Regression: Agent frontmatter fields model, handoffs, argument-hint now marked as unsupported

[armaansood]

Describe the bug

Agent .agent.md files that were working correctly yesterday now show validation warnings on /agent:

Copilot encountered the following issues:
• D:\CosmosDBLivesite\.github\agents\livesite-developer.agent.md: agent uses unsupported fields: model, handoffs
• D:\CosmosDBLivesite\.github\agents\livesite-investigator.agent.md: agent uses unsupported fields: argument-hint, model, handoffs

Affected version

0.0.399

Steps to reproduce the behavior

Create an agent file with model, handoffs, or argument-hint in frontmatter
- Run /agent
- Observe validation warnings

Expected behavior

These fields should be recognized as valid agent frontmatter:

- model - Specify which model the agent should use
- handoffs - Define transitions to other agents with label, target agent, prompt, and send flag
- argument-hint - Provide input hints for the agent

Actual behavior: Fields are flagged as "unsupported" even though they were working in the previous version.

Additional context

No response

[thinkriver]

Comes cross the same issue after upgrade, it was working ok in 0.0.398.

Can I downgrade back to 0.0.398 without impact my configuration ?