Switch to using Sonnet 4 (it's available in VS Code Insiders for me at least). I'm not 100% sure but a Github org admin and/or you might need to enable this model in the Github web interface.

Write good base instructions for your agent[0][1] and keep them up to date. Have your agent help you write and critique it.

Start tasks by planning with your agent (e.g. "do not write any code."), and have your agent propose 2-3 ways to implement what you want. Jumping straight into something with a big prompt is hit or miss, especially with increased task complexity. Planning also gives your agent a chance to read and understand the context/files/code involved.

Apologies if I'm giving you info you're already aware of.

[0] https://code.visualstudio.com/docs/copilot/copilot-customiza...

[1] Claude Code `/init`