Claude Code ships mods, the same tool behind its /diff

The /diff command and AGENTS.md support in Claude Code both started out as mods, the ClaudeDevs account said on X. As of v2.1.287, anyone can use the same mechanism to change how Claude Code behaves, reshape its interface or swap in features of their own.

At a glance

• Claude Code v2.1.287 adds Claude Mods, which the release notes describe as plugins that may now modify deeper behavior, from how the agent acts to the interface and built-in features.
• A mod is a few lines of TypeScript, written by you or by Claude, packed inside a plugin and installed with the /plugin command in the CLI or the desktop app.
• The showcase built-in mod, You should know, runs a side agent that flags what you or Claude might miss, but it works only in first-party sessions with telemetry on.

If you missed the earlier part of this story: on September 3, Anthropic floated the idea of function hooks in a GitHub issue, and that proposal has now shipped as Claude Mods. Plugins themselves are not new in Claude Code. What the v2.1.287 release notes add is that plugins may now modify deeper behavior.

Anthropic built /diff and AGENTS.md support as mods before opening them up

According to the ClaudeDevs post, a mod can do three things: change how Claude Code behaves, customize the UI, or swap in your own features. Anthropic says it used mods to build the /diff command and AGENTS.md support, and that it hopes you find them as useful and as fun as the team does.

You write a mod in a few lines of TypeScript, or you ask Claude to build it for you. Mods ship inside plugins, so you install them with the /plugin command you already use, in either the CLI or the desktop app. For starting points, Anthropic is adding pre-built sample mods to the Claude Code Playground repo, and it links to a getting started guide on its blog.

The first built-in mod, You should know, runs a second agent alongside yours

Besides the mechanism itself, v2.1.287 ships one mod of its own. You should know runs a side agent that, in Anthropic's words, watches your back and flags things you or Claude might miss. To turn it on, run /plugin enable cc-plugin-you-should-know@builtin, the same plugin command used for everything else.

The changelog adds a condition in parentheses: the mod is for first-party sessions with telemetry on. The whole feature gets a single line in the release notes, so they don't say what the side agent reads, when it speaks up or which model runs it.

v2.1.287 also brings plugin fixes, an agents filter and MCP sign-in prompts

Mods are the headline, but the same release changes plugins in other places too. Marketplace errors now explain in plain words why a marketplace was ignored or refused, and what to do about it. Plugin listings now note when a plugin's dependencies were not installed, and updating a plugin retries an install that did not finish. A plugin reload that overlaps a startup --plugin-url download no longer corrupts the cached archive.

Outside plugins, the agents view gets an n:<text> filter that matches session names and tasks. MCP servers on the 2025-11-25 protocol can now send URL prompts, for example to sign in. If a server stops connecting after the update, the notes say to add "bareElicitationCapability": true to its config. Opus 4.7+ and Fable now default to a 1M context window on Bedrock, Vertex, Foundry and the Claude apps gateway, and setting CLAUDE_CODE_DISABLE_1M_CONTEXT=1 keeps 200K.

A mod is TypeScript code that travels inside a plugin

There are only a few pieces. A mod is a small piece of TypeScript, written by hand or generated by Claude. It never installs on its own. It ships inside a plugin, and the plugin goes through /plugin like any other. Once it is enabled, the mod can change behavior, the interface or a feature, which the release notes sum up as plugins modifying deeper behavior.

The closest everyday comparison is a game mod that comes through the launcher you already use for add-ons. You never patch the game's files by hand. You just switch the mod on. Built-in mods take the same route: You should know is enabled with a plugin name that ends in @builtin.

The announcement says little about how mods work under the hood. Neither the post nor the release notes say which parts of Claude Code a mod can reach, what keeps a mod away from permission prompts, or what happens when two mods change the same feature. In our view, the showcase mod comes with an odd limit: You should know runs only in first-party sessions with telemetry on, which appears to rule out anyone using Bedrock, Vertex or other gateways.

Where the sample mods land

Anthropic says it is adding pre-built sample mods to the Claude Code Playground repo, but it has not listed them or given a date. It also hasn't said whether You should know will ever work with telemetry off or with third-party providers. The things to watch next are the getting started guide on Anthropic's blog and the first community plugins built on mods.