Description

ToolInputSchema is defined as a type definition (not a type alias) of ToolArgumentsSchema:

type ToolInputSchema ToolArgumentsSchema // For retro-compatibility

This means ToolInputSchema does not inherit the MarshalJSON method from ToolArgumentsSchema. As a result, when marshaling a Tool struct, the inputSchema field loses important data.

Reproduction

package main

import (
	"encoding/json"
	"fmt"
	"github.com/mark3labs/mcp-go/mcp"
)

func main() {
	schema := mcp.ToolInputSchema{
		Type:       "object",
		Properties: map[string]any{},  // empty but not nil
	}
	
	b, _ := json.Marshal(schema)
	fmt.Println(string(b))
	// Output: {"type":"object"}
	// Expected: {"properties":{},"type":"object"}
}

Impact

This breaks compatibility with systems that require properties to be present in JSON schemas, even when empty. For example, Letta's schema validation fails when properties is missing:

Failed to auto-sync tools from MCP server: {'type': 'object', 'additionalProperties': False}. 
Server was created successfully but tools were not persisted.

Suggested Fix

Change the type definition to a true type alias:

type ToolInputSchema = ToolArgumentsSchema // true alias, inherits methods

Or add a separate MarshalJSON method for ToolInputSchema:

func (tis ToolInputSchema) MarshalJSON() ([]byte, error) {
    return ToolArgumentsSchema(tis).MarshalJSON()
}

Environment

• mcp-go version: v0.41.1 (also verified in v0.43.2)
• Go version: 1.21+