[BUG] Regression (v2.1.160+): subagent spawning fails on Bedrock with "thinking.type must be enabled or adaptive" for Claude 4.x models
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?
Since approximately v2.1.160, spawning subagents via the Agent tool or Workflow tool fails
immediately (0 tokens, ~100–400ms) with a 400 API error when the parent session is running a
Claude 4.x model (Sonnet 4.6, Opus) on AWS Bedrock. The same pattern worked correctly on
earlier versions.
What Should Happen?
Agents should be able to be launched.
Error Messages/Logs
API Error: 400 thinking.type must be "enabled" or "adaptive"Steps to Reproduce
Steps to reproduce
Configure Claude Code VS Code extension to use AWS Bedrock with a Claude 4.x model (e.g. claude-sonnet-4-6 or claude-opus-4-8) as the session model
Open any project and start a Claude Code session
Ask Claude to spawn any subagent — the simplest possible repro:
Use the Agent tool to spawn a subagent with the prompt: "Reply with: OK"
Expected: Subagent responds with "OK"
Actual:
API Error: 400 thinking.type must be "enabled" or "adaptive"
subagent_tokens: 0, duration_ms: ~100-400ms
Claude Model
Sonnet (default)
Is this a regression?
Yes, this worked in a previous version
Last Working Version
v2.1.154
Claude Code Version
2.1.169
Platform
AWS Bedrock
Operating System
macOS
Terminal/Shell
Terminal.app (macOS)
Additional Information
_No response_