end_arr[i]:
-                            delta_message = (
-                                reasoning_parser.extract_reasoning_streaming(
-                                    previous_text,
-                                    current_text,
-                                    delta_text,
-                                    previous_token_ids,
-                                    current_token_ids,
-                                    output_token_ids,
-                                )
-                            )
                             # When encountering think end id in prompt_token_ids
                             # i.e {"enable_thinking": False},
                             # set reasoning status to end.
-                            # Remove the text and token ids related
-                            # to 'reasoning'.
                             if (
                                 res.prompt_token_ids
                                 and reasoning_parser.is_reasoning_end(
@@ -987,30 +975,38 @@ async def chat_completion_stream_generator(
                             ):
                                 reasoning_end_arr[i] = True
                                 current_token_ids = output_token_ids
-                                if delta_message and delta_message.content:
-                                    current_text = delta_message.content
-                                    delta_message.content = None
-                                else:
-                                    current_text = ""
-                            # When encountering think end id in delta_token_ids,
-                            # set reasoning status to end.
-                            # Remove the text and token ids related
-                            # to 'reasoning'.
-                            if reasoning_parser.is_reasoning_end(output_token_ids):
-                                reasoning_end_arr[i] = True
-                                current_token_ids = (
-                                    reasoning_parser.extract_content_ids(
-                                        output_token_ids
+                                # Don't update current_text, keep it as is from delta
+                            else:
+                                delta_message = (
+                                    reasoning_parser.extract_reasoning_streaming(
+                                        previous_text,
+                                        current_text,
+                                        delta_text,
+                                        previous_token_ids,
+                                        current_token_ids,
+                                        output_token_ids,
                                     )
                                 )
-                                if delta_message and delta_message.content:
-                                    current_text = delta_message.content
-                                    delta_message.content = None
-                                else:
-                                    current_text = ""
+
+                                # When encountering think end id in delta_token_ids,
+                                # set reasoning status to end.
+                                # Remove the text and token ids related
+                                # to 'reasoning'.
+                                if reasoning_parser.is_reasoning_end(output_token_ids):
+                                    reasoning_end_arr[i] = True
+                                    current_token_ids = (
+                                        reasoning_parser.extract_content_ids(
+                                            output_token_ids
+                                        )
+                                    )
+                                    if delta_message and delta_message.content:
+                                        current_text = delta_message.content
+                                        delta_message.content = None
+                                    else:
+                                        current_text = ""
 
                         # handle tool calls only after reasoning is done,
-                        else:
+                        if reasoning_end_arr[i]:
                             delta_token_ids = output_token_ids
                             # First time to tool call,
                             # add the remaining text and token ids