The specification generated for enum values does not include the potential values

[yinzara]

When writing a MCP tool that returns a structured JSON output, the list-tools request returns a model that does not state the possible enum values for the enum. This makes it so the input schema does not know what potentially could be sent for each enum value and what the values represent.

[eiriktsarpalis]

I presume this is being caused by #459? If an enum hasn't been configured to serialize as a string, it will default to using integers. This should be reflected in the schema as well.

[mkeeley-tricentis]

I had the same issue; from comment here, you need to annotate the enum to use JsonStringEnumConverter:

[JsonConverter(typeof(JsonStringEnumConverter))]
public enum Animal {
    Cat,
    Dog,
    Elephant,
    Frog,
}

[eiriktsarpalis]

And if you're looking to be AOT compliant you should consider using the generic variant instead:

[JsonConverter(typeof(JsonStringEnumConverter<Animal>))]
public enum Animal {
    Cat,
    Dog,
    Elephant,
    Frog,
}

[yinzara]

No that's not what I'm referring to.

OpenAPIs specification for an enumerated value allows you to indicate what all the enumerated values are for a given enumeration.

So it would say that 1 = Cat and 2 = Dog and so forth.
https://swagger.io/docs/specification/v3_0/data-models/enums/

[yinzara]

Right now if I use an enum in my MCP tool, the input schema doesn't reflect what the potential values are for that enum. I need it do that so that the LLM knows what to send there.

[yinzara]

And what if I can't annotate my enums? What if I'm in a DDD world and my enums need to be configured with some kind of Fluent API or they're on generated gRPC classes?

[yinzara]

You could also refer to:
https://www.speakeasy.com/openapi/schemas/enums#enums-with-names-and-values
If you wanted to use "oneOf" and you could describe both the name and the value

[eiriktsarpalis]

Unless you're talking about the enum keyword (which is included once you configure the converter as described above), can you give me an example of what the expected output should look like?

[yinzara]

What if I can add that annotation to the enums because they're on generated gRPC classes? I tried using the method ``` [JsonSourceGenerationOptions(UseStringEnumConverter = true)] [JsonSerializable(typeof(MyEnumType))] internal partial class EnumJsonContext : JsonSerializerContext; ``` But they're still coming out as numbers and not strings.

…

On Thu, May 29, 2025 at 11:33 PM Eirik Tsarpalis ***@***.***> wrote: *eiriktsarpalis* left a comment (modelcontextprotocol/csharp-sdk#460) <#460 (comment)> Unless you're talking about the enum keyword (which *is* included once you configure the converter as described above), can you give me an example of what the expected output should look like? — Reply to this email directly, view it on GitHub <#460 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAFEA3YS64VBZSW47HHAYGT3A73TDAVCNFSM6AAAAAB6EI4TTCVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZDSMRRGM3DMOJXGA> . You are receiving this because you authored the thread.Message ID: ***@***.***>

[eiriktsarpalis]

If you're using a source generator, you would need to pass the resultant Options in EnumJsonContext as a parameter in the MCP methods. If not using a source generator, you can enable string-based serialization for all enum types like so:

var options = new JsonSerializerOptions(McpJsonUtilities.DefaultOptions) { Converters = { new JsonStringEnumConverter() } };

Assuming you pass that options instance, it will ensure every enum will now use string-based serialization.

[yinzara]

How can I pass that options instance to MCP methods?

[eiriktsarpalis]

They should all accept an optional JsonSerializerOptions parameter.

[yinzara]

I have no idea what that means. It makes no sense.

I have this:

    [McpServerTool(Name = "execute-competition-structure")]
    public static async Task<CompetitionStructure> Execute(
        [Description("The operation to perform")]
        CompetitionStructureOperation operation,
        [Description("The name of the competition structure in the form 'org/{org}/competitionStructures/{competitionStructure}'")]
        string name,
        CompetitionStructureService.CompetitionStructureServiceClient client,
        IAuthContext authContext,
        CancellationToken cancellationToken)
    {
        CompetitionStructure result;

        switch (operation)
        {
            case CompetitionStructureOperation.Disable:
                result = await authContext.DoInContextAsync(() => client.DisableCompetitionStructureAsync(
                    new DisableCompetitionStructureRequest { Name = name },
                    cancellationToken: cancellationToken).ResponseAsync);
                break;

            case CompetitionStructureOperation.Reenable:
                result = await authContext.DoInContextAsync(() => client.ReenableCompetitionStructureAsync(
                    new ReenableCompetitionStructureRequest { Name = name },
                    cancellationToken: cancellationToken).ResponseAsync);
                break;

            default:
                throw new System.ArgumentException($"Unknown operation: {operation}", nameof(operation));
        }

        return result;
    }

My CompetitionStructureOperation enum is from a generated gRPC class and I need to make sure it is a string in the protocol.

How can I do that?