[BUG] MCP tool calls fail: "unknown variant 'adaptive', expected 'enabled' or 'disabled'"

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

MCP tool calls fail during request marshalling when using Claude Code 2.1.32 with Opus 4.6. The Claude API thinking: {type: "adaptive"} configuration is being incorrectly
serialized into the MCP protocol layer.

## Error

503 processing failed: failed to marshal request: unknown variant adaptive, expected enabled or disabled

## Expected Behavior

Claude API thinking configuration should be handled at the Claude API layer, not passed through MCP request marshalling. MCP tools should work regardless of thinking mode.

## Environment

• Claude Code: 2.1.32
• MCP Python SDK: 1.26.0 (latest PyPI)
• Model: Opus 4.6
• Platform: macOS

## Reproduction

1. Use Claude Code 2.1.32 with Opus 4.6 (adaptive thinking enabled)
2. Call any MCP tool
3. Observe marshalling error before tool execution

## Notes

• The MCP SDK has no "adaptive" thinking concept - it only knows enabled/disabled
• Adaptive thinking is a Claude 4.6 API feature, not an MCP feature
• Tested with both mcp==1.26.0 (PyPI) and git main branch - neither supports "adaptive"

What Should Happen?

## What Should Happen

When Claude Code makes an MCP tool call:

1. Claude API layer handles the thinking: {type: "adaptive"} configuration for the model's reasoning process
2. MCP transport layer only receives the tool name and arguments - no thinking configuration
3. The thinking mode should be stripped/isolated before marshalling the MCP request

Current (broken): Thinking config leaks into MCP marshalling → unknown variant 'adaptive'

Expected: Thinking config stays in Claude API layer, MCP only sees tool call data

Error Messages/Logs

503 processing failed: failed to marshal request: unknown variant `adaptive`, expected `enabled` or `disabled`

Steps to Reproduce

Steps to Reproduce

1. Setup MCP server (any server works, using test as example):

cd /path/to/mcp-server
uv sync # or pip install

1. Configure Claude Code to use the MCP server in ~/.claude/claude_desktop_config.json or project config
2. Start Claude Code 2.1.32:

claude --version # Confirm 2.1.32

1. Select Opus 4.6 model:

/model opus

1. Call any MCP tool:

analyze_single_article(url="https://example.com/security-article")

1. Observe error (retries ~10 times then fails):

503 processing failed: failed to marshal request: unknown variant adaptive, expected enabled or disabled
Retrying in 14 seconds… (attempt 6/10)

Key observations:

• Error occurs before the tool executes (during marshalling)
• Happens with any MCP tool, not specific to one server
• Does not occur with Opus 4.5 or when adaptive thinking is disabled
• MCP server logs show no incoming request (fails client-side)

Claude Model

Opus

Is this a regression?

Yes, this worked in a previous version

Last Working Version

4.5
Note for "Additional Information":
This is model-specific, not Claude Code version-specific:

• Opus 4.6: Fails with marshalling error
• Sonnet: Works on same Claude Code 2.1.32

Claude Code Version

Claude Code: 2.1.32

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

VS Code integrated terminal

Additional Information

_No response_