[Bugfix] remove duplicate tokens streamed in required tool choice streaming

[Jason-CKY]

Purpose

each streamed token is duplicated into previous_texts[i] when running a model with tool_parser set, required tool choice and streaming. This will cause JSONDecodeError when running partial_json_parser.loads(current_text) in extract_tool_call_required_streaming method.

This PR removes the duplicate addition in the required tool choice code path introduced in https://github.com/vllm-project/vllm/blame/v0.10.1.1/vllm/entrypoints/openai/serving_chat.py#L873-L876 by removing the changes to previous_texts in the required tool call code path.

The change was part of the feature in 8e8e0b6#diff-f3135631994e5e8f63fff36f5fb493f404a7c253c004183613a007548156e558

The previous_texts[i] is updated in both https://github.com/vllm-project/vllm/blob/v0.10.1/vllm/entrypoints/openai/serving_chat.py#L764-L765 and https://github.com/vllm-project/vllm/blob/v0.10.1/vllm/entrypoints/openai/serving_chat.py#L873-L876 for the case of non-reasoning model, tool call parser set, tool call used with required tool choice and streaming enabled.

Test Plan

Test Result

(Optional) Documentation Update

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• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
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[gemini-code-assist]

Code Review

This pull request addresses a bug in the streaming logic for required tool choice, where previous_texts was being updated twice, leading to duplicated content and a subsequent JSONDecodeError. The change correctly removes the redundant update within the required tool choice code block. This fix relies on the centralized update logic at the end of the loop, which correctly handles both cases with and without a reasoning_parser. The change is a clean and effective fix for the reported issue.

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[Jason-CKY]

@aarnphm this PR is ready for review

[Jason-CKY]

@aarnphm this PR is ready for review

@aarnphm can you take a look at this issue? This is causing all streaming tool calls with tool_choice=required to fail with JSONDecodeError currently.

[chaunceyjiang]

Can you provide the minimal steps to reproduce this?
I'd like to test it locally. This code segment has existed for a long time.

[n0gu-furiosa]

Hi @chaunceyjiang and @Jason-CKY,

I'm not the original author of this PR, but I ran into the same issue and have been eagerly waiting for this to get merged. I also have a few related fixes around the required tool choice, which depend on this PR being merged first. Thanks @Jason-CKY for opening it.

Here are the minimal steps to reproduce:

# Prerequisite: run the server, e.g. `vllm serve meta-llama/Llama-3.1-8B-Instruct`

from openai import OpenAI

client = OpenAI(base_url="http://localhost:8000/v1", api_key="...")

tools = [{
    "type": "function",
    "function": {
        "name": "get_weather",
        "description": "Get the current weather in a given location",
        "parameters": {
            "type": "object",
            "properties": {
                "location": {"type": "string", "description": "City and state, e.g., 'San Francisco, CA'"},
                "unit": {"type": "string", "enum": ["celsius", "fahrenheit"]}
            },
            "required": ["location", "unit"]
        }
    }
}]

for response in client.chat.completions.create(
    model=client.models.list().data[0].id,
    messages=[{"role": "user", "content": "What's the weather like in San Francisco?"}],
    tools=tools,
    tool_choice="required",
    stream=True,
):
    print(response)

Without this fix, the code breaks with the following error (right after the role chunk is sent):

openai.APIError: Expecting property name enclosed in double quotes: line 1 column 3 (char 2)

If you're curious, you can print current_text like below, and you'll quickly see why the bug happens:

diff --git a/vllm/entrypoints/openai/serving_chat.py b/vllm/entrypoints/openai/serving_chat.py
index 579f6f537..d7a2ea5fc 100644
--- a/vllm/entrypoints/openai/serving_chat.py
+++ b/vllm/entrypoints/openai/serving_chat.py
@@ -807,6 +807,7 @@ class OpenAIServingChat(OpenAIServing):
                         previous_text = previous_texts[i]
                         current_text = previous_text + delta_text
                         fn_name_returned = function_name_returned[i]
+                        print(current_text)

Thank you both in advance.

[chaunceyjiang]

Hi @n0gu-furiosa, thank you for providing the reproduction steps. I have noticed this issue. It seems to have been introduced by #20707.

[chaunceyjiang]

Thanks.

I ran the tests locally, and everything worked correctly.

vllm serve /home/jovyan/qwen3-8b  --reasoning-parser qwen3  --enable-auto-tool-choice --tool-call-parser hermes

and

vllm serve /home/jovyan/qwen3-8b   --enable-auto-tool-choice --tool-call-parser hermes

It makes sense to record previous_texts here in a unified way.

/LGTM