Claude Code now has a built-in mod that watches your session and flags what you missed, one command to turn on

Since v2.1.287 (October 1), plugins can change Claude Code's UI and behavior through "Mods". The first built-in one is "You should know": a side agent watches the session and points out things you or Claude missed (in first-party sessions with telemetry on).

/plugin enable cc-plugin-you-should-know@builtin

You can write your own. A mod is a small JS file that runs inside the session and hot-reloads on save.

claude --plugin-dir ./my-mod     # run with hot reload
claude plugin validate ./my-mod  # check manifest and module
claude plugin test ./my-mod      # test against the real runtime

The claude.dev getting-started post (October 1) ships three examples: Token Weather, a band showing how full the context is; Blast Radius, which holds rm -rf and force pushes for approval; and Replay Theater, which steps through the last turn's diffs. Source is in anthropics/claude-code-playground.