bug: A simplest MCP server cannot be invoked via n8n when the input parameters are empty

[SuperGod]

Description

A simplest MCP server cannot be invoked via n8n when the input parameters are empty

Code Sample

package main

import (
        "context"
        "fmt"

        "github.com/mark3labs/mcp-go/mcp"
        "github.com/mark3labs/mcp-go/server"
)

func main() {
        // Create a new MCP server
        s := server.NewMCPServer(
                "Demo 🚀",
                "1.0.0",
                server.WithToolCapabilities(false),
        )

        // Add tool
        tool := mcp.NewTool("hello_world",
                mcp.WithDescription("Say hello to someone"),
        )

        // Add tool handler
        s.AddTool(tool, helloHandler)
        svr := server.NewStreamableHTTPServer(s)
        if err := svr.Start(":8081"); err != nil {
                fmt.Printf("Server error: %v\n", err)
        }
}

func helloHandler(ctx context.Context, request mcp.CallToolRequest) (*mcp.CallToolResult, error) {
        return mcp.NewToolResultText("Hello world"), nil
}

Logs or Error Messages

run it and connect it with n8n(version: 2.1.5), it will call failed with msg:

Problem in node ‘PartialExecutionToolExecutor‘
Error executing tool: Received tool input did not match expected schema ✖ Required → at value

Environment

• Go version (see go.mod): [1.25.5]
• mcp-go version (see go.mod): [0.43.2]
• Any other relevant environment details (debian 12.2)

Additional Context

Possible Solution

when I remove the omitempty tag of json on ToolArgumentsSchema.Properties, and rebuild it ,It will be all ok

diff --git a/mcp/tools.go b/mcp/tools.go
index 0dc7ceed..0c0e1ac1 100644
--- a/mcp/tools.go
+++ b/mcp/tools.go
@@ -637,7 +637,7 @@ func (t Tool) MarshalJSON() ([]byte, error) {
 type ToolArgumentsSchema struct {
        Defs       map[string]any `json:"$defs,omitempty"`
        Type       string         `json:"type"`
-       Properties map[string]any `json:"properties,omitempty"`
+       Properties map[string]any `json:"properties"`
        Required   []string       `json:"required,omitempty"`
 }