microsoft/autogen · error · ArgumentException

Missing parameter type for {parameterName}

Error message

Missing parameter type for {parameterName}

What it means

KernelPluginMiddleware.InvokeFunctionAsync deserializes the LLM's JSON arguments and converts each value using KernelParameterMetadata.ParameterType. If the plugin function's parameter metadata carries no .NET type (ParameterType == null), conversion is impossible and this ArgumentException is thrown.

Source

Thrown at dotnet/src/AutoGen.SemanticKernel/Middleware/KernelPluginMiddleware.cs:47

        _functionCallMiddleware = new FunctionCallMiddleware(functionContracts, functionMap, Name);
    }

    public Task<IMessage> InvokeAsync(MiddlewareContext context, IAgent agent, CancellationToken cancellationToken = default)
    {
        return _functionCallMiddleware.InvokeAsync(context, agent, cancellationToken);
    }

    private async Task<string> InvokeFunctionAsync(Kernel kernel, KernelFunction function, string arguments)
    {
        var kernelArguments = new KernelArguments();
        var parameters = function.Metadata.Parameters;
        var jsonObject = JsonSerializer.Deserialize<JsonObject>(arguments) ?? new JsonObject();
        foreach (var parameter in parameters)
        {
            var parameterName = parameter.Name;
            if (jsonObject.ContainsKey(parameterName))
            {
                var parameterType = parameter.ParameterType ?? throw new ArgumentException($"Missing parameter type for {parameterName}");
                var parameterValue = jsonObject[parameterName];
                var parameterObject = parameterValue.Deserialize(parameterType);
                kernelArguments.Add(parameterName, parameterObject);
            }
            else
            {
                if (parameter.DefaultValue != null)
                {
                    kernelArguments.Add(parameterName, parameter.DefaultValue);
                }
                else if (parameter.IsRequired)
                {
                    throw new ArgumentException($"Missing required parameter: {parameterName}");
                }
            }
        }
        var result = await function.InvokeAsync(kernel, kernelArguments);

View on GitHub (pinned to 027ecf0a37)

Solutions

1. Define plugin methods as concrete public C# methods so KernelFunctionFactory.FromMethod captures ParameterType via reflection.
2. If constructing KernelFunctionMetadata manually, always populate ParameterType for each KernelParameterMetadata.
3. Check for null ParameterType at plugin registration time and fail fast with a clear message naming the function.
4. Avoid dynamic/AOT trimming for assemblies containing plugins, or use source-generated plugin metadata.

Example fix

// before
var metadata = new KernelFunctionMetadata("get_weather")
{
    Parameters = [new KernelParameterMetadata("city")] // no ParameterType
};

// after
var metadata = new KernelFunctionMetadata("get_weather")
{
    Parameters = [new KernelParameterMetadata("city") { ParameterType = typeof(string) }]
};

// Fail fast at plugin registration
foreach (var p in function.Metadata.Parameters)
{
    if (p.ParameterType is null)
        throw new InvalidOperationException($"Plugin '{function.Metadata.Name}': parameter '{p.Name}' has no ParameterType.");
}

catch (ArgumentException ex) when (ex.Message.StartsWith("Missing parameter type"))
{
    logger.LogError(ex, "Plugin metadata incomplete; register plugins from strongly-typed methods");
    throw;
}

When it happens

Trigger: Registering a KernelFunction whose parameter metadata lacks a resolved type — typically dynamically constructed functions (e.g. from PromptFunctions with untyped arguments) or KernelFunctionFactory overloads that do not carry reflection info — and the model then supplies that parameter in its arguments JSON.

Common situations: Building KernelPlugins from delegates defined via method-info-less factories; AOT/trimming scenarios stripping metadata; plugins authored with object/dynamic parameters.

Related errors

• Missing required parameter: {parameterName}
• Unsupported content type
• Only one choice is supported in streaming response
• The method or operation is not implemented.
• unsupported message type, only support TextMessage, ImageMes

AI-assisted analysis of microsoft/autogen@027ecf0a37 (2026-08-15). Data as JSON: /api/errors/febcd770e6381f32. Report an issue: GitHub.