[BUG] Subagent crashes with Abort() when Skill tool invokes non-existent skill

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report
• [x] I am using the latest version of Claude Code

What's Wrong?

When a subagent (via Task tool) uses the Skill tool to invoke a skill that doesn't exist, the entire Claude Code process crashes with Abort() instead of gracefully handling the error.

The Skill tool correctly returns an error (is_error: true), but the subagent fails to process it and crashes the parent process.

What Should Happen?

The Skill tool error should be handled gracefully. The subagent should either:

• Try an alternative approach
• Report the failure back to the parent agent
• Ask for guidance

The parent Claude Code process should never crash due to a subagent receiving a tool error.

Error Messages/Logs

Terminal output ends with:

Abort()

Subagent log shows the tool call and error response, then abruptly ends:

// Line 6 - Subagent calls Skill tool
{"type":"tool_use","id":"toolu_01FkZZH4KTZDP2vpGaoe8SLT","name":"Skill","input":{"skill":"meta-claude:release-notes"}}

// Line 7 - Tool returns error properly
{"type":"tool_result","content":"<tool_use_error>Unknown skill: meta-claude:release-notes</tool_use_error>","is_error":true,"tool_use_id":"toolu_01FkZZH4KTZDP2vpGaoe8SLT"}

// Line 8 - Empty. Log ends. Process crashed.

Steps to Reproduce

1. Create a custom agent in .claude/agents/ with Skill in allowed tools
2. Agent instructions tell it to invoke a skill that doesn't exist (e.g., skill: "release-notes")
3. Delegate to the agent via Task tool
4. Agent calls Skill tool with non-existent skill name
5. Tool returns error, subagent crashes, parent process aborts

Claude Model

Sonnet (subagent)

Is this a regression?

Unknown

Claude Code Version

2.1.7

Platform

Anthropic API (Max subscription)

Operating System

macOS (Darwin 25.1.0)

Terminal/Shell

Default terminal

Additional Information

The subagent log file ends immediately after receiving the is_error: true tool result - no further processing occurs. This suggests the crash happens when the subagent attempts to process a tool error from the Skill tool specifically.

Related but different from #17007 (agent not found in Task tool) - this is about the Skill tool within a subagent failing to handle "skill not found" errors.