Claude Code Mods: Build a Fail-Closed Bash Guard in JS

On October 1, Anthropic shipped mods for Claude Code, and the developer side of the internet has talked about little else since. A mod is a plugin whose JavaScript or TypeScript functions run inside Claude Code. They can draw panes, rewrite prompts, add commands, and, the part this guide cares about, step into a tool call before it runs and refuse it.

Until now, guarding an agent's shell meant a settings hook: a separate script, fed JSON on stdin, that you hoped exited with the right code. A mod is a function with the event in hand. It can wait for a human to answer a question, write to a store that survives the session, and be unit tested from your terminal without logging in or touching the network.