ACP: session/new response reports incorrect currentModelId

[MattKotsenas]

Describe the bug

When starting a new ACP session, the session/new response's models.currentModelId and configOptions (category: model) both report claude-sonnet-4.6 regardless of the actual model being used for inference.

Affected version

1.0.5

Steps to reproduce the behavior

1. Start copilot --acp --stdio (or via a client that connects over ACP)
2. Send initialize then session/new
3. Observe models.currentModelId in the response - it reports claude-sonnet-4.6
4. Send a session/prompt asking the model to identify itself
5. The model responds that it is claude-opus-4.6 (or whatever the configured model actually is)

Expected behavior

currentModelId should reflect the actual model that will handle prompts. If the model changes after session creation (e.g. via --model flag or internal routing), a config_option_update notification with category: "model" should be sent to inform the client.

Additional context

No response

[dkrusky]

The modelId is reporting the model it selected correctly. What is happening is it is not honoring the --model flag or internal post when using any combination of --acp or --stdio.

If you look at your billing, you will see the chats are always being sent using Claude when in this mode. The bug is not in reporting the incorrect model, but instead it is a bug with model selection.

Expected behavior :

When passing --model or internal routing via acp, respect the chosen selected model. Stop overriding the user model with the Premium billable model.