Calling custom agent via prompt ignores configured model in agent.md

[kevinhagenaars]

Describe the bug

When defining a custom agent and setting the model property in it's agent.md file, the agent is called with a completely different model than defined or selected via the /model command in the CLI. It looks like the model that is passed down to the agent when called interactively via a instructive prompt is gpt-4.1 and sometime gpt-4o-mini.

If you call the agent directly from the command prompt via the command `copilot --agent=TestAgent --prompt="Hi Agent!"` it works fine and it uses the model that is defined in the markdown file as seen below.

Affected version

GitHub Copilot CLI 1.0.35.

Steps to reproduce the behavior

1. Create an custom agent with the model property set to a model (e.g. gpt-5.3-codex) and instructions to expose it's model that is used.
2. Save the xxx.agent.md in your system-wide .copilot\agents folder
3. Run copilot in the terminal
4. Type /model and select a different model than the one defined in the agents markdown
5. Use a prompt to call the which you created in step 1
6. Check it's output in the CLI

Expected behavior

The agent that is called interactively via a prompt, should use and run with the model defined in the agent markdown.

Additional context

• OS: Windows 11 Enterprise
• PowerShell 7.6.1
• Visual Studio Code 1.117.0