Web search is super important for frameworks that are not (sufficiently?) in the training data. o3 often pulls info from Swift forums to find and fix obscure Swift concurrency issues for me.
In my experience none of the frontier models I tried (o3, Opus 4, Gemini 2.5 Pro) was able to solve Swift concurrency issues, with or without web search. At least not sufficiently for Swift 6 language mode. They don’t seem to have a mental model of the whole concept and how things (actors, isolation, Tasks) need to play together.