[BUG] MCP tool calls with empty string "" parameter drop ALL parameters

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

When an MCP tool parameter has an empty string value (""), Claude Code's parameter serializer drops all parameters in the tool call — not
just the empty one. The MCP server receives {}.

What Should Happen?

mpty string "" is a valid value and should be passed through to the MCP server like any other string.

Error Messages/Logs

Steps to Reproduce

1. Connect any MCP server with a tool that accepts optional string parameters
2. Call the tool with a mix of required params and one param set to ""
3. Observe: the server receives an empty {} dict; all parameters (including required ones like IDs) are missing

Example with a update_device_metadata tool:

• notes="test" → server receives {device_id: 718830, collector_id: 12071, notes: "test"} — correct
• notes="" → server receives {} — all parameters dropped

The Claude Code permission prompt also shows no parameters when one is an empty string, confirming the issue is in serialization before
the MCP transport.

Claude Model

Opus

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.1.139

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_