[BUG] Subagents look up agent dependencies as skills instead of using the Agent tool

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?

Since approximately 00:00 CET on 2026-04-29, subagents that previously invoked other agents via the Agent tool now check the skills list (from the system reminder) instead. When the target agent is not registered as a skill, the subagent stops with an error.

The agent definitions are unchanged from when they worked
The skills system reminder (listing available skills) appears to be what causes the confusion — subagents now treat it as the authoritative list of callable agents
Workaround: none found

What Should Happen?

The orchestrator invokes subagent via the Agent tool, giving it an isolated context, as it did before.

Error Messages/Logs

The orchestrator reports: "The skill xyz is not available in the current session" and stops. Even when the instructions explicitly say "use the Agent tool with subagent_type worker", the model still attempts a skill lookup instead.

Steps to Reproduce

1. Create ~/.claude/agents/worker.md — a simple agent that does some work
2. Create ~/.claude/agents/orchestrator.md — an agent whose instructions say "call subagent worker for each item"
3. Launch the orchestrator as a subagent via the Agent tool
4. Observe: the orchestrator checks the skills list for worker, fails to find it, and stops instead of using the Agent tool

Claude Model

Sonnet (default)

Is this a regression?

Yes, this worked in a previous version

Last Working Version

day before yesterday

Claude Code Version

 2.1.123 (Claude Code)

Platform

Other

Operating System

Windows

Terminal/Shell

Windows Terminal

Additional Information

_No response_