What a Claude Code mod can do with your permissions

What can a Claude Code mod actually do on my machine, and what should I review before I allow one? A mod runs inside Claude Code, without a sandbox, using the permissions of the user who installed it. Review the events it handles and the API calls it makes in claude plugin validate, then read those lines in the source before deciding whether to load it.

Start from the code that will run

Claude Mods arrived in the October 1, 2026 changelog entry for Claude Code 2.1.287. A mod is a plugin component made from JavaScript or TypeScript event handlers. Claude Code invokes a handler when its named event occurs; the handler can observe the event, change it, or answer it instead of passing it onward.

The permission boundary is the part to keep in view while reading. The mod runs as the user. Its calls can read and write files the user can access, start processes, and make network requests. It can also read environment variables and settings, which may contain credentials. Depending on its hooks, it may see or alter prompts and tool calls, change conversation rows before they are stored, or approve a tool call before a permission prompt appears. A mod can use the user’s plan or API key for its own model requests.

The documentation’s inspection steps begin with obtaining the plugin files and running claude plugin validate against the directory. Before adding the plugin, our suggested practice, not a documented feature, is to export or print the source files and read them on paper or beside the terminal on a second screen. The point is to follow each reported capability into the exact code that uses it, rather than infer behavior from a marketplace description.

Validation does not execute the mod. Its hooks: entries name registered events, and its calls: entries name mods API methods. Those lines describe what the static analysis can read from the hooks module. They are a map for source review, not a promise that the code is harmless or that every behavior in the surrounding plugin has been summarized.

Read the hook names as capabilities

An event name tells you what moment the mod can enter. tool.call runs as Claude is about to use a tool; its handler can let the call continue, refuse it, or provide a result. tool.check participates when Claude Code decides whether a tool call may run. Its decision can be allow, ask, or deny, subject to the precedence rules documented for settings and managed policy.

Prompt events deserve their own source pass. prompt.submit sees a submitted prompt and can pass changed text or context onward, or stop it. prompt.compose, prompt.section, and prompt.context concern the material Claude Code assembles for the model. session.append can rewrite a conversation row before it is stored. Read the corresponding handler and any helper it calls to understand which fields it observes and returns.

Other hooks mark session, turn, command, and interface activity: session.start, session.send, session.receive, turn.step, command.run, and ui.render are examples from the reference. They are not an exhaustive list for a particular mod; use its validation output and the reference page for that build.

The event handler receives the mods API as $. The API call list narrows down what it can do outside its own module. $.fs.read and $.fs.write are file access; $.process.run and $.process.spawn start programs; $.http.fetch makes HTTP requests. $.env.get and $.settings.read expose environment and settings values, while $.env.set changes an environment variable for Claude Code and commands or MCP servers started afterward.

Trace the data flow for $.model.complete, which uses the user’s plan or API key; $.prompt.submit, which starts a prompt; $.session.send, which messages another session or subagent; and $.mcp.call, which invokes a connected server tool under session permission rules. The reference treats each API method as an event too, so a policy mod can observe or refuse calls from later mods.

Follow each reported call into its source

Our suggested practice, not a documented feature, is to keep the validation output beside the source while reviewing. Use a small checklist tied to the names you actually see.

• For tool.call, inspect the handler’s filters, branches, and return paths. Does it rewrite tool arguments, return a result without calling next, or let the ordinary permission check happen?
• For tool.check, follow the decision logic and its inputs. The code may approve a call that would otherwise prompt, so understand when it returns a different decision.
• For prompt.submit or other prompt hooks, trace every changed text or context field and any condition that drops or replaces a prompt.
• For session.append, see which conversation rows are selected and what replacement content is returned.
• For $.fs, $.process, and $.http, inspect the paths, command arguments, destinations, and data passed into each operation. Relative file paths resolve from the session working directory, but the API has the user’s permissions.
• For $.env, $.settings, $.session, and $.mcp, identify which values or messages are read, changed, or forwarded, and where the data goes next.
• For $.model.complete, $.prompt.submit, and $.session.send, trace what content leaves the current step, who receives it, and whether usage or another session is involved.

Review the rest of the plugin package too: the hooks and calls lines do not describe its other components. Track which source you inspected; keeping a copy or recording a source revision is our suggested practice, not a documented feature.

Organization policy changes who can load one

For teams deploying managed settings, allowManagedModsOnly is an option on Claude Code’s built-in guard. When enabled, user-provided mods do not load, while mods that count as the organization’s can still load. The admin page describes what qualifies as managed and how the guard handles user-installed mods, mods loaded from --plugin-dir, and mods written during a session. This is an organization setting, not a source-review substitute.

disableSideloadFlags addresses a different path. It rejects --plugin-dir and --plugin-url at startup and also keeps session-created mods from loading. The setting also rejects --agents and --mcp-config, so administrators should read its broader scope before deploying it. Marketplace restrictions can work with it when the intent is to permit plugins only from approved marketplaces.

These controls determine who can introduce a mod and which loading paths remain available; they do not add a sandbox. The docs say user mods can still access files, processes, and the network with the user’s permissions. Translate organization policy into an individual source review accordingly.

Try the change away from the main checkout

A separate clone or branch trial is our suggested practice, not a documented feature. Keep the main repository set aside, load the candidate in the disposable copy, and try the workflows the mod claims to affect. It does not change the mod’s permissions or isolate its process from the user’s account.

The docs describe claude plugin test for exercising hooks, with calls stubbed as needed. This checks programmed behavior; it does not prove that a mod can only do what a test covers. Source review and a broader repository trial remain separate steps, and the trial is your team’s procedure rather than a Claude Code security feature.

If this is part of a larger repo policy, review controls such as CODEOWNERS or CI as our suggested practice, not a documented Mods feature. The repo hijack explainer covers a separate way repository content can affect Claude Code. The defaults manual is relevant when checking how Claude Code’s approval settings work. Neither changes the permission boundary described here.

A codebase to practise on

If you want a codebase to practise these habits on, the Arcade kit page is a games-showcase template built with React Native, Expo, and Hono. It is listed at $99, and its free Arcade demo can be opened before choosing the kit. It is a separate codebase for practising source review; it does not relate to Claude Code mods. The SaaS Dashboard kit and Fitness kit are other options. Trying any of them on a separate branch or clone is our suggested practice, not a feature of the templates.

FAQ

Does validation run a mod?

No. The overview says claude plugin validate lists hooks and API calls without running the mod.

What does an event name tell me?

It identifies a point where the handler can run. The reference describes the event’s timing and what a handler can return.

Can an administrator prevent user-installed mods from loading?

Yes. The admin page documents allowManagedModsOnly for that purpose and describes disableSideloadFlags for sideload paths.

Does a separate clone sandbox the mod?

No. A separate clone is our suggested practice for trying a candidate. The documentation says the mod still runs with the user’s permissions.

Sources

• Claude Code changelog, fetched 3 Oct 2026
• Claude Code plugins guide, fetched 3 Oct 2026
• Mods overview, fetched 3 Oct 2026
• Create a mod, fetched 3 Oct 2026
• Mods reference, fetched 3 Oct 2026
• Manage mods for your organization, fetched 3 Oct 2026
• Test a mod, fetched 3 Oct 2026
• Troubleshoot a mod, fetched 3 Oct 2026
• React to events with a mod, fetched 3 Oct 2026
• Use the mods API, fetched 3 Oct 2026
• Repo hijack explainer, fetched 3 Oct 2026
• Defaults manual, fetched 3 Oct 2026
• Arcade kit page, fetched 3 Oct 2026