Hooks

Purpose

Enforce a rule deterministically rather than asking the model to remember it. A hook runs every time, regardless of the model's attention, its context, or its mood — which is exactly what a rule requires.

When to Use

• A rule must hold every time: formatting, a forbidden command, a required check.
• The agent keeps forgetting an instruction despite it being in the instructions.
• Automating a step that should happen around every edit or command.
• Blocking a dangerous action before it executes.

Capabilities

• Lifecycle events: before and after a tool call, on session start, on stop.
• Deterministic enforcement: formatting, linting, blocking.
• Injecting context at the right moment.
• Notification and logging.

Inputs

• The rule and when it must apply.
• The event that should trigger it.
• What should happen when it fails: block, warn, or fix.

Outputs

• Hook configuration that runs reliably.
• Rules enforced without depending on the model's memory.
• A fast hook — one that does not make the agent unpleasant to use.

Workflow

1. Identify the rule that keeps being broken — If an instruction is in the instruction file and is still violated, it is a hook, not an instruction. Instructions are guidance; hooks are enforcement.
2. Choose the event — Before a tool call to block or validate; after one to format or verify; on session start to inject context.
3. Make it fast — A hook runs on every matching event. Two seconds on every file edit is a hostile environment. Under 500ms, or run it asynchronously.
4. Fail informatively — A hook that blocks must say why and what to do instead. A silent block is maddening and the model cannot recover from it.
5. Prefer fixing over blocking — A hook that formats the file is better than one that rejects the edit and demands the model format it.
6. Test the hook itself — A broken hook blocks everything, and the failure mode is confusing.

Best Practices

• A rule the model must remember is a rule the model will eventually forget. If it matters, enforce it in a hook.
• Do not put slow operations in a synchronous hook. A full test suite on every edit is a way to make the agent unusable.
• Blocking hooks must explain themselves. exit 2 with an explanatory message on stderr is what lets the model correct course.
• Match narrowly. A hook that fires on every tool call, including reads, wastes time on operations it does not care about.
• Auto-formatting after an edit is the single most valuable hook: it removes an entire class of diff noise and lint failures for free.
• Guard against the destructive command (rm -rf, a force push, a production deploy) in a pre-tool hook. This is cheap insurance.

Examples

Format on write — the highest-value hook:

{
  "hooks": {
    "PostToolUse": [
      {
        "matcher": "Edit|Write",
        "hooks": [
          {
            "type": "command",
            "command": "if echo \"$CLAUDE_FILE_PATHS\" | grep -qE '\\.(ts|tsx)$'; then pnpm prettier --write $CLAUDE_FILE_PATHS 2>/dev/null; fi"
          }
        ]
      }
    ]
  }
}

Every file the agent writes is formatted. The model never has to think about it, the diff is always clean, and the lint step never fails on formatting.

Blocking a destructive command, with an explanation the model can act on:

#!/usr/bin/env bash
# PreToolUse hook on Bash. Exit 2 blocks the call; stderr is shown to the model.
set -euo pipefail

command=$(jq -r '.tool_input.command' <<<"$(cat)")

if [[ "$command" =~ (^|[[:space:]])rm[[:space:]]+(-[a-zA-Z]*f[a-zA-Z]*[[:space:]]+)*/ ]]; then
  echo "Blocked: 'rm -rf' targeting an absolute path is not permitted." >&2
  echo "If you need to remove build artifacts, use 'pnpm clean'." >&2
  exit 2
fi

if [[ "$command" =~ git[[:space:]]+push.*--force([[:space:]]|$) ]]; then
  echo "Blocked: force-push is not permitted on this repository." >&2
  echo "Use 'git push --force-with-lease' if you must rewrite a branch you own." >&2
  exit 2
fi

exit 0

The model receives the message, understands why it was blocked, and uses the suggested alternative — rather than retrying the same command.

Notes

• Exit code 2 from a PreToolUse hook blocks the tool call and returns stderr to the model. Any other non-zero code is treated as a hook error, which is a different and less useful outcome.
• Hooks run with your credentials and permissions. A hook is arbitrary code executing on every matching event — review them as carefully as any other code with that reach.
• The instinct to add many hooks should be resisted. Each one is latency on every operation. Add the ones that prevent real, observed failures.