Custom agents in `.claude/agents/` don't receive MCP tools when spawned as sub-agents via `task` tool

[renatogbp]

Describe the bug

Custom agents defined in .claude/agents/ with mcps: declared in their YAML frontmatter do not receive MCP tool connections when spawned as sub-agents via the task tool. The agent loads and runs, but MCP tools are absent from its tool inventory.

Workaround: Moving agent files to .copilot/agents/ (with symlinks in .claude/agents/) resolves the issue — sub-agents then receive all MCP tools correctly.

This may be related to #2630, but is a distinct issue: #2630 covers MCP propagation failing entirely for sub-agents. This bug is specifically about agent file location affecting MCP resolution.

Affected version

1.0.26

Steps to reproduce the behavior

1. Create a custom agent in .claude/agents/my-agent.md with mcps: frontmatter:

   ---
   name: my-agent
   description: Agent that uses MCP tools
   mcps:
     my-mcp:
       type: local
       command: some-mcp-command
       args: [mcp, my-server]
   ---
   Agent instructions here.

2. Configure .mcp.json at repo root with the same MCP servers (they work in the main interactive session).

3. Spawn the agent as a sub-agent via the task tool:

   task(agent_type="my-agent", prompt="List all your available tools")
   

4. Result: Agent loads but reports no MCP tools — only basic tools (view, edit, grep, powershell, etc.)

5. Move agent to .copilot/agents/my-agent.md, symlink .claude/agents/my-agent.md → .copilot/agents/my-agent.md.

6. Reload session and repeat step 3.

7. Result: Agent now reports all MCP tools correctly.

Expected behavior

Agents in .claude/agents/ should receive MCP tools from their mcps: frontmatter when spawned as sub-agents, the same as agents in .copilot/agents/.

Additional context

• OS: Windows 11
• Shell: PowerShell 7
• Copilot CLI version: 1.0.26
• The main interactive session always has MCP tools regardless of agent location
• Session reload was required after moving files (mid-session moves hit a stale file path cache)