Opus 5 is the default now, and subagents can nest three deep

Claude Code 2.1.219 shipped three changes worth pulling out of the changelog.

Opus 5 (claude-opus-5) is now the default Opus model, with a 1M context window. The context number is the part that matters for how I actually use it. My always-on session runs for days at a time, and the thing that used to end a session wasn't the task getting hard — it was the context filling up with tool output from work that was already finished.

Fast mode now applies to Opus 5 and Opus 4.8. Worth being precise about what this is, because the name invites the wrong assumption: fast mode is still Opus, with faster output. It isn't a silent downgrade to a smaller model. Toggle it with /fast.

Subagents can spawn nested subagents, up to depth 3 by default. This is the one that changes my setup. I run a principal-agent pattern — one always-on session on a Mac mini in Chicago that dispatches per-product subagents for magerblog, beatbrain, prxps, loooom, and kotsu. Until now that tree was two levels: the principal delegates once, and whatever it delegated to had to finish the job alone. A subagent that needed a code review or a scraper check had to do it inline.

The honest tradeoff: every level of nesting is another process with its own cold-start context, re-deriving things the parent already knew. Depth 3 is a budget, not a target. The failure mode isn't running out of levels — it's a tree where the leaves are solving the wrong problem confidently, and the root has no way to tell.

It lines up with the unhobbling shift from earlier this week. The model gets trusted with more judgment; the harness gets room to build deeper structures. Both bets are that the thing at the bottom of the tree can read the room.