Accept plan fails to transition from plan model to default model (Opus 5.5)

[roijvallabb]

Describe the bug

When testing Opus 5.5 in plan mode today I saw that the implementation using GPT 5.6/6 Luna was using unexpected amount of tokens, I also noticed that the reasoning looked more like Anthropic rather than GPT, although when asked the model reported it was using Luna.

I have verified that when transitioning from Plan mode via prompt (to autopilot in this case), the problem occurs. If you select to exit plan mode and prompt manually, and then ask it to implement the plan, it uses Luna as expected. Any prompt after the automatic transition also seems to use default model.

Prompt (problematic behavior)

Prompt (normal behavior)

Session autopilot

Session prompt manually

Affected version

GitHub Copilot CLI 1.0.88

Steps to reproduce the behavior

1. Enter plan mode
2. Simple prompt like 'Setup a C# benchmarking project including creating a git repository. Commit your changes but don't push'
3. Accept plan and build on autopilot
4. Reasoning and AIC usage correlate to plan model

Expected behavior

1. Enter plan mode
2. Simple prompt like 'Setup a C# benchmarking project including creating a git repository. Commit your changes but don't push'
3. Accept plan and build on autopilot
4. Reasoning and AIC usage correlate to default model

Additional context

No response