Allow tools to have an output schema #273

marianogonzalez · 2025-04-03T22:07:14Z

marianogonzalez
Apr 3, 2025

Pre-submission Checklist

I have verified this would not be more appropriate as a feature request in a specific repository
I have searched existing discussions to avoid duplicates

Your Idea

Tools right now are open ended when it comes to the produced output. This makes sense when looked from an LLM point of view.

However, a fair amount of "written by a human" tools will tend to return data in a fixed schema (e.g: data driven tools that will return JSON in a fixed schema). As MCP adoption grows larger in the enterprise, the ability to catalog tools and have a strong input-output contract about them is greatly desirable. For tools used in Agent to Agent communication, this could even be used as a way of hint an LLM to produce data in a certain format and schema, without the need of prompt engineering.

This should of course be an optional feature, since a) this is not always needed, and b) need to preserve backwards compatibility.

Example 1: A tool that transforms an address to coordinates

"tools": [
      {
        "name": "geocode",
        "description": "Transforms an address to coordinates",
        "inputSchema": {
          "type": "object",
          "properties": {
            "address": {
              "type": "string",
              "description": "The location address"
            }
          },
          "required": ["address"]
        }
      },
     "output": {
      "type": "text"
      "mimeType": "application/json",
      "schema": "{\"$schema\":\"http://json-schema.org/draft-07/schema#\",\"type\":\"object\",\"properties\":{\"latitude\":{\"type\":\"number\",\"description\":\"The latitude component of a location's coordinates\"},\"longitude\":{\"type\":\"number\",\"description\":\"The longitude component of a location's coordinates\"}},\"required\":[\"latitude\",\"longitude\"]}"
     }
    ]

Example 2: Same tool, but returning the result as an embedded resource instead

"tools": [
      {
        "name": "geocode",
        "description": "Transforms an address to coordinates",
        "inputSchema": {
          "type": "object",
          "properties": {
            "address": {
              "type": "string",
              "description": "The location address"
            }
          },
          "required": ["address"]
        }
      },
     "output": {
      "type": "resource"
      "mimeType": "application/json",
      "schema": "{\"$schema\":\"http://json-schema.org/draft-07/schema#\",\"type\":\"object\",\"properties\":{\"latitude\":{\"type\":\"number\",\"description\":\"The latitude component of a location's coordinates\"},\"longitude\":{\"type\":\"number\",\"description\":\"The longitude component of a location's coordinates\"}},\"required\":[\"latitude\",\"longitude\"]}"
     }
    ]

Scope

KennyVaneetvelde · 2025-04-12T14:59:21Z

KennyVaneetvelde
Apr 12, 2025

I would also love this but why not something like this:

"tools": [
  {
    "name": "example_tool",
    "description": "A sample tool demonstrating input and output schemas",
    "inputSchema": {
      "type": "object",
      "properties": {
        "input_one": {
          "type": "string",
          "description": "some input string",
          ...
        },
        ...
      },
      "required": ["input_field"]
    },
    "outputSchema": {
      "type": "object",
      "description":"The output schema",
      "properties": {
        "result": {
          "type": "number",
          "description": "An output result",
          ...
        },
        ...
      },
      "required": ["result"]
    }
  }
]

Though I would be overjoyed if MCP suddenly decided to just simply support the OpenAPI specs, it seems they wanna do their own thing, but an output schema is absolutely required IMO

2 replies

marianogonzalez Apr 14, 2025
Author

@KennyVaneetvelde A tool can produce differnet types of results, including text, blob and embedded resources. IMO, an output schema feature should aknowledge that and also supporting tools that return a PNG image or text tools that return XML.

KennyVaneetvelde Apr 14, 2025

@marianogonzalez I agree, my main point was about having the consistency, so having it named "outputSchema" and be on the same level as the input schema...

It could still be other types than "object"

But good point nevertheless!

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Allow tools to have an output schema #273

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marianogonzalez Apr 3, 2025

Pre-submission Checklist

Your Idea

Scope

Replies: 1 comment · 2 replies

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KennyVaneetvelde Apr 12, 2025

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marianogonzalez Apr 14, 2025 Author

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KennyVaneetvelde Apr 14, 2025

marianogonzalez
Apr 3, 2025

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KennyVaneetvelde
Apr 12, 2025

marianogonzalez Apr 14, 2025
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