MCP server doesn't support returning enums

[glaforge]

Expected Behavior

I tried implementing an MCP tool that returned an enum.

For example, let's say you have a tool that looks like this:

@Tool(name = "sun-state")
public SunState sunState() {
    return SunState.TOTAL_ECLIPSE;
}

My enum looks like this:

public enum SunState {
    FULL, ANNULAR_ECLIPSE, PARTIAL_ECLIPSE, TOTAL_ECLIPSE
}

Actual Behaviour

In the case above, when using the MCP inspector, the output of that tool will be a string surrounded by quotes, corresponding to the name of the enum:

"\"TOTAL_ECLIPSE\""

Instead of just `"TOTAL_ECLIPSE"`, maybe?

Now if I want to make this enum `@Introspected` and have a `@JsonSchema`, my `enum` becomes:

```java
@JsonSchema(
    title = "Sun state",
    description = "Sun state",
    uri = "/sunState"
)
@Introspected
public enum SunState {
    FULL, ANNULAR_ECLIPSE, PARTIAL_ECLIPSE, TOTAL_ECLIPSE;
}

The server runs fine, but then, when I list the tools in the MCP inspector, it tells me:

[
  {
    "received": "string",
    "code": "invalid_literal",
    "expected": "object",
    "path": [
      "tools",
      2,
      "outputSchema",
      "type"
    ],
    "message": "Invalid literal value, expected \"object\""
  }
]

However, if I turn the enum into a similar record:

@JsonSchema
@Introspected
public record SunState(String state) {
}

It works just fine.

Steps To Reproduce

As stated above, trying to annotate, return, an enum, from an MCP tool.

Environment Information

No response

Example Application

No response

Version

4.9.3