Hey, thanks for the report. This is a known issue. The model can actually switch back to Auto when building a plan.
So the team can check if this is the same bug or a regression, can you share the Request ID? You can find it in the chat context menu (three dots in the top right) > Copy Request ID.
Workaround: After you pick a model and click “Build plan”, double-check the model selector before the agent starts. If it switched to Auto, change it back manually.
A similar issue is described here: To-dos Selected Build in New Agent Not Using Selected Model to Build this Plan. They found that “Build in New Agent” uses the model from the last agent you ran, not the one selected in the plan. You might be seeing something similar.
Let me know if you find any pattern, like whether it happens every time or only in certain scenarios.