[AabhasK]

What does this PR do?

transformers serve treats an assistant message as a tool call whenever the tool_calls key is present, even when its value is null or [] (get_processor_inputs_from_messages in cli/serving/utils.py).

• "tool_calls": null raises a TypeError, which surfaces as an HTTP 500. This is the shape message.model_dump() from the OpenAI client produces.
• "tool_calls": [] silently drops the assistant's content from the prompt.

This PR checks that tool_calls is non-empty instead of checking that the key exists. It is a standalone PR, I did not find an existing issue for it.

Reproduction

Start the server:

transformers serve Qwen/Qwen2.5-0.5B-Instruct --port 8000

Then run:

from openai import OpenAI

client = OpenAI(base_url="http://localhost:8000/v1", api_key="unused")
model = "Qwen/Qwen2.5-0.5B-Instruct"

messages = [{"role": "user", "content": "My name is Aurelio. Say hello to me."}]
reply = client.chat.completions.create(model=model, messages=messages).choices[0].message

for tool_calls in (None, []):
    history = messages + [
        {"role": "assistant", "content": reply.content, "tool_calls": tool_calls},
        {"role": "user", "content": "What is my name?"},
    ]
    try:
        out = client.chat.completions.create(model=model, messages=history, temperature=0)
        print(f"tool_calls={tool_calls}: {out.usage.prompt_tokens} prompt tokens ->", out.choices[0].message.content)
    except Exception as e:
        print(f"tool_calls={tool_calls}: {type(e).__name__}: {e}")

Output on main:

tool_calls=None: InternalServerError: Error code: 500
tool_calls=[]: 55 prompt tokens -> Hello! My name is Qwen, and I am an artificial intelligence developed by Alibaba Cloud. How can I assist you today?

Output with this PR:

tool_calls=None: 74 prompt tokens -> Your name is Aurelio.
tool_calls=[]: 74 prompt tokens -> Your name is Aurelio.

Test

Added test_llm_empty_tool_calls_keeps_content to tests/cli/test_serve.py. It fails on main and passes with this PR.

Who can review?

@SunMarc @Rocketknight1

[github-actions]

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