microsoft/autogen · error · JsonException

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

Error message

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

What it means

The same JsonPropertyNameEnumConverter throws JsonException('Unable to convert "X" to enum T') when the JSON string does not match any [JsonPropertyName] label on the target enum's fields — i.e. Anthropic sent an enum value (or an error string) that this AutoGen.Anthropic build has no member for. It is the classic symptom of API-side enum expansion (new stop_reason values, new content types) outpacing the package.

Source

Thrown at dotnet/src/AutoGen.Anthropic/Converters/JsonPropertyNameEnumCoverter.cs:26

namespace AutoGen.Anthropic.Converters;

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.Anthropic to the latest version (enum coverage for new API values lands in updates).
2. Disable beta features / the prompt-caching beta header that unlocks enum values the package doesn't know.
3. If you maintain a fork, add the new value with its [JsonPropertyName] label to the affected enum.
4. Capture the raw JSON to identify the offending string and enum, then check the Anthropic changelog for the field.

Example fix

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

// after (fork option: fall back to a catch-all member instead of throwing)
// add to the enum: [JsonPropertyName("server_tool_use")] ServerToolUse
return (T)Enum.Parse(typeToConvert, field.Name);

try { var resp = await client.CreateChatCompletionAsync(request, ct); } catch (JsonException ex) when (ex.Message.StartsWith("Unable to convert \"")) { /* Anthropic sent an enum label this package version lacks -> upgrade AutoGen.Anthropic; meanwhile disable the beta feature that produces it */ throw new InvalidOperationException("Anthropic API value not supported by this AutoGen.Anthropic version — update the package.", ex); }

When it happens

Trigger: Anthropic adds a stop_reason / content type / event type (e.g. 'max_tokens' vs 'max_tokens_to_stop', 'refusal', 'server_tool_use') not present in the package's enum; an error string arrives where an enum was expected; the beta header enables features whose enum labels the package predates.

Common situations: Old AutoGen.Anthropic package against a new model/API version; account enrolled in Anthropic betas; sending the request through a gateway that substitutes its own enum labels.

Related errors

• Unknown content type
• Value was null.
• Failed to deserialize response
• system message has already been set and only one system mess
• Unexpected message type: {message?.GetType()}

AI-assisted analysis of microsoft/autogen@027ecf0a37 (2026-08-15). Data as JSON: /api/errors/3c025ab7b7cd1ab4. Report an issue: GitHub.