Intercept and Rewrite Tool Inputs Before Claude Executes Them

The PreToolUse hook doesn't just let you allow or block tool calls — you can also modify the input before Claude executes it. This is handy for enforcing conventions, adding flags Claude might miss, or redirecting file writes to safer locations.

Here's a hook that automatically adds --no-interaction to any artisan command Claude tries to run:

#!/bin/bash
# .claude/hooks/artisan-flags.sh

INPUT=$(cat)
COMMAND=$(echo "$INPUT" | jq -r '.tool_input.command')

if echo "$COMMAND" | grep -q 'artisan'; then
  MODIFIED=$(echo "$COMMAND" | sed 's/artisan /artisan --no-interaction /g')
  jq -n --arg cmd "$MODIFIED" '{
    hookSpecificOutput: {
      hookEventName: "PreToolUse",
      permissionDecision: "allow",
      updatedInput: {
        command: $cmd
      }
    }
  }'
else
  exit 0
fi

Register the hook for Bash tool calls:

{
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "Bash",
        "hooks": [
          {
            "type": "command",
            "command": ".claude/hooks/artisan-flags.sh"
          }
        ]
      }
    ]
  }
}

The updatedInput field in your response replaces the tool input Claude was going to use. You can modify any field — command for Bash, file_path for Write/Edit, url for WebFetch.

You can also add additionalContext alongside updatedInput to tell Claude why you changed the input.

PreToolUse hooks are a clean way to enforce conventions without adding instructions to every prompt.

via Claude Code Docs — Hooks