[BUG] Custom subagents in .claude/agents/ not recognized by VS Code extension

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] I can reproduce this bug consistently

Bug Description

Custom subagents defined in .claude/agents/ work correctly in the CLI (claude command in terminal) but are completely unavailable in the VS Code extension. The Task tool in the VS Code extension only recognizes hardcoded built-in subagent types and rejects custom agent names.

Steps to Reproduce

1. Create a custom subagent at .claude/agents/tdd-planner.md with valid YAML frontmatter:

---
name: tdd-planner
description: Use this agent for planning features using TDD methodology.
tools: Read, Grep, Glob, Bash, WebFetch, WebSearch, AskUserQuestion, TodoWrite
disallowedTools: Write, Edit, NotebookEdit, Task
---

You are a TDD Planning Agent...

1. In the CLI (terminal): Run claude from the project directory and ask "Use the tdd-planner to plan a feature"

• Result: Agent is detected, spawned, and executes correctly following the custom system prompt

1. In the VS Code extension: Ask the same question

• Result: The Task tool attempts subagent_type: "tdd-planner" and fails with:

``
Agent type 'tdd-planner' not found. Available agents: Bash, general-purpose, statusline-setup, Explore, Plan, claude-code-guide
``

• There is also no automatic delegation based on the agent's description field

Expected Behavior

Custom subagents in .claude/agents/ should be available in the VS Code extension with the same behavior as the CLI:

• Automatic delegation based on the agent's description field
• Tool restrictions from YAML frontmatter (tools, disallowedTools) enforced
• Custom system prompt used for the agent's behavior

Actual Behavior

The VS Code extension's Task tool only accepts hardcoded built-in subagent types. Custom agents are completely ignored.

Workaround

Spawning a general-purpose agent and instructing it to read the .claude/agents/*.md file and follow its instructions. This does not enforce YAML frontmatter tool restrictions — the agent has full tool access regardless of disallowedTools.

Impact

Teams cannot share project-specific subagents through version control and have them work consistently across both CLI and VS Code extension, which is the documented use case for .claude/agents/ (project-scoped agents checked into the repo).

Environment

• Claude Code VS Code Extension
• Platform: Linux (Ubuntu 24.04)
• Model: Claude Opus 4.6
• Claude Code CLI: Custom agents work correctly in CLI from the same project directory

Related

• #23874 (Task tools disabled in VS Code extension due to isTTY check — possibly same root cause)
• Subagents documentation