microsoft/semantic-kernel · error · NotSupportedException

The provided response format type '{responseFormat.GetType()

Error message

The provided response format type '{responseFormat.GetType()}' is not supported.

What it means

The response format object is expected to be either a GeneratedImageFormat enum or a string. If it is non-null and neither of those types (e.g. an int, a custom class, a JObject), NotSupportedException is thrown with the actual type name. This is a type-level guard before any string matching occurs.

Source

Thrown at dotnet/src/Connectors/Connectors.OpenAI/Core/ClientCore.TextToImage.cs:169

            return null;
        }

        if (responseFormat is GeneratedImageFormat format)
        {
            return format;
        }

        if (responseFormat is string formatString)
        {
            return formatString.ToUpperInvariant() switch
            {
                "URI" or "URL" => GeneratedImageFormat.Uri,
                "BYTES" or "B64_JSON" => GeneratedImageFormat.Bytes,
                _ => throw new NotSupportedException($"The provided response format '{formatString}' is not supported.")
            };
        }

        throw new NotSupportedException($"The provided response format type '{responseFormat.GetType()}' is not supported.");
    }
}

View on GitHub (pinned to c028a0c7dc)

Solutions

Convert the value to a string or GeneratedImageFormat enum before assignment.
If loading from configuration, explicitly type the property as string and let the connector parse it.
Use the strongly-typed GeneratedImageFormat enum to prevent type ambiguity.

Example fix

// before — raw config object, type unknown
settings.ResponseFormat = configSection["responseFormat"];

// after — coerce to string or enum
settings.ResponseFormat = configSection["responseFormat"]?.ToString();
// or
settings.ResponseFormat = GeneratedImageFormat.Bytes;

Defensive patterns

Strategy: type-guard

Validate before calling

object? CoerceResponseFormat(object? raw)
{
    return raw switch
    {
        null => null,
        GeneratedImageFormat f => f,
        string s => s,
        System.Text.Json.JsonElement je => je.GetString(),
        _ => throw new ArgumentException($"ResponseFormat must be string or GeneratedImageFormat, got {raw.GetType().Name}")
    };
}

Type guard

bool IsValidResponseFormat(object? value) =>
    value is null || value is GeneratedImageFormat || value is string;

Try / catch

try { await imageService.GetTextToImageAsync(prompt, settings); }
catch (NotSupportedException ex) when (ex.Message.Contains("response format type"))
{
    settings.ResponseFormat = GeneratedImageFormat.Uri;
    await imageService.GetTextToImageAsync(prompt, settings);
}

Prevention

Always type execution-settings properties as string or GeneratedImageFormat, never object.
When binding from JSON configuration, explicitly convert JsonElement to string.

When it happens

Trigger: Passing a response format from configuration or a dynamic source that deserializes to an unexpected type — e.g. a JsonElement, an anonymous object, an enum that is not GeneratedImageFormat, or a numeric value.

Common situations: Binding execution settings from JSON/IConfiguration where the deserializer produces a JsonElement or a different enum. Passing a loosely-typed 'object' from configuration parsing without converting it first. Version mismatch where a renamed enum type no longer matches.

Related errors

AI-assisted analysis of microsoft/semantic-kernel@c028a0c7dc (2026-08-13). Data as JSON: /api/errors/e20409867392a66a. Report an issue: GitHub.