# Dart Sass 2 will turn old warnings into build failures — maintainers publish a readiness test

Dart Sass maintainers are targeting December 2026 for a breaking v2 release that promotes most older deprecation warnings to errors and changes slash parsing, while explicitly deferring @import removal to v3.

Sass projects can test for upcoming Dart Sass 2 build failures now using published fatal-deprecation flags, rather than waiting for the release.

- Status: Active
- Published: 2026-10-12T06:08:25+13:00
- Updated: 2026-10-12T06:08:25+13:00
- Categories: Web Development, Frameworks, Developer Tools
- Tags: build tooling, CSS, Dart Sass, Sass
- Canonical HTML: https://beyondthe.news/dossiers/dart-sass-2-december-2026-fatal-deprecations-slash-division

## What changed

The Sass maintainers published their Dart Sass 2 roadmap, targeting December 2026 rather than announcing a completed release. Most deprecations introduced before Dart Sass 1.105.0 will become compilation errors in v2. Slash will be treated as a CSS-style separator instead of a division operator; numeric division must use math.div() or calc(). Crucially, @import, global-builtin, color-module-compat and legacy if() remain deprecations until Sass 3. The team published --fatal-deprecations=1.79.0,compile-string-relative-url,misplaced-rest,with-private,function-name,adjacent-compounds and equivalent JS API options for early compatibility checks.

## Why it matters

Sass remains part of many frontend build pipelines. A major upgrade that turns tolerated warnings into errors can break CI and production asset builds unexpectedly. Teams can use the maintainer-provided flags and migrator now to identify incompatibilities while keeping Sass 1.x deployed.

## A planned breaking release, not a new feature cycle

The project aims for December 2026, but has not promised an exact date. Most deprecations already emitted by Sass 1.x will become hard failures in Sass 2.

## Important exceptions

@import and related global builtin and color-module compatibility warnings remain until Sass 3. The legacy Sass if() function also remains usable with a warning.

## Slash semantics change

The / operator will follow CSS separator semantics; arithmetic should use math.div() or calc() instead. Existing styles relying on ambiguous slash division deserve tests.

## Run the compatibility gate today

The Sass CLI's fatal-deprecations list and JS API fatalDeprecations option can elevate future errors in current builds. The official Sass migrator can automate common changes.

## Key details

- Target: December 2026, not yet GA.
- Most existing deprecations become errors in 2.0.
- @import removal deferred until Dart Sass 3.
- Slash operator becomes a separator; math.div() is the arithmetic alternative.
- Maintainer-provided fatal-deprecations flags enable readiness checks.

## Builder takeaways

- Add the official fatal-deprecation flags to a CI branch now.
- Use Sass migrator for known patterns and verify generated CSS.
- Do not prematurely remove @import solely because v2 is approaching; plan for v3 separately.
- Pin production Sass versions until build compatibility is verified.

## What to watch

- Confirmed Sass 2.0 release date and final compatibility notes.
- Bundler and framework plugins that pin Sass versions.

## Uncertainties

- December is a target rather than a guaranteed release date.
- New compatibility advisories may appear before the final v2 release.

## Sources

- [The Road to Dart Sass 2](https://sass-lang.com/blog/the-road-to-dart-sass-2/) — Sass maintainers · primary release · 2026-10-07T00:00:00+13:00. Exact exception list, slash semantics and compatibility test flags.
- [Frontend Focus issue 761](https://frontendfoc.us/issues/761) — Frontend Focus · specialist coverage · 2026-10-07T00:00:00+13:00. Independent specialist coverage of planned migration.