CodeScene case study: coding agent refactors 300K lines of C in three weeks

CodeScene has published a case study describing how a coding agent refactored a 300,000-line C codebase in three weeks. The codebase comes from an open-source decompilation project of Street Fighter III: 3rd Strike. According to CodeScene, the work produced 2,903 commits across 726 files, modified 252,055 lines of code, raised the codebase's Code Health score from 5.6 to 10.0, and cost roughly $4,000 in tokens. Adam Tornhill, founder of CodeScene and author of Your Code as a Crime Scene, wrote that after 30 years of working on large systems, this was the first time he had seen what he calls large-scale superhuman AI performance.

Two mechanisms supported the effort, according to the case study. The first is a quality signal: the CodeHealth MCP Server gives the agent a deterministic score to optimize against and is used to judge whether a transformation had a positive effect. The second is a correctness safeguard: a replay-trajectory testing framework compares rollback state hashes frame by frame, checking system behavior after every change.

Perhaps the more novel result is what the agents built along the way. Rather than applying a fixed refactoring catalog, they gradually accumulated a refactoring playbook that ended up with 22 recipes and 82 supporting notes. These include common transformations such as Extract Function and Guard Clauses, as well as recipes specific to this codebase. Failed attempts were recorded by the system, forming an auditable evolution log. Engineers involved said the changes were merged into a branch through 54 PRs. The report also notes that outside observers have questioned whether the approach can be used on production code.