[BUG] API Error 400: tools.17.custom.input_schema invalid JSON schema - occurs with empty mcpServers

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?

Description

Claude Code throws a JSON schema validation error on every interaction, even with zero MCP servers configured. This appears to be a built-in tool with an invalid schema, not a user configuration issue.

What Should Happen?

Expected Behavior

Claude Code should work normally without MCP servers configured.

Environment

• Claude Code version: 2.1.19
• macOS: 26.2 (Build 25C56)
• Node.js: v24.13.0
• VS Code: Latest (installed via App Store or download)

Troubleshooting Attempted

• Fresh extension reinstall
• Empty MCP server config ({"mcpServers": {}})
• Removed all plugins from ~/.claude/plugins
• Cleared Claude desktop app configs
• Tested with individual MCP servers disabled one by one
• Killed all MCP-related processes

Additional Context

The error specifically mentions tools.17.custom.input_schema, suggesting a built-in tool (index 17) has a schema that doesn't comply with JSON Schema draft 2020-12. This occurs regardless of user MCP configuration.

Error Messages/Logs

## Error Message
API Error: 400 {"type":"error","error":{"type":"invalid_request_error","message":"tools.17.custom.input_schema: JSON schema is invalid. It must match JSON Schema draft 2020-12 (https://json-schema.org/draft/2020-12). Learn more about tool use at [https://docs.claude.com/en/docs/tool-use."}}](https://docs.claude.com/en/docs/tool-use.%22%7D%7D)

Steps to Reproduce

Steps to Reproduce

1. Fresh install of Claude Code extension (v2.1.19)
2. Set ~/.claude/settings.json to {"mcpServers": {}}
3. Open any project in VS Code
4. Try to send any message to Claude Code
5. Error occurs on every attempt

Claude Model

Opus

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

2.1.19

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_