[BUG] Background subagents missing many tools

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?

Background subagents spawned via the Agent tool with run_in_background: true receive only 7 tools: Bash, Read, Write, Edit, Glob, Grep, Skill. They are missing TaskCreate, TaskUpdate, TaskList, TaskGet, SendMessage, CronCreate, CronList, CronDelete, ToolSearch, and all deferred tools.

Foreground subagents receive 76+ tools including all of the above.

What Should Happen?

Per the subagent docs: "subagents inherit all tools from the main conversation." The tools: frontmatter field should control which tools are available. Background agents should have the same tools as foreground agents minus MCP tools.

Steps to Reproduce

1. Create ~/.claude/agents/TaskWorker.md:

---
name: TaskWorker
description: Worker with task tools
tools: Bash, Read, Write, Edit, Glob, Grep, Skill, ToolSearch, TaskCreate, TaskUpdate, TaskList, SendMessage
background: true
---
Worker agent.

1. Spawn it: Agent(subagent_type: "TaskWorker", mode: "bypassPermissions", prompt: "Call TaskCreate with subject 'test'. List ALL your tools.")

1. Result: Agent reports only 7 tools. TaskCreate unavailable.

1. Same agent spawned without run_in_background (foreground): All tools available, TaskCreate works.

Claude Model

Opus

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.1.88

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Cursor

Additional Information

_No response_