prompt-intercept-pattern

Prompt Intercept Pattern

A Claude Code plugin that demonstrates hook-intercepted commands — slash commands that run code without consuming an API turn.

The pattern

Normal slash commands send their content to Claude as a prompt. This pattern short-circuits that: a UserPromptSubmit hook intercepts the prompt, runs your code, and blocks the API call. Claude never sees it.

Three pieces:

1. Stub command (commands/prompt-intercept-pattern.md) — registers the command in /help with autocomplete. disable-model-invocation: true prevents programmatic invocation. The file body is never sent to Claude.
2. Hook config (hooks/hooks.json) — wires UserPromptSubmit to the bash script.
3. Hook script (hooks/user_prompt_submit.sh) — matches the command, does work, outputs {"decision":"block","reason":"message for user"}.

prompt-intercept-pattern/
├── .claude-plugin/
│   └── plugin.json
├── commands/
│   └── prompt-intercept-pattern.md   # stub for /help
└── hooks/
    ├── hooks.json                    # wires the hook
    └── user_prompt_submit.sh         # does the work

Try it

claude --plugin-dir /path/to/prompt-intercept-pattern

Then type /prompt-intercept-pattern hello world. You'll see "Echoed: hello world" with no API call.

Adapt it

Open hooks/user_prompt_submit.sh and find the Side effects section. Replace the echo logic with your own: clipboard copy, file writes, notifications, whatever. The message variable is displayed to the user.

To rename the command, update three places:

1. The filename and name: field in commands/
2. The case match in user_prompt_submit.sh
3. The args parameter strip in user_prompt_submit.sh

Real Examples

• https://github.com/kylesnowschwartz/claude-bumper-lanes/blob/main/bumper-lanes-plugin/tools/bumper-lanes/internal/hooks/prompt_handler.go
• https://github.com/kylesnowschwartz/SimpleClaude/blob/main/plugins/sc-hooks/hooks/handlers/copy_message_handler.rb

When to use this

Any command where the work is a side effect and sending a prompt to Claude is waste:

• Copy text to clipboard
• Toggle a setting
• Send a notification
• Write to a file
• Hit an external API

Requirements

• Claude Code with plugin support
• jq for JSON handling

License

MIT