A report on Li Chengqian, Xiao Yu and Zhangsun Wuji

By 626, Li Shimin was fearful that he would be killed by Li Jiancheng, and his staff members Fang Xuanling, Du Ruhui, and Zhangsun Wuji were repeatedly encouraging Li Shimin to attack Li Jiancheng and Li Yuanji first—while Wei Zheng was encouraging Li Jiancheng to attack Li Shimin first.

Emperor Gaozu, in response, issued summonses to Li Jiancheng and Li Yuanji for the next morning, convening the senior officials Pei Ji, Xiao Yu, and Chen Shuda to examine Li Shimin's accusations.

He made his wife Princess Zhangsun the empress, and their oldest son Li Chengqian the crown prince.

His older brother Li Chengqian was crown prince, but Li Tai and his associates had design on that position, eventually pushing Li Chengqian to plot treason in 643.

Emperor Taizong, in shock, convened the senior officials Zhangsun Wuji (Empress Zhangsun's brother), Fang Xuanling, Xiao Yu, and Li Shiji, as well as officials from the supreme court, the legislative bureau, and the examination bureau, to investigate, and the extent of the plot was revealed.

Later in 626, when Emperor Taizong personally ranked the contributions of the generals and officials in order to grant them fiefs, Emperor Taizong ranked five of them — Fang, Zhangsun Wuji, Du Ruhui, Yuchi Gong, and Hou Junji to be contributors of the highest grade, and Fang was created the Duke of Han.

In 630, when Emperor Taizong began to have his crown prince Li Chengqian formally rule on a number of matters of state, he commissioned Fang and Li Gang (李綱) to sit with the Crown Prince and assist him in making the decisions.

Later in 643, when Li Chengqian, fearing that Emperor Taizong would depose him in favor of his more favored brother Li Tai the Prince of Wei, was revealed to have plotted with Hou Junji to overthrow Emperor Taizong, Emperor Taizong had Zhangsun, Fang, Xiao Yu, Li Shiji, along with the responsible officials at the legislative and examination bureaus and the supreme court, investigate the matter.

Emperor Gaozong of Tang (21 July 628 – 27 December 683 ), personal name Li Zhi, was the third emperor of the Tang dynasty in China, ruling from 649 to 683; after January 665, he handed power over the empire to his second wife Empress Wu (the future Wu Zetian), and her decrees were carried out with greater force than the decrees of Emperor Gaozong's. Emperor Gaozong was the youngest son of Emperor Taizong and Empress Zhangsun; his elder brothers were Li Chengqian and Li Tai.

The powerful chancellor Zhangsun Wuji – Empress Zhangsun's brother—suggested that he make Li Zhi crown prince, a possibility that Li Tai was apprehensive about.

He made Zhangsun and two other senior chancellors, Fang Xuanling and Xiao Yu, senior advisors to Li Zhi, and made another chancellor, Li Shiji, the head of Li Zhi's household.

Soon, however, with many objections to the system, the strongest of which came from Zhangsun Wuji, Emperor Taizong cancelled the scheme, although Li Shiji's title remained Duke of Ying.

Subsequently, in summer 643, when Li Chengqian the Crown Prince, was revealed to have plotted to overthrow Emperor Taizong, as he was fearful that Emperor Taizong would replace him with his brother Li Tai the Prince of Wei, Emperor Taizong had Li Shiji, along with other key officials Zhangsun Wuji, Fang Xuanling, Xiao Yu, as well as officials in charge of the supreme court, the legislative bureau, and the examination bureau, investigate.

In 643, he was implicated in a plot by the crown prince, Li Chengqian, to overthrow Emperor Taizong, and was executed.

Hou, along with Li Shimin's brother-in-law Zhangsun Wuji, Zhangsun's uncle Gao Shilian, and the general Yuchi Gong, advised Li Shimin to act first and ambush Li Jiancheng and another brother who supported Li Jiancheng, Li Yuanji the Prince of Qi.

However, Li Chengqian's plot was betrayed by his guard Gegan Chengji (紇干承基), and after an investigation ordered by Emperor Taizong and conducted by Zhangsun Wuji, Fang Xuanling, Xiao Yu, Li Shiji, as well as responsible officials from the supreme court and the legislative and examination bureaus of government, Li Chengqian was removed.