Custom agents intermittently unavailable in new sessions

/chat のセッション開始フローと .github/agents からのカスタムエージェントの検出から始め、新しいセッション間で断続的に発生する Unknown agent_type の結果を再現します。成功したセッションと失敗したセッションを比較し、その後、作業が受け付けられる前に検出が確実に完了すること、または明示的な失敗と復旧アクションが表示されることを確認します。

Title: Custom agents intermittently unavailable in new sessions — suspected startup race

Description:

Custom agents defined in my repository’s .github/agents/ directory intermittently fail to register when starting a session in the GitHub Copilot desktop app.

In affected sessions, dispatching a custom agent returns Unknown agent_type, even though its definition exists in the repository. Other sessions correctly expose all 28 custom agents. This appears to be a session initialization or agent-discovery problem; a startup race is suspected but not confirmed.

Steps to reproduce:

1. Open a repository containing custom agents under .github/agents/.

2. Start a new session in the Copilot app.

3. Ask the agent to invoke one of those custom agents.

4. Intermittently, dispatch rejects the custom agent with Unknown agent_type. Other sessions load the agents successfully.

Expected behavior:

Custom-agent discovery should complete before the session accepts work requiring those agents. If discovery fails, the app should show an explicit error and recovery action instead of allowing execution to continue with missing capabilities.

Actual behavior and impact:

Sessions can start without the repository’s custom agents being available. Automated work then fails or continues without the intended specialist agents. This makes workflows unreliable and forces me to check agent availability and restart sessions manually.

Environment:

• GitHub Copilot desktop app on macOS

• App version: 1.1.21

• Repository opened in a local worktree

• Intermittent; exact trigger and root cause unknown

Please investigate session-start ordering and custom-agent registration, and provide a clear indication when discovery has failed or is incomplete.