I created a workspace local extension in VS Code that uses the VS Code API to let Claude Code open files in VS Code as tabs and save them (to apply save participants like Prettier in case it is not used via the CLI) and to get diagnostics (like for TypeScript where there is no option to get workspace-wide diagnostics and you have to go file by file). I taught Claude Code to use this extension via a skill file and it works perfectly, much more reliably than its own IDE LSP integration.