Returning serde_json::Value is not allowed?

[SteveL-MSFT]

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Your Question

I have a tool that will return a value that is any serde_json::Value. So I define my struct like this:

pub struct ExpressionResponse {
    pub result: serde_json::Value,
}

This compiles fine, but using MCP Inspector. When I list tools I get this error:

{
  "error": "[\n  {\n    \"code\": \"custom\",\n    \"path\": [\n      \"tools\",\n      1,\n      \"outputSchema\",\n      \"properties\",\n      \"result\"\n    ],\n    \"message\": \"Invalid input\"\n  }\n]"
}

If I change it to a String and convert the Value to a string, it works fine. However, this also means potential number values or objects, etc... will just be strings. I'd rather not return it as a JSON string since it seems unnatural to have the resulting JSON contain a property that contains a JSON string. What's the right way to do this? Thanks.

[SteveL-MSFT]

Found a workaround:

#[derive(Serialize, JsonSchema)]
#[serde(untagged)]
pub enum ExpressionResult {
    Value(serde_json::Value),
}

#[derive(Serialize, JsonSchema)]
pub struct ExpressionResponse {
    pub result: ExpressionResult,
}

This works, seems unnecessary, but end result is equivalent.