This study investigated the development of syntactic complexity in second language (L2) learner essays, focusing on how key syntactic features evolve as L2 learners practice writing through the use of an automated written corrective feedback (AWCF) tool. Using a corpus of essays written across three writing sessions, five indices of syntactic complexity were analyzed: mean length of T-unit (MLT), mean length of clause (MLC), dependent clauses per clause (DC/C), coordinate phrases per clause (CP/C), and complex nominals per clause (CN/C). Linear mixed-effects models were employed, with Session as a fixed effect and participant as a random effect. Results revealed significant growth in MLT, MLC, and CP/C, indicating learners' increasing use of longer and more coordinated syntactic structures. DC/C showed marginal gains, while CN/C remained unchanged. These findings suggest that while some dimensions of syntactic complexity are responsive to short-term writing practice and corrective feedback, others may require extended instruction and developmental time. The study also discusses the potential role of AWCF, specifically from Grammarly, in facilitating syntactic development. Implications for pedagogy and future research directions are explored.