Task tool fails with 'Tool names must be unique' error when MCP servers are enabled

Bug Description

The Task tool (used for spawning sub-agents) fails with error tools: Tool names must be unique when MCP servers are enabled, specifically Playwright MCP in my case.

Steps to Reproduce

1. Enable any MCP server (e.g., Playwright):

``bash
claude mcp add playwright
``

1. Try to use the Task tool with a sub-agent:

``
Task tool with subagent_type=Explore
``

1. Observe the error

Expected Behavior

Task tool should work correctly even when MCP servers are enabled, with sub-agents having access to all available tools including MCP tools.

Actual Behavior

API Error: 400

{
  "type": "error",
  "error": {
    "type": "invalid_request_error",
    "message": "tools: Tool names must be unique."
  }
}

Root Cause Analysis

When Task tool spawns a sub-agent:

1. The sub-agent tries to load all available tools
2. MCP server tools are already loaded in the parent session
3. Tool names collide when the sub-agent tries to re-register them
4. This causes the "Tool names must be unique" error

Workaround

Temporarily disable MCP servers:

claude mcp disable playwright

Environment

• Claude Code Version: 2.0.30 (native build)
• OS: Ubuntu 24.04.3 LTS (Linux 6.14.0-33-generic)
• MCP Server: Playwright MCP (@playwright/mcp)
• Active MCP Servers: 1 (playwright)

Additional Context

• Other tools (Glob, Grep, Read, Bash) work fine with MCP enabled
• Only Task tool is affected
• This blocks parallel multi-agent workflows when MCP is active

Suggested Fix

Sub-agents should either:

1. Inherit the parent's tool registry without re-registering
2. Skip duplicate tool registration with a warning
3. Namespace MCP tools differently for parent/child contexts