[BUG] Opus 4.6 high effort parallel tool calls interrupted in CLI — Sonnet 4.6 works fine

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?

Summary

Claude Opus 4.6 with effortLevel: high consistently fails mid-task in the CLI
when it spawns aggressive parallel tool call batches. Switching to Sonnet 4.6
(same task, same codebase) works without issues.

## Behavior

• Opus 4.6 + high effort: tool call batches are throttled or timed out mid-execution,

session is interrupted

• Sonnet 4.6: completes the same tasks without interruption

## Hypothesis
Opus 4.6 at high effort fans out more parallel tool calls than the backend can
handle, triggering a throttle or timeout. Sonnet 4.6 uses a more conservative
parallel call pattern and stays under the threshold.

## Reproduction

1. Set model to claude-opus-4-6, effortLevel: high in settings.json
2. Run a task that requires multiple file reads/searches (e.g. exploring a medium-sized codebase)
3. Observe interrupted parallel tool batches

## Workaround

• Switch to Sonnet 4.6, OR
• Set effortLevel: medium on Opus 4.6

## Environment

• Claude Code CLI
• Model: claude-opus-4-6
• effortLevel: high

What Should Happen?

Claude Opus 4.6 at high effort has been able to handle most- if not all tasks, without issue until this started happening yesterday. Instead, ,it is stuck in a persistent loop where it shows "⎿  Interrupted · What should Claude do instead? "

Error Messages/Logs

⎿  Interrupted · What should Claude do instead?

Steps to Reproduce

Steps to Reproduce

1. Install Claude Code CLI (latest version)
2. Set model and effort in ~/.claude/settings.json:

{
"model": "claude-opus-4-6",
"effortLevel": "high"
}

1. Open a medium-to-large codebase (500+ lines across multiple files)
2. Run a task that requires broad file exploration, e.g.:

read the handoff notes and summarize what needs to be fixed

1. Observe: Opus 4.6 spawns a large parallel tool call batch (Glob + multiple Read calls simultaneously). The session is interrupted mid-batch — tool calls do not complete, session hangs or errors.
2. To confirm it's model/effort-specific, change settings to:

{
"model": "claude-sonnet-4-6"
}

1. Run the identical prompt — Sonnet 4.6 completes the task without interruption.

Expected: Opus 4.6 high effort completes parallel tool batches successfully.

Actual: Parallel tool batch is throttled/timed out mid-execution; session interrupted.

Claude Model

Opus

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

2.1.79 and 2.1.80

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_