[BUG] API Error 400 - MCP tool schema uses oneOf/allOf/anyOf at top level

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?

Getting error when starting a conversation with MCP servers enabled:

API Error: 400
{"type":"error","error":{"type":"invalid_request_error",
"message":"tools.68.custom.input_schema: input_schema does not support
oneOf, allOf, or anyOf at the top level"},"request_id":"req_011CaiZnfWRMz6iht1ztGqsn"}

Steps to reproduce:

1. Enable Meta Ads MCP integration on claude.ai
2. Open Claude Code desktop app (Windows)
3. Start any conversation

Expected: Conversation starts normally
Actual: API 400 error blocks all conversations

The issue appeared after enabling the Meta Ads MCP server.
Request ID: req_011CaiZnfWRMz6iht1ztGqsn

<img width="925" height="463" alt="Image" src="https://github.com/user-attachments/assets/cef4b173-b1c0-4771-8a95-92ebf1ce5061" />

What Should Happen?

Conversation starts normally without any API errors.

Error Messages/Logs

API Error: 400
  {
    "type": "error",
    "error": {
      "type": "invalid_request_error",
      "message": "tools.68.custom.input_schema: input_schema does not support oneOf, allOf, or anyOf at the top level"
    },
    "request_id": "req_011CaiZnfWRMz6iht1ztGqsn"
  }

Steps to Reproduce

1. Enable Meta Ads MCP integration on claude.ai
2. Open Claude Code desktop app (Windows)
3. Start any conversation

Claude Model

Sonnet (default)

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

Unknown

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

Other

Additional Information

The error occurs with all MCP servers enabled (Meta Ads, Ahrefs, Canva,
Figma, Gmail, Google Calendar, Google Drive, HubSpot, Klaviyo, Make,
Motion, Notion, Slack, Supermetrics, Tavily, etc.).

The issue started after enabling the Meta Ads MCP integration.
Disabling all MCP integrations resolves the error.

The error references tools.68, suggesting the 68th tool in the list
has an invalid input_schema using oneOf, allOf, or anyOf at the top level,
which is not supported by the Anthropic API.