You Should Know: The Claude Code Mod That Reads Over Your Shoulder

There is a clip going around of a terminal with one line in it: /plugin enable cc-plugin-you-should-know@builtin. Four thousand comments asking what it does. Here is the answer, plus the parts the clip leaves out.

The short version: it is an official Anthropic mod, it ships inside Claude Code, it is off until you turn it on, and if you run multi-hour agentic sessions it is worth the thirty seconds.

Claude Code 2.1.287, released October 1, 2026, added two things at once: Claude Mods, a plugin tier that is allowed to change deeper behaviour than a skill or a hook, and You Should Know, the first built-in mod that uses it.

You Should Know spins up a side agent that observes the main session's output. It is not editing your code and it is not in the loop on tool calls. It reads what Claude produced and, when it finds something you are likely to scroll past, it posts a short note above your prompt. That is the whole interface: a line of text where you would normally type.

The example Anthropic used when announcing it is the right one. Claude adds a retry to a payment call. Reasonable in isolation. In a session where the call is not idempotent, that retry is a customer getting charged twice, and it is buried at the bottom of a wall of green diff. The side agent's job is to say: you should know this happened.

Three corrections to what is circulating. It is a mod, not a marketplace plugin — the @builtin suffix means it ships with the CLI, nothing is downloaded, nothing is third-party. It is opt-in and stays enabled across sessions once you turn it on. And the changelog notes it runs for first-party sessions with telemetry on, so if you have set DISABLE_TELEMETRY it is unlikely to fire.

Update first. The mod does not exist below 2.1.287, and the error you get on an older build — Plugin "cc-plugin-you-should-know@builtin" is not installed in this project — reads like a typo in the name. It is not. The name is correct; the version is old. Run claude update, then at the prompt:

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

To turn it off, swap enable for disable, or open /plugin, go to Installed, and find it under Built-in. The other built-ins listed there — cc-plugin-diff, cc-plugin-agents-md, cc-plugin-telemetry — are the same mechanism. Claude Code has been running on mods for a while; this is just the first one that was announced.

Short sessions do not need it. If you are asking for a function and reading the diff, you are the side agent.

It earns its keep on the sessions where you are not reading the diff: an overnight migration, a multi-repo refactor, a build you kicked off in auto mode and checked on an hour later. Those are the sessions where the output outruns attention, and attention is the thing that was catching the double charge. Handing that to a second agent is the same move as handing the typing to the first one.

The open question is signal-to-noise. A watcher that flags everything is a watcher you mute in a day. We have it running on two long-lived client builds and will report back on whether the notes stay worth reading. The honest early read: it is quiet, and when it speaks it has so far been right.

You Should Know is small. Mods are not. Until this release, extending Claude Code meant skills (instructions), hooks (shell commands at lifecycle points), and MCP servers (tools). None of those could add a pane, take over a slash command, or sit beside the session and watch it. Mods can, and the documentation now ships a plugin-authoring skill so you can write one.

That is the part to pay attention to. A side agent that reads output for payment retries is a demo. A side agent that reads output for your team's rules — the migration that must never run without a backup, the env var that must never be logged, the API that must never be called twice — is a product. The first one is built. The second one is now a weekend.