What probably matters to youSection of the release
What
Plugins gain a new function hook, register_agent (alongside the existing hooks for registering commands and tools), letting a plugin define a custom agent.
The plugin SDK exposes an agent.register call that takes a name, description, and prompt (plus other fields), validating that the name only contains letters, digits, underscores, or hyphens and is no longer than 64 characters.
Agents registered this way at runtime are merged into the session's list of available agents, tagged as coming from a plugin (source: "plugin", registeredAtRunTime: true).
The session/engine layer gains a refreshAgents method, mirroring the existing refreshCommands, to re-read custom agent definitions on demand.
Why
This lets plugins extend Claude Code with their own specialized agents without requiring a restart, the same way plugins can already register custom commands and tools.
