---
name: "Claude Code hooks: Prettier, protect-files, Stop prompt, git if-filter"
description: "Use when CLAUDE.md is not enough and a hook must deterministically block or format."
---

# Claude Code hooks: Prettier, protect-files, Stop prompt, git if-filter

## When
Use when Prettier, protect-files, Stop prompt, or git if-filter must always run.

## Stop
Never mix exit 2 and JSON in one hook. Never treat a hook as undoing work already done.

## Steps
1. **Exit 0** — no objection via exit code.
2. `PreToolUse`: does **not** approve the tool; the normal permission flow still applies.
3. `UserPromptSubmit`, `UserPromptExpansion`, `SessionStart`, `PostModelSwitch`: stdout treated as plain text is added to context.
4. **Exit 2** — block the action. Write a reason to stderr. For `PreToolUse` it is fed to Claude as feedback. Some events cannot be blocked (`SessionStart`: stderr shown to the user, execution continues).
5. **Any other exit** — usually a non-blocking error unless stdout is valid JSON that passes schema validation (then JSON alone decides).
6. `"ok": true`: the action proceeds
7. `"ok": false`: what happens depends on the event:
8. `Stop` and `SubagentStop`: the `reason` is fed back to Claude so it keeps working, unless the response also sets `"impossible": true` to mark the condition as one that can never be satisfied, in which case Claude Code allows the stop and the turn ends

## Load
.claude/settings.json hooks. Shell. No send/pay/deploy from the hook itself.