[BUG] tools.N.custom.defer_loading: Extra inputs are not permitted (400) when MCP tools are configured

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?

Claude Code 2.1.49 throws a 400 API error when MCP tools are configured and the dynamic tool loading feature (tengu_tool_pear) is active. The tool serializer writes defer_loading: true directly into the API payload, but the Anthropic API's strict schema validation rejects it as an unknown field inside the custom tool sub-object.

What Should Happen?

defer_loading should either:

1. Be gated behind the appropriate beta header (same pattern as input_examples), or
2. Be stripped from the payload before the API call and used only as a client-side hint

Error Messages/Logs

API Error: 400 {"error":{"message":"{\"message\":\"tools.21.custom.defer_loading: 
Extra inputs are not permitted\"}. Received Model Group=claude-sonnet-4-6\n
Available Model Group Fallbacks=None","type":"None","param":"None","code":"400"}}

Steps to Reproduce

In the tool serialization function (xPR in the embedded bundle), defer_loading: true
is unconditionally written into the API tool payload when R.deferLoading is truthy:

if (R.deferLoading) B.defer_loading = !0;

deferLoading is set to true for any MCP tool when the dynamic tool loading feature
flag is active:

deferLoading: q && (DE(GT) || aK8(GT))
// DE(GT) returns true for all MCP tools (isMcp === true)

The defer_loading field ends up inside the custom sub-object when the API
processes the tool array. Since defer_loading is not part of the custom tool
schema, the API rejects it with "Extra inputs are not permitted".

By contrast, other conditional fields like input_examples are correctly gated
behind a beta header check:

if (R.betas?.includes(ZaT) && T.input_examples) B.input_examples = T.input_examples;
.0
defer_loading is missing an equivalent guard.

Claude Model

Sonnet (default)

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

2.1.49

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_