Disabling output schema validation for mcp inspector?

[ivannpaz]

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Question Category

• Protocol Specification
• SDK Usage
• Server Implementation
• General Implementation
• Documentation
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Your Question

I'm migrating a couple of MCP servers, which were previously implemented with the great work done before at https://github.com/mark3labs/mcp-go

There's one MCP in particular which uses Gemini for image generation, and which I would normally debug via
npx @modelcontextprotocol/inspector.

I used to have input and output schema in that implementation as well, though in all honesty, it was me the one doing the BindArguments phase to translate input into my struct. Now, I had a couple of extra params in the input model to render structured, or not.

When structured was disabled, I would normally do something like the following, and got my image back displayed straight there in the inspector:

return mcp.NewToolResultImage(responseText, img.Data, img.MIMEType), nil

Now, when moving to fully schema oriented, I cannot find a way to disable output validation depending on any switch I could turn on and off.

In short, this library does what it should do, and Im perhaps trying to bend it in a very personal way for my debugging purposes, but wanted to understand if there was a way to achieve this, or just give up and go debug the output of the MCP elsewhere?

[findleyr]

Thanks for carefully describing your use case (and I agree that mark3labs/mcp-go represents a lot of great work).

Can you try setting your tool.OutputSchema to e.g. json.RawMessage("{}"), so that every output is valid?

[ivannpaz]

Let me add a bit more context on how I'm currently building the tool server attachment:

(weirdly formatted so it fits in narrow column 🤣 )

func (t *Tool) Attach(srv *mcp.Server) {
	mcp.AddTool(
		srv,
		&mcp.Tool{
			Description: "Generate product previews via generative AI models",
			Name:        "genimage",
		},
		t.handler(),
	)
}

func (t *Tool) handler() mcp.ToolHandlerFor[ImageGenerationRequest, ImageGenerationResponse] {
	return func(
		ctx context.Context,
		request *mcp.CallToolRequest,
		input ImageGenerationRequest,
	) (*mcp.CallToolResult, ImageGenerationResponse, error) {

		var response ImageGenerationResponse
		
		// etc etc
		

[ivannpaz]

btw, if I attempt to add the suggestion:

func (t *Tool) Attach(srv *mcp.Server) {
	mcp.AddTool(
		srv,
		&mcp.Tool{
			Description:  "Generate product previews via generative AI models",
			Name:         "genimage",
			OutputSchema: json.RawMessage("{}"), // Here...
		},
		t.handler(),
	)
}

this explodes upon starting the service right away

panic: AddTool "genimage": output schema must have type "object" (got <nil>)

goroutine 1 [running]:
github.com/modelcontextprotocol/go-sdk/mcp.(*Server).AddTool(0x14000162cc0, 0x140001157a0, 0x1400046acc0)

[findleyr]

Whoops, that's my bad suggestion! The error message is quite right: output schema must be an object. Try:

OutputSchema: json.RawMessage(`{"type":"object","additionalProperties":true}`)

[ivannpaz]

My goodness, this worked! Thank @findleyr ! 💯

In short, these were the changes for the record:

// Your suggestion which I can add or not programatically for testing purposes
mcp.AddTool(
	srv,
	&mcp.Tool{
		Description:  "Generate product previews via generative AI models",
		Name:         "genimage",
		OutputSchema: json.RawMessage(`{"type":"object","additionalProperties":true}`),
	},
	t.handler(), // this still uses the generics for in and out schema, but they do not override the one above
)

// I still collect both types, one for the actual structured type, another one for the mcp version
var generatedImages []GeneratedImage
var images []mcp.ImageContent

// and later on, if do not have `structured` enabled, I can do my early return with:
return &mcp.CallToolResult{
	Content: []mcp.Content{
		&mcp.ImageContent{
			MIMEType: img.MIMEType,  // img is images[0] from above
			Data:     img.Data,
		},
	},
}, ImageGenerationResponse{}, nil

[findleyr]

Great!

This worked not by disabling the validation, but by changing the validated schema to be trivial.

We should add this to our documentation. As much as possible, I'd like to work within the protocol rather than introduce e.g. options to disable validation.

[ivannpaz]

Makes total sense, if Im adding validation, is because I want it. Thing is I got used to do some previewing which I didn't want to let go 😅 Thanks for your help @findleyr

[jba]

I thought you could do this by having the handler return type be any?

…

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[ivannpaz]

That would defeat the purpose of having fully type safe flow. This "requirement" of mine is only needed for tests directed at the MCP server, but I do want fully validated input and output schemas 99% of the time though.