What changed
Google added built-in and Markdown-defined subagents with separate context, tools and instructions, plus explicit and automatic routing and parallel execution.
Why it matters
Specialized contexts can reduce main-session clutter and narrow permissions, while parallelism can shorten independent research, review and code-analysis tasks.
What to do
Define a read-only investigator and test reviewer with disjoint tools, run them on independent tasks and inspect their summaries and permission boundaries.
Caveat
Parallel agents consume more quota and can conflict or overwrite work when tasks are not independent or write scopes overlap.