Phrases Expressions of Agreement and Disagreement

I'm not entirely sure about that.

Meaning

I'm hesitant to agree completely.

Translation language

John: This is the best solution.
Alice: <b>I'm not entirely sure about that</b>. Are there any alternatives?
John: Let's explore other possibilities.
Alice: Good idea. <b>I'm not entirely sure about that</b> being the optimal choice.