Filesystem MCP: move_file tool returns array instead of string (Error -32602)

[shintaro0403]

Filesystem MCP: move_file tool returns array instead of string (Error -32602)

Summary

When executing the move_file tool from @modelcontextprotocol/server-filesystem, error code -32602 (Invalid params) occurs.

Root Cause: The tool returns an array instead of a string, causing MCP protocol validation to fail.

Error Details

Error Code: -32602 (JSON-RPC Invalid params)

Error Message: Output validation error: Invalid structured content for tool move_file: [ { "expected": "string", "code": "invalid_type", "path": ["content"], "message": "Invalid input: expected string, received array" } ]

Problem Location: The content field is an array (should be a string)

Steps to Reproduce

1. Configure @modelcontextprotocol/server-filesystem as an MCP server
2. Call the move_file tool with:
   • source: Source file path
   • destination: Destination file path
3. Error occurs

Example: { "source": "C:\path\to\file.txt", "destination": "C:\path\to\new\file.txt" }

Expected Behavior

The move_file tool should return a string message when file movement succeeds.

Example: "Successfully moved file from source to destination"

Actual Behavior

The move_file tool returns an array, causing MCP protocol validation to fail.

Environment

OS: Windows 10 (build 26100) Node.js: 22.21.1 (Built-in Claude Desktop) Package: @modelcontextprotocol/server-filesystem@latest MCP Client: Claude Desktop 1.0.1307

Log Excerpts

Request: 2025-11-30T09:57:51.155Z [filesystem] [info] Message from client: { "method": "tools/call", "params": { "name": "move_file", "arguments": { "source": "C:\Users\skiku\Documents\MCP_Learning\chapter01\test1.txt", "destination": "C:\Users\skiku\Documents\MCP_Learning\chapter01\テストファイル\test1.txt" } }, "jsonrpc": "2.0", "id": 26 }

Response (Error): 2025-11-30T09:57:51.174Z [filesystem] [info] Message from server: { "jsonrpc": "2.0", "id": 26, "result": { "content": [{ "type": "text", "text": "MCP error -32602: Output validation error: Invalid structured content for tool move_file: ..." }], "isError": true } }

Additional Information

• Other file operation tools (read_file, write_file, create_directory, etc.) work correctly
• The move_file tool appears in the available tools list but fails at runtime
• This issue prevents file movement functionality from being used

Related Issues

I checked GitHub Issues but found no known problems related to the move_file tool. This appears to be an unreported bug.

Suggested Labels

• bug
• server-filesystem
• schema-validation