Sass @use with-block cannot override importing stylesheet variables

Answer to the Exact Question

In every released version of Dart Sass and LibSass up to 2026‑10, the @use ... with($var: …) syntax can only target variables that are exported by the imported module. It cannot replace a variable that you have defined in the stylesheet that imports the module. No compiler flag or experimental option currently enables this behaviour.

What the Specification Says

The Sass language specification explicitly limits the with block to module‑level variables. Importer variables are omitted from the override list, and the spec marks the possibility of overriding importer variables as an open design decision. Consequently, all current compilers follow that rule.

Confirmed Facts

• All released Dart Sass versions (including 1.77.x) and LibSass do not allow importer‑variable overrides.
• The compiler error message for an unsupported override is typically:

  Error: $foo is not a variable that can be overridden with a with‑block.
  

• There is no documented compiler flag that changes this behaviour.
• If a future release adds this feature, the spec does not yet define precedence; most likely the module default would win unless the spec is updated.

Practical Verification Steps

1. Check your compiler version:

   sass --version
   

2. Create a module _module.scss that defines $color: blue; and exports it.
3. In your main stylesheet, define $color: red; and then try:

   @use "module" with ($color: green);
   

4. Compile. If you see the error above, the override is not supported. If it compiles, the module value will be green, and your local $color: red; remains unchanged.

Recommendation for Authors

Until the Sass specification clarifies the behaviour or a compiler implements importer‑variable overrides, treat with as a tool only for module variables. If you need to influence a variable that lives in the importing stylesheet, consider passing the value through a mixin or using a @forward with a custom namespace.

Missing Diagnostic Detail

If you are using a pre‑release or custom build of Sass, let us know the exact compiler and version number, as experimental flags could change the outcome.