dotnet/machinelearning · error · NotSupportedException

Only text content is supported, but got {item.GetType().Name

Error message

Only text content is supported, but got {item.GetType().Name}

What it means

This BuildPrompt overload works on ChatMessage (Semantic Kernel style) and only renders TextContent items into the LLaMA prompt. It throws NotSupportedException when a message item is any other content type (function call/result, image, binary), since the plain-text LLaMA template cannot represent them.

Source

Thrown at src/Microsoft.ML.GenAI.LLaMA/Llama3_1ChatTemplateBuilder.cs:69

        sb.Append($"<|start_header_id|>assistant<|end_header_id|>{Newline}");
        var input = sb.ToString();

        return input;
    }

    public string BuildPrompt(ChatHistory chatHistory)
    {
        // build prompt from chat history
        var sb = new StringBuilder();

        sb.Append("<|begin_of_text|>");
        foreach (var message in chatHistory)
        {
            foreach (var item in message.Items)
            {
                if (item is not TextContent textContent)
                {
                    throw new NotSupportedException($"Only text content is supported, but got {item.GetType().Name}");
                }

                var text = textContent.Text?.Trim() ?? string.Empty;

                var prompt = message.Role switch
                {
                    _ when message.Role == AuthorRole.System => $"<|start_header_id|>system<|end_header_id|>{Newline}{text}<|eot_id|>{Newline}",
                    _ when message.Role == AuthorRole.User => $"<|start_header_id|>user<|end_header_id|>{Newline}{text}<|eot_id|>{Newline}",
                    _ when message.Role == AuthorRole.Assistant => $"<|start_header_id|>assistant<|end_header_id|>{Newline}{text}<|eot_id|>{Newline}",
                    _ => throw new NotSupportedException($"Unsupported role {message.Role}")
                };

                sb.Append(prompt);
            }
        }

        sb.Append($"<|start_header_id|>assistant<|end_header_id|>{Newline}");

View on GitHub (pinned to 7b76e69cf9)

Solutions

1. Flatten function-call/result items to text messages before calling BuildPrompt.
2. Disable automatic function invocation (FunctionChoiceBehavior.None) or remove tool items from history.
3. Filter messages whose Items contain non-TextContent entries.
4. Serialize non-text items to a textual form (e.g. JSON of arguments/results) if they must be included.

Example fix

// before
var prompt = builder.BuildPrompt(chatHistory); // history has FunctionResultContent items
// after
var textOnly = chatHistory.Select(m => new ChatMessage(m.Role,
    string.Join("\n", m.Items.OfType<TextContent>().Select(t => t.Text))));
var prompt = builder.BuildPrompt(textOnly);

if (chatHistory.Any(m => m.Items.Any(i => i is not TextContent)))
    throw new InvalidOperationException("History contains non-text items; flatten before BuildPrompt.");

static bool IsTextOnly(ChatMessage m) => m.Items.All(i => i is TextContent);
var safe = chatHistory.Where(IsTextOnly);

try { prompt = builder.BuildPrompt(chatHistory); }
catch (NotSupportedException ex) when (ex.Message.StartsWith("Only text content"))
{ prompt = builder.BuildPrompt(FlattenToText(chatHistory)); }

When it happens

Trigger: Calling BuildPrompt(IEnumerable<ChatMessage>, ChatOptions?, bool) when any message's Items collection contains non-text content — e.g. FunctionCallContent, FunctionResultContent, or image content added by Semantic Kernel's automatic function calling.

Common situations: Enabling auto function invocation so tool-call/result items appear in chat history; multimodal messages fed to a text-only LLaMA pipeline; chat histories persisted with rich content items.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

• Unsupported role {message.Role}
• Please provide a message with content.
• Please provide a message with a valid role. The valid roles
• Invalid role.
• Failed to generate a reply.

AI-assisted analysis of dotnet/machinelearning@7b76e69cf9 (2026-09-11). Data as JSON: /api/errors/b364559ab78815a0. Report an issue: GitHub.