v2.1.23+ causes 100% CPU hang with multiple parallel instances and hooks

Description

Claude Code hangs with 100% CPU usage when running multiple parallel instances with hooks enabled. This regression was introduced somewhere between v2.1.22 and v2.1.23.

Environment

• OS: Linux (WSL2, Ubuntu)
• Claude Code version: 2.1.27 (also affects 2.1.25)
• Installation: Native client (not npm)
• Profile: Custom config directory via CLAUDE_CONFIG_DIR

Steps to Reproduce

1. Enable hooks in settings (disableAllHooks: false)
2. Have multiple plugins with hooks enabled (e.g., PreToolUse hooks for Bash, Write, Edit)
3. Start 2+ Claude Code instances in parallel
4. One or more instances will hang at 100% CPU

Expected Behavior

Multiple parallel instances should work without hanging, as they did in v2.1.22.

Actual Behavior

• Instances hang with ~100% CPU usage
• Even simple commands like /plugin cause hangs
• The UI becomes unresponsive
• Process must be killed with kill -9

Workaround

Using v2.1.22 resolves the issue completely:

CLAUDE_CONFIG_DIR=$HOME/.claude-work /path/to/versions/2.1.22

With v2.1.22, everything works perfectly - all hooks, all plugins, multiple parallel instances.

Potentially Related Changes

Looking at the changelog, these changes between 2.1.22 and 2.1.27 might be relevant:

• v2.1.27: "Content-level ask now takes precedence over tool-level allow" - permission handling change
• v2.1.23: "Fixed race condition causing 400 errors with prompt caching scope" - might have introduced new race conditions
• v2.1.23: "Fixed per-user temp directory isolation on shared systems"

Hook Configuration

The hooks are standard PreToolUse/PostToolUse hooks that return JSON with permissionDecision. Example hook output:

{"hookSpecificOutput":{"hookEventName":"PreToolUse","permissionDecision":"allow"}}

Additional Context

• The issue does NOT occur with v2.1.22 regardless how many sessions in parallel and regardless which hooks are enabled or disabled
• CPU usage stays at around 90-100% until the process is manually killed
• No error messages are displayed before the hang
• The issue does occur on the first user prompt then it hangs completely