microsoft/autogen · error · InvalidOperationException

Parameter name cannot be null

Error message

Parameter name cannot be null

What it means

While converting a FunctionContract to a Mistral FunctionDefinition, each parameter must produce a named JSON-schema property; a FunctionContract parameter whose Name is null cannot become a property key and this InvalidOperationException is thrown.

Source

Thrown at dotnet/src/AutoGen.Mistral/Extension/FunctionContractExtension.cs:29

public static class FunctionContractExtension
{
    /// <summary>
    /// Convert a <see cref="FunctionContract"/> to a <see cref="FunctionDefinition"/> that can be used in funciton call.
    /// </summary>
    /// <param name="functionContract">function contract</param>
    /// <returns><see cref="FunctionDefinition"/></returns>
    public static FunctionDefinition ToMistralFunctionDefinition(this FunctionContract functionContract)
    {
        var functionDefinition = new FunctionDefinition(functionContract.Name ?? throw new Exception("Function name cannot be null"), functionContract.Description ?? throw new Exception("Function description cannot be null"));
        var requiredParameterNames = new List<string>();
        var propertiesSchemas = new Dictionary<string, JsonSchema>();
        var propertySchemaBuilder = new JsonSchemaBuilder().Type(SchemaValueType.Object);
        foreach (var param in functionContract.Parameters ?? [])
        {
            if (param.Name is null)
            {
                throw new InvalidOperationException("Parameter name cannot be null");
            }

            var schemaBuilder = new JsonSchemaBuilder().FromType(param.ParameterType ?? throw new ArgumentNullException(nameof(param.ParameterType)));
            if (param.Description != null)
            {
                schemaBuilder = schemaBuilder.Description(param.Description);
            }

            if (param.IsRequired)
            {
                requiredParameterNames.Add(param.Name);
            }

            var schema = schemaBuilder.Build();
            propertiesSchemas[param.Name] = schema;

        }
        propertySchemaBuilder = propertySchemaBuilder.Properties(propertiesSchemas);

View on GitHub (pinned to 027ecf0a37)

Solutions

1. Ensure every parameter in FunctionContract.Parameters has a non-null Name matching the method signature.
2. Use reflection-based contract generation instead of manual construction so names are always populated.
3. Validate contracts before registration (guard loop) to fail with a better error message.

Example fix

// before
var contract = new FunctionContract
{
    Name = "GetWeather",
    Parameters = [new FunctionParameterContract { Description = "city", ParameterType = typeof(string) }],
};

// after
var contract = new FunctionContract
{
    Name = "GetWeather",
    Parameters = [new FunctionParameterContract { Name = "city", Description = "city", ParameterType = typeof(string), IsRequired = true }],
};

var badParams = (contract.Parameters ?? []).Where(p => string.IsNullOrEmpty(p.Name)).ToList();
if (badParams.Count > 0) throw new ArgumentException($"{contract.Name}: {badParams.Count} parameter(s) missing Name");

static bool AllParametersNamed(FunctionContract c) => (c.Parameters ?? []).All(p => !string.IsNullOrEmpty(p.Name));

When it happens

Trigger: Registering a tool whose parameter list contains a FunctionContract parameter with Name == null — typically hand-built contracts, partially initialized parameter objects, or parameter metadata extraction that returned null.

Common situations: Manually constructing FunctionContract.Parameters and omitting Name; refactoring that drops the name assignment; edge cases in reflection-based contract providers (e.g. weird parameter expressions).

Related errors

• Function name cannot be null
• Missing MISTRAL_API_KEY environment variable
• Failed to review code block
• Missing MISTRAL_API_KEY environment variable.
• Please set OPENAI_API_KEY environment variable.

AI-assisted analysis of microsoft/autogen@027ecf0a37 (2026-08-15). Data as JSON: /api/errors/3477a0e818de795e. Report an issue: GitHub.