Qwen3.5 only output reasoning and no content

Hi I am having troubles using qwen3.5 4B with vllm.

Here is my config

docker run -d \
    --runtime nvidia \
    --gpus 1 \
    --name vllm-demo \
    -v ./cache_models:/root/.cache/huggingface \
    --env "HF_TOKEN=$HF_TOKEN" \
    -p 8000:8000 \
    --ipc=host \
    vllm/vllm-openai:latest \
    Qwen/Qwen3.5-4B \
    --mm-encoder-tp-mode data \
    --mm-processor-cache-type shm \
    --reasoning-parser qwen3 \
    --enable-prefix-caching

After I start the docker container, I tried it with below code

# Modify OpenAI's API key and API base to use vLLM's API server.
openai_api_key = "EMPTY"
openai_api_base = "http://localhost:8000/v1"

messages = [
    {"role": "system", "content": "You are a helpful assistant."},
    {"role": "user", "content": "Who won the world series in 2020?"},
    {
        "role": "assistant",
        "content": "The Los Angeles Dodgers won the World Series in 2020.",
    },
    {"role": "user", "content": "Where was it played?"},
]


def main(stream: bool):
    client = OpenAI(
        # defaults to os.environ.get("OPENAI_API_KEY")
        api_key=openai_api_key,
        base_url=openai_api_base,
    )
    chat_completion = client.chat.completions.create(
        messages=messages,
        model="Qwen/Qwen3.5-4B",
        stream=stream,
        max_tokens=3000,
        # extra_body={"reasoning_effort": "none"}
    )
    return chat_completion


out = main(stream=False)
print(dict(out.choices[0].message))

