Stella did not the ability to read minds to know what Cyneah was planning. She had seen the look on the latter's face countless times in the past— whether it was reflected on Caelan's or the mirror she was looking at.
For this reason, Stella was aware that Cyneah planned to leave the security of the Tower of Conclave.
Others might automatically think Cyneah's plan was to use herself as bait to save Kyran. After all, she already heard from Stella that the enemy boss had ordered her capture long before the empire's situation escalated. Since Cyneah cared a lot about Kyran she would definitely not hesitate to sacrifice herself if it would mean he would be saved.
However, it was precisely because Cyneah cared a lot about Kyran that Stella was convinced the former would never sacrifice herself.
