# Tailwind CSS v4.1.18 — Release & Upgrade Intelligence
- **Product:** Tailwind CSS (https://tailwindcss.com)
- **Version:** v4.1.18
- **Released:** Dec 11, 2025
- **Impact Score:** 5/10
- **Breaking Changes:** Not verified
- **Canonical Analysis:** https://changelogs.us/tailwind/updates/v4.1.18/

## Executive Summary (TL;DR)
Tailwind CSS v4.1.18. Source highlight: Ensure validation of `source(…)` happens relative to the file it is in (#19274)

## Bug Fixes
- Ensure validation of `source(…)` happens relative to the file it is in (#19274)
- Include filename and line numbers in CSS parse errors (#19282)
- Skip comments in Ruby files when checking for class names (#19243)
- Skip over arbitrary property utilities with a top-level `!` in the value (#19243)
- Support environment API in `@tailwindcss/vite` (#18970)
- Preserve case of theme keys from JS configs and plugins (#19337)

## Upgrade Command
```bash
npm install --save-exact tailwindcss@4.1.18
```