microsoft/semantic-kernel · error · ArgumentException

A function name can contain only ASCII letters, digits, dash

Error message

A function name can contain only ASCII letters, digits, dashes and underscores: '{name}' is not a valid name.

What it means

Thrown by MistralFunction.ValidFunctionName when the function name does not match the pattern ^[0-9A-Za-z_-]*$ or exceeds 64 characters. Mistral's tool/function API only accepts names containing ASCII letters, digits, dashes, and underscores. This is an ArgumentException during tool registration.

Source

Thrown at dotnet/src/Connectors/Connectors.MistralAI/Client/MistralFunction.cs:114

    #region private

#if NET
    [GeneratedRegex("^[0-9A-Za-z_-]*$")]
    private static partial Regex AsciiLettersDigitsUnderscoresRegex();
#else
    private static Regex AsciiLettersDigitsUnderscoresRegex() => s_asciiLettersDigitsUnderscoresRegex;
    private static readonly Regex s_asciiLettersDigitsUnderscoresRegex = new("^[0-9A-Za-z_-]*$");
#endif

    private static void ValidFunctionName(string name)
    {
        Verify.NotNull(name, nameof(name));
        Verify.True(name.Length <= 64, "The name of the function must be less than or equal to 64 characters.", nameof(name));

        if (!AsciiLettersDigitsUnderscoresRegex().IsMatch(name))
        {
            throw new ArgumentException($"A function name can contain only ASCII letters, digits, dashes and underscores: '{name}' is not a valid name.");
        }
    }

    private static MistralParameters ToMistralParameters(KernelFunctionMetadata metadata)
    {
        var parameters = new MistralParameters();

        if (metadata.Parameters is { Count: > 0 })
        {
            foreach (var parameter in metadata.Parameters)
            {
                parameters.Properties.Add(parameter.Name, parameter.Schema ?? GetDefaultSchemaForTypelessParameter(parameter.Description));
                if (parameter.IsRequired)
                {
                    parameters.Required.Add(parameter.Name);
                }
            }
        }

View on GitHub (pinned to c028a0c7dc)

Solutions

Rename the KernelFunction to use only [A-Za-z0-9_-] and keep it under 64 characters.
Use the KernelFunction attribute or method overloads to override the exposed name: [KernelFunction("get_user_info")].
Sanitize function names before registering the plugin with Mistral.

Example fix

// before
[KernelFunction("Get User Info")]
public string GetUserInfo() { ... }

// after
[KernelFunction("get_user_info")]
public string GetUserInfo() { ... }

Defensive patterns

Strategy: validation

Validate before calling

static readonly Regex ValidName = new(@"^[0-9A-Za-z_-]{1,64}$");
if (!ValidName.IsMatch(functionName))
    throw new InvalidOperationException($"Invalid function name: {functionName}");

Type guard

static bool IsValidMistralFunctionName(string name) =>
    !string.IsNullOrEmpty(name) && name.Length <= 64 &&
    Regex.IsMatch(name, @"^[0-9A-Za-z_-]*$");

Prevention

Use snake_case or kebab-case for function names exposed to LLMs.
Apply [KernelFunction("sanitized_name")] when C# method names are unsuitable.
Sanitize auto-generated function names before registration.

When it happens

Trigger: Registering a KernelFunction whose name contains spaces, dots, parentheses, non-ASCII (Unicode) characters, or is longer than 64 characters; importing a plugin with C# method names that include special characters.

Common situations: Plugin method names with dots like 'Get.User.Info'; localized function names with accented characters; auto-generated function names from descriptions or prompts that include spaces; very long descriptive method names.

Related errors

AI-assisted analysis of microsoft/semantic-kernel@c028a0c7dc (2026-08-13). Data as JSON: /api/errors/0b86cb7c3b36ea89. Report an issue: GitHub.