[Bugfix] catch AssertionError in MistralTokenizer as ValueError

[gcalmettes]

When the MistralTokenizer is used, input preprocessing is delegated to mistral-common which uses assert statements to stop the process if the received input is not conformed to what is expected.

Currently AssertionError are not catched when inputs are preprocessed in vLLM and therefore sending an incorrectly formatted message array to a model relying on the MistralTokenizer will result in a 500 Internal Server Error error.

This PR convert AssertionError from the MistralTokenizer to ValueError, so they can be properly caught

# server started with: docker run --rm -d --gpus all --shm-size=1G --ulimit memlock=-1 vllm/vllm-openai:v0.8.3 --served-model-name mistral-small  --model=mistralai/Mistral-Small-3.1-24B-Instruct-2503 --tokenizer-mode mistral --load-format mistral --config-format mistral

from openai import OpenAI

client = OpenAI(
    base_url="http://localhost:8003/v1",
    api_key= "API_KEY",
)

response = client.chat.completions.create(
    model="mistral-small",
    messages=[
        {
            "role": "system",
            # system role content does not support the List[str] format like for user role
            "content": [ 
                {"type": "text", "text": "You are an expert at Maths"},
            ],
        },
        {"role": "user", "content": "What is 1+1?"},
    ]
)

print(response.choices[0].message.content)

assertion error result in 500 Internal Server Error:

  File "/usr/local/lib/python3.12/dist-packages/fastapi/routing.py", line 301, in app
    raw_response = await run_endpoint_function(
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/dist-packages/fastapi/routing.py", line 212, in run_endpoint_function
    return await dependant.call(**values)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/dist-packages/vllm/entrypoints/utils.py", line 63, in wrapper
    return handler_task.result()
           ^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/dist-packages/vllm/entrypoints/utils.py", line 85, in wrapper
    return await func(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/dist-packages/vllm/entrypoints/openai/api_server.py", line 476, in create_chat_completion
    generator = await handler.create_chat_completion(request, raw_request)
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/dist-packages/vllm/entrypoints/openai/serving_chat.py", line 181, in create_chat_completion
    ) = await self._preprocess_chat(
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/dist-packages/vllm/entrypoints/openai/serving_engine.py", line 409, in _preprocess_chat
    request_prompt = apply_mistral_chat_template(
                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/dist-packages/vllm/entrypoints/chat_utils.py", line 1198, in apply_mistral_chat_template
    return tokenizer.apply_chat_template(
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/dist-packages/vllm/transformers_utils/tokenizers/mistral.py", line 372, in apply_chat_template
    encoded = self.mistral.encode_chat_completion(request)
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/dist-packages/mistral_common/tokens/tokenizers/mistral.py", line 245, in encode_chat_completion
    return self.instruct_tokenizer.encode_instruct(instruct_request)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/dist-packages/mistral_common/tokens/tokenizers/sentencepiece.py", line 232, in encode_instruct
    new_tokens = self.encode_system_message(msg)
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/dist-packages/mistral_common/tokens/tokenizers/sentencepiece.py", line 609, in encode_system_message
    assert isinstance(message.content, str), "Message content must be normalized"

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[DarkLight1337]

Perhaps this is better solved by updating mistral_common? cc @patrickvonplaten

[DarkLight1337]

IMO assertion errors should not be treated like user errors

[gcalmettes]

I agree with you @DarkLight1337 .

However I have still to see an external PR on mistal-common to be merged.
E.g.: this PR regarding the compliance with OpenAI API was made last september (and as you can see I re-ping them on it), and finally we had to make a workaround in vLLM to handle the inconsistency.

[DarkLight1337]

To avoid mistakenly catching assertion errors inside vLLM, can we add a try-catch block to wrap mistral tokenizer and convert assertion errors to value errors?

[gcalmettes]

Yes this is a good idea. I'll update the PR to go in that direction

[DarkLight1337]

This should work for now. Please fix pre-commit

[gcalmettes]

@DarkLight1337 I made the change you proposed and added a note to explain why we are doing an Error type conversion