Skill tool call returns 'Tool result missing due to internal error' and agent gets stuck

Bug Description

When invoking the Skill tool, Claude Code intermittently returns [Tool result missing due to internal error] and the agent gets stuck — it does not retry, does not proceed, and does not inform the user. The only recovery is Ctrl+C.

This has been happening repeatedly since 2026-04-04 (yesterday). Before that, it was rare/nonexistent.

Reproduction Steps

1. Open a Claude Code session with hooks and skills configured
2. Invoke a skill via the Skill tool: Skill("knowledge-evolve")
3. The tool returns: [Tool result missing due to internal error]
4. Agent gets stuck — no output, no retry, no error message to user
5. Only Ctrl+C recovers the session
6. Retrying the same Skill call after Ctrl+C usually succeeds

Expected Behavior

• Agent should automatically retry (at least once) on internal error
• If retry fails, agent should inform the user instead of hanging silently

Actual Behavior

• Tool returns internal error
• Agent produces no output and hangs indefinitely
• User must Ctrl+C to recover

Environment

• Claude Code version: 2.1.92
• OS: Windows 11 Home 10.0.26200
• Shell: Git Bash
• Node.js: v24.12.0
• Model: claude-opus-4-6 (1M context)
• Hooks: 53 total (43 global + 10 project), including PreToolUse and PostToolUse hooks
• MCP servers: 3 configured (kastell, context7, prompts-chat)
• Plugins: 2 (context-mode, telegram)

Error Message (exact)

<error>[Tool result missing due to internal error]</error>

This is the raw tool result returned by CC runtime when calling the Skill tool.

Frequency

• Rare before 2026-04-04
• Multiple times per session since 2026-04-04
• Affects Skill tool primarily, but may affect other tools

Possible Related Issues

• #39830 — Parallel agent calls losing tool_results (similar symptom, different trigger)
• #33949 — SSE streaming hangs indefinitely
• #26224 — General hanging/freezing

Workaround

User must Ctrl+C and manually re-invoke the skill. The CLAUDE.md directive to "immediately retry on internal error" is not followed because the agent gets stuck before it can act on the error.