[BUG] tools.26 schema error (oneOf/allOf/anyOf) in v2.1.63

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?

Title: tools.26 schema error (oneOf/allOf/anyOf) in v2.1.63

Version: 2.1.63 (latest channel)
OS: Linux (Debian)
Model: Opus 4.6 via Claude Max

Steps to reproduce:

1. Fresh install of Claude Code 2.1.63
2. Launch claude in any directory
3. Send any message (e.g., "hi")
4. Sometimes works on first message, then fails
5. Enabling /plan mode triggers it reliably
6. Once triggered, every subsequent message fails

Error:
API Error: 400 {"type":"error","error":{"type":"invalid_request_error",
"message":"tools.26.custom.input_schema: input_schema does not support
oneOf, allOf, or anyOf at the top level"}}

Attempted fixes (none worked):

• Deleted ~/.claude/ entirely
• Deleted ~/.local/share/claude/ and reinstalled
• Disabled all plugins
• No MCP servers configured
• Clean directory with no project files

What Should Happen?

Claude should function not fail

Error Messages/Logs

API Error: 400 {"type":"error","error":{"type":"invalid_request_error","message":"tools.26.custom.input_schema: input_schema does not support oneOf, allOf, or anyOf at the top 
     level"},"request_id":"req_011CYf65QCBdcNR96xqQdvd3"}

Steps to Reproduce

Start Claude
Type anythhing

Claude Model

Opus

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

< 2.1.63

Platform

Anthropic API

Operating System

Ubuntu/Debian Linux

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_