microsoft/semantic-kernel · error · NotSupportedException

Unsupported content type {content.GetType().Name}. Cannot co

Error message

Unsupported content type {content.GetType().Name}. Cannot convert to ResponseContentPart.

What it means

KernelContentExtensions.ToResponseContentPart handles four KernelContent subtypes (TextContent, ImageContent, BinaryContent, FileReferenceContent). Any other KernelContent-derived type falls through the switch and throws NotSupportedException.

Source

Thrown at dotnet/src/Agents/OpenAI/Extensions/KernelContentExtensions.cs:21

using System;
using OpenAI.Responses;

namespace Microsoft.SemanticKernel.Agents.OpenAI;

/// <summary>
/// Extensons methods for <see cref="KernelContent"/>.
/// </summary>
internal static class KernelContentExtensions
{
    internal static ResponseContentPart ToResponseContentPart(this KernelContent content)
    {
        return content switch
        {
            TextContent textContent => textContent.ToResponseContentPart(),
            ImageContent imageContent => imageContent.ToResponseContentPart(),
            BinaryContent binaryContent => binaryContent.ToResponseContentPart(),
            FileReferenceContent fileReferenceContent => fileReferenceContent.ToResponseContentPart(),
            _ => throw new NotSupportedException($"Unsupported content type {content.GetType().Name}. Cannot convert to {nameof(ResponseContentPart)}.")
        };
    }

    internal static ResponseContentPart ToResponseContentPart(this TextContent content)
    {
        return ResponseContentPart.CreateInputTextPart(content.Text);
    }

    internal static ResponseContentPart ToResponseContentPart(this ImageContent content)
    {
        if (content.Uri is not null)
        {
            return ResponseContentPart.CreateInputImagePart(content.Uri);
        }

        if (content.Data is not null)
        {
            var dataUri = new Uri($"data:{content.MimeType};base64,{Convert.ToBase64String(content.Data.Value.ToArray())}");

View on GitHub (pinned to c028a0c7dc)

Solutions

1. Ensure every message item is one of TextContent, ImageContent, BinaryContent, or FileReferenceContent.
2. Project custom content into TextContent (e.g., serialize to text) before adding it to the message.
3. Upgrade the Agents.OpenAI package so newly supported content types are handled.
4. Unit-test message construction with the exact content types you will send.

Example fix

// before
message.Items.Add(myCustomContent);
// after
message.Items.Add(new TextContent(myCustomContent.ToString()));

static bool IsSupportedContent(KernelContent c) =>
    c is TextContent or ImageContent or BinaryContent or FileReferenceContent;

static bool IsSupportedContent(KernelContent c) =>
    c is TextContent or ImageContent or BinaryContent or FileReferenceContent;

try { var part = content.ToResponseContentPart(); }
catch (NotSupportedException ex) when (ex.Message.Contains("Unsupported content type")) {
    // project unknown content to TextContent instead
}

When it happens

Trigger: A ChatMessageContent item is a content type the converter does not know, e.g., a custom KernelContent subclass or a content type added after this converter was written.

Common situations: Adding a custom content class to a message; version skew where a newer Semantic Kernel content type is not yet mapped; embedding an unsupported media type.

Related errors

• Invalid OpenAI client type '{connection.Type}' was specified
• Unsupported role {message.Role.Label}. Only system, user, de
• ImageContent cannot be converted to ResponseContentPart. Onl
• AudioContent cannot be converted to ResponseContentPart. Onl
• FileReferenceContent cannot be converted to ResponseContentP

AI-assisted analysis of microsoft/semantic-kernel@c028a0c7dc (2026-08-13). Data as JSON: /api/errors/ebac3d61393402c2. Report an issue: GitHub.