Custom agents intermittently unavailable in new sessions

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.

Start with the /chat session-start flow and custom-agent discovery from .github/agents, reproducing the intermittent Unknown agent_type result across new sessions. Compare successful and failed sessions, then verify that discovery reliably completes before work is accepted or that an explicit failure and recovery action is shown.