But whatever max_tokens I used in openai client I only got content=None

{'content': None,
 'refusal': None,
 'role': 'assistant',
 'annotations': None,
 'audio': None,
 'function_call': None,
 'tool_calls': [],
 'reasoning': 'Thinking Process:\n\n1.  **Analyze the Request:** The user is asking about the location where the 2020 World Series was played.\n\n2.  **Retrieve Knowledge:** Access knowledge about the 2020 MLB World Series.\n    *   Teams: Los Angeles Dodgers vs. Tampa Bay Rays.\n    *   Location: Due to the pandemic (COVID-19) and empty stadiums, the game was played without spectators at Minute Maid Park in Houston, but the hosting city is crucial. Wait, no. Two series? No.\n    *   Correction: In 2020, it was a "designated playoff series" where teams were designated. But actually, the question is about the home fields of both games. The Series was a best of five, played behind closed doors.\n    *   Wait, check specific details. The 2020 World Series games were played at Minute Maid Park in Houston, Texas? No, that was the ALDS.\n    *   Let\'s verify the 2020 World Series venue.\n    *   Game 1-2 were at T-Mobile Park (Seattle)? No.\n    *   Actually, both teams were winning their respective divisions, so they played games in Seattle? Wait, the Dodgers were a Dodgers player\'s team, but where did they play?\n    *   Wait, the question is about the World Series.\n\n    *   *Correction*: The 2020 World Series between the LA Dodgers and the Tampa Bay Rays was played at **Tropicana Field** in **St. Petersburg, Florida** before all 5 games? No.\n    *   Let\'s recall. It was played in multiple venues?\n    *   Actually, looking at a category specific fact. 2020 World Series was played: Game 1 at Minute Maid Park in Houston? No.\n    *   Wait, I recall something about the games being played in a specific location due to the empty stadium policy. Actually, the 2020 World Series was in "St. Petersburg, Florida" for all 5 games?\n    *   Let me check my internal knowledge base carefully.\n    *   Fact: The 2020 World Series was played in **St. Petersburg, Florida** (Tropicana Field)? No, that was a different tournament or a minor league thing.\n    *   Fact: The 2020 World Series took place at **Minute Maid Park in Houston**?\n    *   Let\'s think about the specific stadiums.\n    *   Game 1: Dodgers at Tampa Bay.\n    *   Game 2: Tampa Bay at Los Angeles.\n    *   Wait, the 2020 World Series was hosted in **St. Petersburg, Florida**? No.\n    *   Wait, I remember a trivia point about it being the first time MLB played all games in a specific city, but I might be mixing it up with the Philippine World Series or something.\n    *   Wait, no. The 2020 World Series was played entirely at **T-Mobile Park**? No.\n    *   Okay, let\'s step back. The teams were Los Angeles Dodgers and Tampa Bay Rays.\n    *   Games 1-4? No.\n    *   The 2020 World Series games were played in: **T-Mobile Park** (Seattle), **Minute Maid Park** (Houston)?\n    *   Actually, I think it was **Hamilton**? No.\n    *   Wait, I am confusing it with the 2021 All-Star Game or something.\n    *   Let me recall: The 2020 World Series took place behind closed doors. Where were the stadiums located?\n    *   Game 1 and 2 were at Minute Maid Park in Houston?\n    *   No, the 2020 World Series was played in **St. Petersburg**? No.\n    *   Okay, let\'s check.\n    *   2020 World Series: Los Angeles Dodgers vs. Tampa Bay Rays.\n    *   Stadiums:\n        *   Game 1: **Tropicana Field**? No.\n        *   Game 2: **Tropicana Field**?\n        *   Actually, in the 2020 World Series, the games were played entirely at **T-Mobile Park** in Seattle? No.\n        *   Wait, I remember now. Both games were played in **St. Petersburg, Florida**? No.\n\n    *   Okay, let\'s be precise.\n    *   The stadiums used were:\n        *   Game 1: **Tampa Bay Rays at Game One** -> **Tropicana Field**. Wait, Rays home is Tropicana Field.\n        *   Game 2: **Dodgers** -> **Dodger Stadium**.\n        *   Game 3: **Dodgers** -> **Dodger Stadium**.\n        *   Game 4: **Rays** -> **Tropicana Field**.\n        *   Game 5: ...\n    *   Wait, the 2020 World Series was played in **St. Petersburg**? No.\n    *   Okay, I need to stop guessing and recall the exact venue details.\n    *   In 2020, due to the pandemic and the specific scheduling, the **World Series** was played at **Tropicana Field** (St. Petersburg, Florida) for all games? No, that was weird.\n    *   Wait, I am thinking of the first all-home games (Hideout).\n    *   Let me check the specific location of the 2020 World Series games.\n    *   It was played behind closed doors in: **T-Mobile Park**, **Minute Maid Park**, **Wrigley Field**, **Cowboys Field**? No.\n    *   Wait, I am mixing up the MLB playoffs with the World Series.\n    *   Actually, the 2020 World Series took place at:\n        *   Game 1: Tropicana Field (St. Petersburg, FL)?\n        *   Game 2: Dodger Stadium (LA)?\n        *   Game 3: Dodger Stadium (LA)?\n        *   Game 4: Tropicana Field (St. Petersburg, FL)?\n        *   Game 5: Tropicana Field (St. Petersburg, FL)?\n    *   Wait, the winner was the Dodgers.\n    *   Wait, the games were played behind closed doors.\n    *   Let\'s search memory for "2020 World Series venue".\n    *   Common knowledge: The games were played in **"St. Petersburg, Florida"**? No.\n    *   Wait, a very specific fact: **The 2020 World Series was played entirely at T-Mobile Park in Seattle?** No.\n    *   Wait, the 2020 World Series was played at **Tropicana Field**?\n    *   Wait, actually, it seems the series was played at multiple stadiums, but one specific one was mentioned? No.\n    *   Wait, I recall the series was played in **Houston** (Minute Maid Park). No.\n\n    *   *Wait, major correction*: The 2020 World Series was played in **St. Petersburg, Florida** (Tropicana Field). Wait, is that right? No.\n    *   Okay, let\'s try again.\n    *   **Verdict**: The 2020 World Series was played in **St. Petersburg, Florida**? No.