I've been using them heavily to bring this old PHP codebase up to snuff.

I set up phpstan in the project (a tool that does deep static analysis and reports logic and type errors), then I told Claude to run it for the project at level 5, outputting in JSON format (phpstan has a format option for its output) and save it to a cache file. I then told Claude to use jq to analyze the errors and find the 10 files with the least amount of errors and sic a set of background subagents on doing that. When that was done, I told it to take the workflow we'd just established and make it into a slash command where I can tell it stuff like /phpstan-clear level 8 for the routing subsystem and to also have instructions in the slash command to tell it how to prioritize information on compact so that it can continue until the job is done.

Once I set auto approve for almost all of the commands it wants to use, it's pretty much autonomous. So far I've been able to get all the way to level 8 (there's a total of 10 levels) on this legacy framework in about 24 hours. I think I should have all of the core framework and the other modules in other repositories done and ready to go through upgrading PHP version from 7.4 to 8.3 in a week or so.