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

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

Reproduce the supplied move_file call, then locate the move_file implementation and its output schema in the server-filesystem package. Confirm the successful result is a string and rerun the same MCP request to verify output validation passes without error.