\n    *   **Verdict**: The 2020 World Series was played in **Los Angeles, California**? No.\n    *   **Verdict**: The 2020 World Series was played in **St. Petersburg**? No.\n    *   Okay, the correct answer is: The games were played at **T-Mobile Park**? No.\n    *   Wait, the 2020 World Series was played in **St. Petersburg** because the Rays had their home games? No.\n    *   Wait, the 2020 World Series was played at **Tropicana Field**?\n    *   Wait, actually, the 2020 World Series was played at **T-Mobile Park** in Seattle. No.\n    *   Wait, I am stuck in a loop.\n    *   Let\'s try to access specific details. 2020 World Series, Dodgers vs. Rays.\n    *   Games were played in **St. Petersburg, Florida**? No.\n    *   Games were played in **T-Mobile Park**? No.\n    *   Games were played in **Minute Maid Park**? No.\n    *   Games were played in **Dodger Stadium**? No.\n    *   Wait, the 2020 World Series had a unique format. All 5 games? No, best of 5. No, best of 3? No, best of 5.\n    *   Wait, I recall the series was played in **St. Petersburg, Florida**.\n    *   Wait, actually, I think the series was played in **stadiums in St. Petersburg**? No.\n    *   Okay, let\'s look at the facts.\n    *   **Fact Check**: The 2020 World Series was played in **St. Petersburg, Florida**?\n    *   Wait, I found a reference in my memory that the 2020 World Series was played **behind closed doors**.\n    *   Wait, the specific venue was **Tropicana Field**?\n    *   Wait, no. The 2020 World Series was played in **Tx** (Texas).\n    *   Wait, I think the correct answer is **Houston, Texas**. (Minute Maid Park).\n    *   Wait, no. The 2020 World Series was played in **St. Petersburg**.\n    *   Wait, I need to be 100% sure.\n    *   Let\'s recall: The 2020 MLB playoffs were held without fans.\n    *   Games were played at **St. Petersburg**? No.\n    *   Okay, I think the series was played at **T-Mobile Park** in Seattle? No.\n    *   Wait, I\'m confusing the 2020 ALDS with the 2020 World Series.\n    *   Let\'s think. The 2020 World Series was Los Angeles Dodgers vs. Tampa Bay Rays.\n    *   The games were played at **Tropicana Field** and **Dodger Stadium**?\n    *   Wait, the first two games?\n    *   Wait, actually, all games were played at **T-Mobile Park**? No.\n    *   Wait, the 2020 World Series was played in **St. Petersburg, Florida**?\n    *   Actually, I recall a trivia question where the answer was **St. Petersburg, Florida** because of Tropicana Field. But that feels wrong for the Dodgers vs. Rays.\n    *   Wait, actually, that year, the 2020 World Series was played in **St. Petersburg, Florida** (Tropicana Field). No, that was a specific playoff series, maybe the ALDS?\n    *   Okay, let\'s accept that I might be misremembering the "St. Petersburg" bit.\n    *   Let\'s try a different angle. The teams were LA Dodgers and Tampa Bay Rays.\n    *   Usually, the Boston vs Yankees style?\n    *   Okay, let\'s recall the actual set of games.\n    *   Game 1: **Tampa Bay** vs **LA Dodgers**.\n    *   Wait, where was **Tampa Bay**\'s game played? **Tropicana Field**?\n    *   Game 2: **LA Dodgers** vs **Tampa Bay**.\n    *   Wait, where was the Dodgers\' game played? **Dodger Stadium**?\n    *   Wait, I think the games were played at **Minute Maid Park** in Houston. That was the 2020 ALDS.\n    *   Wait, no.\n    *   Wait, the 2020 World Series was played in **St. Petersburg, Florida**?\n    *   Actually, I think I remember a trivia point: **The 2020 World Series was played in St. Petersburg, Florida.** No, that\'s wrong.\n    *   Wait, let\'s check the location of the stadiums used.\n    *   Actually, the 2020 World Series was played in **St. Petersburg, Florida**? No.\n    *   Wait, I found a memory: It was played in **St. Petersburg, Florida**.\n    *   Wait, actually, it was played at **T-Mobile Park**?\n    *   Wait, the correct answer is **St. Petersburg, Florida**.\n    *   Wait, no. It was played in **Tx**.\n    *   Okay, I need to resolve this.\n    *   Actually, the correct answer is: **The 2020 World Series was played in **St. Petersburg, Florida.** (at Tropicana Field). Wait, that was the 2020 ALDS? No.\n    *   Wait, actually, the 2020 World Series was played at **T-Mobile Park** in Seattle? No.\n    *   Wait, I think I remember now. The 2020 World Series was played in **St. Petersburg, Florida**. No.\n    *   Okay, let\'s try to verify via logic.\n    *   Los Angeles Dodgers Home: Dodger Stadium.\n    *   Tampa Bay Rays Home: Tropicana Field.\n    *   Games were played behind closed doors.\n    *   Did they rotate?\n    *   Game 1: Dodger Stadium.\n    *   Game 2: Tropicana Field?\n    *   Wait'}

It seem like the reasoning parser of qwen3 did not work properly here.
Could someone help me please?

Your issue is that the response only fills the reasoning field (not content), and the reasoning is very long and repetitive. This is a known problem with Qwen3.5 models and the --reasoning-parser qwen3 flag: the model outputs the reasoning, but the parser fails to extract the final answer into the content field, leaving content as None. This behavior has been reported for Qwen3 and Qwen3.5 models in vLLM, especially when using the reasoning parser, and is not caused by your max_tokens setting or client code. See Qwen3.5-2B output content is always None on RTX5090 and Qwen3 fails parsing reasoning content.

To work around this, you can try disabling the reasoning parser (remove --reasoning-parser qwen3), or set enable_thinking=False in chat_template_kwargs via the extra_body parameter in your client call. This should make the model output the answer in the content field as expected. If you need reasoning outputs, you may need to wait for further updates to the Qwen3 reasoning parser in vLLM, as this is a parser/model compatibility issue. Would you like more detail or example code for these workarounds?

Sources:

Hint: Mention @RunLLM in the post for followups.