microsoft/semantic-kernel · error · ArgumentException

Unsupported parameter type: {typeString}

Error message

Unsupported parameter type: {typeString}

What it means

Thrown by the type-mapping switch in BedrockFunctionSchemaExtensions when a .NET parameter type string does not match any supported Bedrock type. The supported set includes primitives (String, Boolean, numeric types) and their array forms. Any other type string (objects, custom classes, unsupported arrays, dictionaries) hits the default arm and throws.

Source

Thrown at dotnet/src/Agents/Bedrock/Extensions/BedrockFunctionSchemaExtensions.cs:99

            "Int32" => Amazon.BedrockAgent.Type.Integer,
            "UInt32" => Amazon.BedrockAgent.Type.Integer,
            "Int64" => Amazon.BedrockAgent.Type.Integer,
            "UInt64" => Amazon.BedrockAgent.Type.Integer,
            "Single" => Amazon.BedrockAgent.Type.Number,
            "Double" => Amazon.BedrockAgent.Type.Number,
            "Decimal" => Amazon.BedrockAgent.Type.Number,
            "String[]" => Amazon.BedrockAgent.Type.Array,
            "Boolean[]" => Amazon.BedrockAgent.Type.Array,
            "Int16[]" => Amazon.BedrockAgent.Type.Array,
            "UInt16[]" => Amazon.BedrockAgent.Type.Array,
            "Int32[]" => Amazon.BedrockAgent.Type.Array,
            "UInt32[]" => Amazon.BedrockAgent.Type.Array,
            "Int64[]" => Amazon.BedrockAgent.Type.Array,
            "UInt64[]" => Amazon.BedrockAgent.Type.Array,
            "Single[]" => Amazon.BedrockAgent.Type.Array,
            "Double[]" => Amazon.BedrockAgent.Type.Array,
            "Decimal[]" => Amazon.BedrockAgent.Type.Array,
            _ => throw new ArgumentException($"Unsupported parameter type: {typeString}"),
        };
    }
}

View on GitHub (pinned to c028a0c7dc)

Solutions

1. Change the parameter type to a supported primitive (String, Boolean, Int32, Double, etc.) or a supported one-dimensional array (e.g., String[]).
2. For complex inputs, accept a JSON string parameter and deserialize inside the function.

Example fix

// before
[KernelFunction]
public string Create(MyCustomObject data) { ... } // unsupported type

// after — accept a primitive and parse
[KernelFunction]
public string Create([Description("JSON object")] string dataJson)
{
    var data = JsonSerializer.Deserialize<MyCustomObject>(dataJson);
    ...
}

static readonly HashSet<string> Supported = new()
{
    "String","Boolean","Int32","Int64","Double","Single","Decimal",
    "String[]","Boolean[]","Int32[]","Int64[]","Double[]","Single[]","Decimal[]"
};
// before registering the plugin, check each parameter type name against Supported

static bool IsBedrockSupportedTypeName(string typeName) =>
    typeName is "String" or "Boolean" or "Int16" or "UInt16" or "Int32" or "UInt32"
        or "Int64" or "UInt64" or "Single" or "Double" or "Decimal"
        or "String[]" or "Boolean[]" or "Int16[]" or "UInt16[]" or "Int32[]"
        or "UInt32[]" or "Int64[]" or "UInt64[]" or "Single[]" or "Double[]" or "Decimal[]";

try { agent = await bedrockClient.CreateOrUpdateAgentAsync(...); }
catch (ArgumentException ex) when (ex.Message.Contains("Unsupported parameter type"))
{
    logger.LogError("A kernel function parameter type is not supported by Bedrock. Use primitives or their arrays.");
    throw;
}

When it happens

Trigger: Defining a kernel function parameter whose .NET type is not in the supported mapping — e.g., a custom class, Dictionary, Guid, DateTime, enum, nullable struct, or a jagged/multidimensional array. The extension extracts the type name as a string and fails to match it.

Common situations: Plugin methods with complex object parameters; using Guid/DateTime/TimeSpan parameters; enums; nullable value types reported as 'Nullable`1'; multidimensional arrays; Dictionary or KeyValuePair parameters.

Related errors

• Invalid parameter type for function {agentToolDefinition.Id}
• The option key '{key}' is required for a Bedrock function pa
• Message role must be either Assistant or User.
• No function results were returned.
• runtimeClient

AI-assisted analysis of microsoft/semantic-kernel@c028a0c7dc (2026-08-13). Data as JSON: /api/errors/643032280299f7d6. Report an issue: GitHub.