This study investigates the impact of stock liquidity on the characteristics of auditor-client contracts, considering the moderating role of input-based audit quality in firms listed on the Tehran Stock Exchange. Stock liquidity is recognized as a critical factor influencing investor decisions and financing costs, potentially affecting the demand for and pricing of audit services. In this research, stock liquidity is treated as the independent variable, while the characteristics of auditor-client contracts, proxied by audit fees, serve as the dependent variable. Additionally, the moderating role of input-based audit quality in this relationship is examined. The study utilizes data from 164 sample firms listed on the Tehran Stock Exchange for 10 years from 1393 to 1402. Data and information were collected through an archival method, and input-based audit quality was measured using two proxies: auditor firm size and auditor specialization. The findings indicate that stock liquidity has a positive and significant relationship with audit fees at the 5% level. Firms with higher stock liquidity, reflecting better financial health, tend to pay higher audit fees. Furthermore, auditor firm size and auditor specialization positively moderate the relationship between stock liquidity and audit fees, leading to increased audit costs. The results provide valuable insights for firms and auditors in making decisions related to audit quality.