Anthropic published `mods/`, the source of Claude Code's built-in plugins, and with it a second plugin architecture

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On 2026-09-09 Anthropic added a `mods/` folder to the public anthropics/claude-code repo containing the full source of three plugins compiled into the binary: `sec-default`, `diff` and `telemetry`. A mod is a plugin whose behaviour is a hooks module rather than a `hooks.json` of shell commands: one `register(on, options)` entry registering functions shaped `($, e, next)` over the engine's own events. The folder had 52 commits by 2026-09-13 and none of this appears in the CHANGELOG, so the repo is ahead of the release notes.