I'd estimate we're near a million LoC (will double check tomorrow, but wouldn't be surprised if it was over that to be honest). Huge monorepo, ~1500 engineers, all sorts of bespoke/custom tooling integrated, fullstack (including embedded code), a mix of languages (predominantly Java & JS/TS though).

In my case the AI is actively detrimental unless I hand hold it with every single file it should look into, lest it dive into weird ancient parts of the codebase that bear no relevance to the task at hand. Letting the latest and "greatest" agents loose is just a recipe for frustration and disaster despite lots of smart people trying their hardest to make these infernal tools be of any use at all. The best I've gotten out of it was some light Vue refactoring, but even then despite AGENTS.md, RULES.md and all the other voodoo people say you should do it's a crapshoot.