Always getting a "Ċ" before EOS when using Qwen

[rolevax]

Description

I run a simple chatbot with Qwen and got a "Ċ" at every end of the generated texts:

User> hello
Assistant: Hello! How can I assist you today?Ċ<|im_end|>
User> who are you?
Assistant: I'm an artificial ... (blablabla) ... How can I assist you specifically today?Ċ<|im_end|>

The code is like:

using LLama.Common;
using LLama;
using LLama.Abstractions;
using System.Text;

internal class Program
{
    private static readonly string SystemMessage = """
    You are a helpful assistant.
    """;

    private static async Task Main(string[] args)
    {
        string modelPath = @"my/local/dir/qwen2-7b-instruct-q6_k.gguf";

        var parameters = new ModelParams(modelPath)
        {
            Seed = 1337,
            GpuLayerCount = 29,
        };
        using var model = LLamaWeights.LoadFromFile(parameters);
        using var context = model.CreateContext(parameters);
        var executor = new InteractiveExecutor(context);

        var chatHistory = new ChatHistory();
        chatHistory.AddMessage(AuthorRole.System, SystemMessage);

        ChatSession session = new(executor, chatHistory);
        session.WithHistoryTransform(new QwenHistoryTransform());

        InferenceParams inferenceParams = new()
        {
            MaxTokens = 256,
            AntiPrompts = ["<|im_end|>", "<|endoftext|>"],
        };

        string? userInput;
        while ((userInput = ReadLine()) != null)
        {
            var stream = session.ChatAsync(new ChatHistory.Message(AuthorRole.User, userInput), inferenceParams);
            await foreach (var text in stream)
            {
                Console.Write(text);
            }
        }
    }

    private static string? ReadLine() {
        Console.Write("\nUser> ");
        return Console.ReadLine();
    }
}

class QwenHistoryTransform : IHistoryTransform
{
    private const string Bos = "<|im_start|>";
    private const string Eos = "<|im_end|>";

    public string HistoryToText(ChatHistory history)
    {
        var sb = new StringBuilder(1024);

        foreach (var message in history.Messages)
        {
            sb.Append(Bos);
            sb.Append(GetRoleName(message.AuthorRole));
            sb.Append('\n');
            sb.Append(message.Content);
            sb.Append(Eos);
            sb.Append('\n');
        }

        return sb.ToString();
    }

    private static string GetRoleName(AuthorRole authorRole)
    {
        return authorRole switch
        {
            AuthorRole.User => "user",
            AuthorRole.Assistant => "assistant",
            AuthorRole.System => "system",
            _ => throw new Exception($"Unsupported role: {authorRole}"),
        };
    }

    public ChatHistory TextToHistory(AuthorRole role, string text)
    {
        return new ChatHistory([new ChatHistory.Message(role, text)]);
    }

    public IHistoryTransform Clone()
    {
        return new QwenHistoryTransform();
    }
}

So how can I find where the "Ċ" comes from? What is it related to?

The model is from https://huggingface.co/Qwen/Qwen2-7B-Instruct-GGUF

[sangyuxiaowu]

#858