microsoft/autogen · error · JsonException

Unable to convert "{value}" to enum {typeToConvert}.

Error message

Unable to convert "{value}" to enum {typeToConvert}.

What it means

JsonPropertyNameEnumConverter<T>.Read iterates the enum's fields and matches the incoming JSON string against each field's [JsonPropertyName] name. If no field's attribute name equals the string, conversion is impossible and this JsonException (echoing the value and enum type) is thrown.

Source

Thrown at dotnet/src/AutoGen.Mistral/Converters/JsonPropertyNameEnumConverter.cs:26

namespace AutoGen.Mistral;

internal sealed class JsonPropertyNameEnumConverter<T> : JsonConverter<T> where T : struct, Enum
{
    public override T Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
    {
        string value = reader.GetString() ?? throw new JsonException("Value was null.");

        foreach (var field in typeToConvert.GetFields())
        {
            var attribute = field.GetCustomAttribute<JsonPropertyNameAttribute>();
            if (attribute?.Name == value)
            {
                return (T)Enum.Parse(typeToConvert, field.Name);
            }
        }

        throw new JsonException($"Unable to convert \"{value}\" to enum {typeToConvert}.");
    }

    public override void Write(Utf8JsonWriter writer, T value, JsonSerializerOptions options)
    {
        var field = value.GetType().GetField(value.ToString());
        var attribute = field?.GetCustomAttribute<JsonPropertyNameAttribute>();

        if (attribute != null)
        {
            writer.WriteStringValue(attribute.Name);
        }
        else
        {
            writer.WriteStringValue(value.ToString());
        }
    }
}

View on GitHub (pinned to 027ecf0a37)

Solutions

1. Update AutoGen.Mistral (and Mistral SDK) to a version whose enums include the new value — the message tells you exactly which string is missing.
2. If updating is blocked, add the missing [JsonPropertyName] member to a local fork of the enum.
3. Capture the raw HTTP response to confirm which endpoint/field carried the unknown value before upgrading.

Example fix

// before (SDK enum)
public enum FinishReasonEnum { [JsonPropertyName("stop")] Stop, [JsonPropertyName("length")] Length }
// API returns "content_filter" -> JsonException

// after (add the member)
public enum FinishReasonEnum { [JsonPropertyName("stop")] Stop, [JsonPropertyName("length")] Length, [JsonPropertyName("content_filter")] ContentFilter }

try { var response = JsonSerializer.Deserialize<ChatCompletionResponse>(json, options); } catch (JsonException ex) when (ex.Message.Contains("Unable to convert")) { /* capture value + upgrade SDK */ }

When it happens

Trigger: The Mistral API returns an enum string the SDK does not know — e.g. a newly added finish reason, tool type, or model capability not present in the shipped enum fields of the AutoGen.Mistral version in use.

Common situations: Mistral ships a new finish_reason/tool variant after the package was released; using a preview/beta Mistral model with extended enums; typos in hand-crafted JSON fed to the client.

Related errors

• Value was null.
• Failed to deserialize response
• Failed to create gallery
• Invalid JSON response from server. Check if the MCP route is
• Invalid JSON response from server. Check if the MCP route is

AI-assisted analysis of microsoft/autogen@027ecf0a37 (2026-08-15). Data as JSON: /api/errors/08d1a32782c46c7d. Report an issue: GitHub.