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.

[bharat3645]

This is a schemars-side quirk rather than an rmcp or protocol restriction.

For a field typed serde_json::Value, schemars 1.x emits the JSON-Schema boolean true as that property's schema (JSON Schema 2020-12 lets true/false stand in as whole schemas, true meaning "accept anything"). So your struct produces:

{
  "type": "object",
  "properties": { "result": true },
  "required": ["result"]
}

The tool's outputSchema validator rejects that bare true — it expects each property to be an object-form schema — which is the Invalid input you get at outputSchema.properties.result.

Your untagged-enum wrapper works only as a side effect: it turns the property into a $ref to { "anyOf": [true] }, which is object-form, so it passes. Equivalent at runtime, just with an extra type and an odd anyOf: [true]. (Reproduced with schemars 1.1.)

If you'd rather not add the wrapper, point the field at an object-form "any" schema directly with schema_with:

use schemars::{Schema, SchemaGenerator};

fn any_json(_: &mut SchemaGenerator) -> Schema {
    // {} is an object-form schema that still means "any JSON value"
    Schema::try_from(serde_json::json!({})).unwrap()
}

#[derive(Serialize, JsonSchema)]
pub struct ExpressionResponse {
    #[schemars(schema_with = "any_json")]
    pub result: serde_json::Value,
}

which emits:

"properties": { "result": {} }

If you'd prefer it self-documenting over a bare {}, return {"type": ["object","array","string","number","boolean","null"]} from the same function — also object-form, also passes validation. Either way result stays a real JSON value (number, object, etc.), no stringify needed.