Claude Opus 4.5 is significantly stronger than earlier Claude models for handling large-scale refactoring, especially when you treat it as an “engineer with tools” instead of a passive text generator. It can build multi-step plans, maintain consistent architectural decisions, and reason across many files with fewer contradictions. Developers working with multi-language or long-standing codebases will notice that Opus 4.5 is better at understanding how interfaces, services, and abstractions interact across a repository. This makes it well-suited for tasks such as migrating auth flows, redesigning error handling, introducing new logging frameworks, or performing dependency cleanup.
A practical way to use Opus 4.5 for refactoring is to run it in a plan → retrieve → edit → validate loop. First, ask the model to propose a structured plan for the refactor, then allow it to request specific files or run local tools such as “read_file,” “write_file,” or “run_tests.” Opus 4.5’s improved reasoning helps it maintain invariants—like naming conventions or interface contracts—across dozens of changes. This structured loop usually performs more reliably than giving the model a giant prompt containing half the codebase, which often leads to mismatched or outdated edits.
In large monorepos, retrieval becomes essential, and this is where a vector database like Milvus or Zilliz Cloud fits in naturally. Instead of loading hundreds of files into the model’s context, embed the repository into the vector database and expose a “search_codebase” tool. Opus 4.5 can then fetch only the most relevant snippets on demand. This reduces token usage while improving the consistency of the edits, because the model always operates on updated and semantically relevant context. With this pattern, Opus 4.5 becomes a very capable partner for systematic refactoring, even in complex, long-lived codebases.