Input validation doesn't catch missing fields

[iwyrkore]

Describe the bug
The input validation for a tool call doesn't seem to catch missing required fields. It does catch invalid fields that are not in the schema.

To Reproduce
Steps to reproduce the behavior:

1. I added a second field the the basic example (and also output the tool info):

Run this file:

// Copyright 2025 The Go MCP SDK Authors. All rights reserved.
// Use of this source code is governed by an MIT-style
// license that can be found in the LICENSE file.

package main

import (
	"context"
	"fmt"
	"log"

	"github.com/modelcontextprotocol/go-sdk/mcp"
)

type SayHiParams struct {
	Name  string `json:"name"`
	Title string `json:"title"`
}

func SayHi(ctx context.Context, req *mcp.CallToolRequest, args SayHiParams) (*mcp.CallToolResult, any, error) {
	return &mcp.CallToolResult{
		Content: []mcp.Content{
			&mcp.TextContent{Text: "Hi " + args.Title + " " + args.Name},
		},
	}, nil, nil
}

func main() {
	ctx := context.Background()
	clientTransport, serverTransport := mcp.NewInMemoryTransports()

	server := mcp.NewServer(&mcp.Implementation{Name: "greeter", Version: "v0.0.1"}, nil)
	mcp.AddTool(server, &mcp.Tool{Name: "greet", Description: "say hi"}, SayHi)

	serverSession, err := server.Connect(ctx, serverTransport, nil)
	if err != nil {
		log.Fatal(err)
	}

	client := mcp.NewClient(&mcp.Implementation{Name: "client"}, nil)
	clientSession, err := client.Connect(ctx, clientTransport, nil)
	if err != nil {
		log.Fatal(err)
	}

	res, err := clientSession.CallTool(ctx, &mcp.CallToolParams{
		Name:      "greet",
		Arguments: map[string]any{"name": "user"},
	})
	if err != nil {
		log.Fatal(err)
	}
	fmt.Println(res.Content[0].(*mcp.TextContent).Text)

	clientSession.Close()
	serverSession.Wait()

	// Output: 
        // {"tools":[{"description":"say hi","inputSchema":{"type":"object","required":["name","title"],"properties":{"name":{"type":"string"},"title":{"type":"string"}},"additionalProperties":false},"name":"greet"}]}
Hi  user
        // Hi  user
}

Expected behavior
I would have expected that it would generate a validation error for the missing field "title" in the tool call.

Additional context
The schema looks correct:

"inputSchema":{"type":"object","required":["name","title"],"properties":{"name":{"type":"string"},"title":{"type":"string"}},"additionalProperties":false}

[findleyr]

Thanks again.

I'm sure this is again an artifact of validating against the Go value, not the JSON.
But this is harder to fix than #447. It seems like we need to always double marshal/unmarshal when performing validation.

[iwyrkore]

Yes it seems you're right.

For this one, maybe you can unmarshal into map[string]any first and validate that? Again, slow. :-(

[findleyr]

Yeah, I don't think we have any choice other than doing a double unmarshal, or rewriting a significant portion of encoding/json. The latter is actually something we'd talked about doing, but it's not going to get done tomorrow (and we need a fix tomorrow!)

I doubt the performance penalty is actually that significant, though obviously we want to be as performant as possible. I'll look into it more tomorrow.