He was praised for his quick thinking but criticized by historians for his being overly attentive to the emperors' desires.
Yang Su made Feng his assistant, and they constructed a magnificent palace, at much human and material cost.
Yang Su was fearful that Emperor Wen would punish him, but Feng correctly predicted that when Emperor Wen's wife Empress Dugu arrived at the palace, Yang Su would not only not receive punishment, but would be rewarded.
As Yu was unfamiliar with civil service, Feng effectively handled the civil service matters, and the edicts that he drafted were calculated to please Emperor Yang; he would also block the submissions made by officials who had different opinions than Emperor Yang.
He was therefore blamed by the Old Book of Tang for Emperor Yang's increasing favors for Yu and the deteriorating state of Sui governance.
Yuwen Huaji subsequently abandoned Jiangdu, leading the elite Xiaoguo Army (驍果) back north.
He made Feng the head of the legislative bureau (內史令, Neishi Ling), a position that Feng continued to hold after Yuwen poisoned Yang Hao and declared himself the emperor of a new Xu state at Liaocheng (聊城, in modern Liaocheng, Shandong).
In 621, Dou came to Wang's aid, and Emperor Gaozu, initially believing that Li Shimin could not prevail under those circumstances, secretly ordered retreat.
Instead of retreating, Li Shimin sent Feng back to Chang'an to analyze to Emperor Gaozu that Wang was in desperate straits, but that if allowed to survive, could regroup and become a serious threat later.
Pleased, Emperor Gaozu praised Feng, comparing his advice to that of the Jin dynasty official Zhang Hua, whose advice had permitted Emperor Wu of Jin to destroy Eastern Wu.
For his contributions, Feng was created the Duke of Pingyuan and continued to serve as Li Shimin's assistant.
They soon came in conflict, however, as Feng would often agree with Xiao on important matters, and then change his recommendations once they were before Emperor Taizong.
In 627, Feng became ill while handling official matters at the executive bureau, and Emperor Taizong visited him personally, and then used an imperial wagon to take him back to his mansion.