# Biome @biomejs/biome@2.6.6 — Release & Upgrade Intelligence
- **Product:** Biome (https://biomejs.dev)
- **Version:** @biomejs/biome@2.6.6
- **Released:** Nov 06, 2026
- **Impact Score:** 6/10
- **Breaking Changes:** Not verified
- **Canonical Analysis:** https://changelogs.us/biome/updates/biomejs-biome-2.6.6/

## Executive Summary (TL;DR)
Biome CLI v2.6.6 introduces a large suite of new nursery rules for JavaScript, TypeScript, CSS, Svelte, and package.json configurations, alongside significant linting performance enhancements. It also fixes several parsing bugs and a rule-group configuration overflow issue.

## New Features
- Added nursery rule noArrayMethodsOnArguments to report array method calls on the arguments object.
- Added nursery rule noConstantComparisons to catch impossible or always-true conditional statements.
- Added nursery rule noErasingOperation to disallow multiplying, dividing, or bitwise-ANDing with 0 in ways that have no effect.
- Added nursery rule noNegatedArrayPredicate to suggest moving negations into Array#some() and Array#every() callbacks.
- Added nursery rule noNumberArgumentOutOfRange to check invalid number literal arguments in toString(), toFixed(), toExponential(), and toPrecision().
- Added nursery rule noSharedBranchCode to report duplicated code blocks at the start or end of if/else branches.
- Added nursery rule noSubtractionComparison to recommend direct comparison instead of subtracting and checking against 0.
- Added nursery rule noSvelteIgnoredUnsubscribe to prevent memory leaks from unstopped Svelte store subscriptions.
- Added nursery rule noUninvokedArrayCallback to report array methods called on arrays with empty slots.
- Added nursery rule useNextScriptForGoogleAnalytics to enforce Next.js third-party Google Analytics/Tag Manager components.
- Added nursery rule usePackageJsonCpu to mandate a 'cpu' field in package.json files.
- Added nursery rule usePackageJsonEngines to mandate an 'engines' field in package.json files.

## Improvements
- Improved general linting performance by skipping syntax nodes that no enabled rule checks.
- Improved CSS linting performance by reducing memory allocations for selector comparison rules.
- Added MDX ESM blocks, expressions, and JSX tags parsing support to the markdown parser.

## Bug Fixes
- Fixed an issue where configuring rules in groups with more than 256 rules enabled or disabled the wrong rule.
- Fixed useGenericFontNames incorrectly reporting font-family inside @font-palette-values.
- Fixed useExhaustiveSwitchCases to report non-exhaustive switches over values read from for...of loops and array element properties.
- Fixed SCSS negative interpolation list items being incorrectly parsed as a subtraction operator.

## Upgrade Command
```bash
npm install --save-dev --save-exact @biomejs/biome@2.6.6
```