CLI crashes when Skill tool is invoked with non-existent skill name

Description

Claude Code CLI crashes ungracefully when the Skill tool is invoked with a skill name that doesn't exist. The terminal becomes unresponsive (empty but still running, no keyboard input accepted), requiring the user to kill the process manually.

Steps to Reproduce

1. Have an agent configuration where prompt-engineer exists as an agent (in ~/.claude/agents/prompt-engineer.md) but NOT as a skill
2. Have a subagent attempt to invoke: Skill tool with {"skill": "prompt-engineer", "args": "..."}
3. Claude Code crashes - the interface completely disappears, terminal becomes empty

Expected Behavior

The Skill tool should return an error message like:

Error: Skill "prompt-engineer" not found. Available skills: aws-auth, cache-awareness, git, pr-review, ...

Actual Behavior

• The CLI crashes silently
• Terminal becomes unresponsive (empty screen, no input accepted)
• Process appears to still be running but cannot be interacted with
• Requires manual kill (Ctrl+C or kill command)

Environment

• Claude Code version: 2.1.12
• OS: macOS (Darwin 24.5.0, Apple Silicon)
• Installation: Homebrew

Session Log Evidence

The crash occurs consistently at this point in the session log:

{
  "type": "tool_use",
  "name": "Skill",
  "input": {
    "skill": "prompt-engineer",
    "args": "create vj-engineer agent..."
  }
}

The session ends abruptly after this entry with no error recorded.

Workaround

Ensure agents are invoked via the Task tool (with subagent_type), not the Skill tool. The Skill tool should only be used for actual skills defined in ~/.claude/skills/.

Additional Context

This crash is reproducible - it occurred across multiple sessions (at least 4 times) when attempting variations of prompts that trigger subagent delegation to prompt-engineer.