Less 5 alpha.1: Jess-powered compiler preview

.coderabbit.yaml

@@ -0,0 +1,2 @@
+reviews:
+  max_files: 500

.github/TESTING_PUBLISHING.md

@@ -32,7 +32,7 @@ Dry-run mode will:
 # Check current version
 node -p "require('./packages/less/package.json').version"
 
-# Run dry-run to see what version would be created
+# Run dry-run to see what version would be published
 DRY_RUN=true pnpm run publish
 ```
 
@@ -55,9 +55,9 @@ git checkout alpha
 DRY_RUN=true GITHUB_REF_NAME=alpha pnpm run publish
 
 # This will show:
-# - Version validation (must contain -alpha.)
-# - Master sync check
-# - Version comparison with master
+# - The committed alpha version
+# - Alpha branch validation
+# - A dry-run publish plan without npm mutation
 ```
 
 ### 4. Test Version Override
@@ -127,12 +127,25 @@ git checkout alpha
 DRY_RUN=true GITHUB_REF_NAME=alpha pnpm run publish
 
 # Should show:
-# - Alpha version increment (e.g., 5.0.0-alpha.1 → 5.0.0-alpha.2)
+# - The committed alpha version (for example 5.0.0-alpha.1)
 # - Publishing with 'alpha' tag
 # - Pre-release creation
-# - All alpha validations passing
+# - A release plan only; it does not prove the prerequisite Jess alpha has
+#   been published or that the worktree is clean
 ```
 
+### Test Less 5 Alpha Readiness
+
+```bash
+pnpm run test:alpha
+```
+
+This is the Less 5 alpha release gate. It checks the supported alpha contract
+and prints the unsupported alpha.1 inventory. The broad legacy corpus remains
+available through `pnpm --dir packages/less run test:legacy-node`, but it is
+not expected to be green yet and is intentionally not the alpha.1 publish gate
+until the unsupported buckets are drained.
+
 ### Test Version Validation
 
 ```bash
@@ -147,7 +160,9 @@ DRY_RUN=true GITHUB_REF_NAME=alpha pnpm run publish
 
 Before actually publishing:
 
-- [ ] Run dry-run mode to verify version calculation
+- [ ] Prepare, commit, and push the exact alpha version before publishing
+- [ ] Run `pnpm run test:alpha` to verify the supported Less 5 alpha contract
+- [ ] Run dry-run mode to verify the release plan (not release readiness)
 - [ ] Verify branch restrictions work (try from wrong branch)
 - [ ] Test alpha validations (if testing alpha branch)
 - [ ] Check that version override works (if needed)

.github/workflows/ci.yml

@@ -22,33 +22,33 @@ jobs:
             node: 'lts/*'
           - os: ubuntu-latest
             node: 'lts/-1'
-          - os: ubuntu-latest
-            node: 'lts/-2'
-          - os: ubuntu-latest
-            node: 'lts/-3'
-
-    runs-on: ${{ matrix.os }}
+          - os: ubuntu-latest
+            node: 'lts/-2'
+
+    runs-on: ${{ matrix.os }}
     # This has copy/paste steps and should be refactored using DRY
     steps:
       - uses: actions/checkout@v4
       - name: Install pnpm
         uses: pnpm/action-setup@v4
       - uses: actions/setup-node@v4
         with:
-          node-version: ${{ matrix.node }}
-          cache: 'pnpm'
-      - name: Install dependencies
-        run: pnpm install --frozen-lockfile --ignore-scripts
-      - name: Print put node & npm version
-        run: node --version && pnpm --version
-      - name: Run node tests (ESM + CJS)
+          node-version: ${{ matrix.node }}
+          cache: 'pnpm'
+      - name: Install dependencies
+        run: pnpm install --frozen-lockfile --ignore-scripts
+      - name: Print put node & npm version
+        run: node --version && pnpm --version
+      - name: Run Less v5 alpha support contract
+        run: pnpm run test:alpha
+      - name: Run Less v5 Node module smoke tests
         run: pnpm run test:node
       - name: Run release automation tests
         if: matrix.os == 'ubuntu-latest' && matrix.node == 'lts/*'
         run: pnpm run test:release
 
   copilot-review:
-    name: Request Copilot review
+    name: Request Copilot review
     runs-on: ubuntu-latest
     continue-on-error: true
     if: github.event_name == 'pull_request' && (github.event.action == 'opened' || github.event.action == 'reopened')
@@ -61,7 +61,7 @@ jobs:
         run: |
           gh api \
             --method POST \
-            -H "Accept: application/vnd.github+json" \
-            /repos/${{ github.repository }}/pulls/${{ github.event.pull_request.number }}/requested_reviewers \
-            -f "reviewers[]=copilot-pull-request-reviewer" \
-          || echo "::warning::Could not request Copilot review (the token may lack pull-requests: write access, or Copilot PR reviews may not be enabled for this repository)"
+            -H "Accept: application/vnd.github+json" \
+            /repos/${{ github.repository }}/pulls/${{ github.event.pull_request.number }}/requested_reviewers \
+            -f "reviewers[]=copilot-pull-request-reviewer" \
+          || echo "::warning::Could not request Copilot review (the token may lack pull-requests: write access, or Copilot PR reviews may not be enabled for this repository)"

.github/workflows/create-release-pr.yml

@@ -65,9 +65,9 @@ jobs:
           CURRENT=$(node -p "require('./packages/less/package.json').version")
 
           if [ "$BRANCH" = "alpha" ]; then
-            NPM_VERSION=$(npm view less dist-tags.alpha)
+            NPM_VERSION=$(npm view less dist-tags.alpha 2>/dev/null || echo "")
           else
-            NPM_VERSION=$(npm view less version)
+            NPM_VERSION=$(npm view less version 2>/dev/null || echo "")
           fi
 
           DEFAULT_NEXT=$(node scripts/release-metadata.js next-version "$BRANCH" "$CURRENT" "$NPM_VERSION")
@@ -83,6 +83,11 @@ jobs:
           if [ -n "$EXISTING_TITLE" ]; then
             NEXT=$(node scripts/release-metadata.js parse-title "$BRANCH" "$EXISTING_TITLE")
             RELEASE_BRANCH="$EXISTING_BRANCH"
+            if ! node scripts/release-metadata.js validate "$BRANCH" "$NEXT" "$NPM_VERSION" 2>/dev/null; then
+              echo "Existing release PR title v${NEXT} is no longer publishable; falling back to v${DEFAULT_NEXT}"
+              NEXT="$DEFAULT_NEXT"
+              RELEASE_BRANCH=$(node scripts/release-metadata.js branch "$BRANCH" "$NEXT")
+            fi
           else
             NEXT="$DEFAULT_NEXT"
             RELEASE_BRANCH=$(node scripts/release-metadata.js branch "$BRANCH" "$NEXT")
@@ -121,7 +126,7 @@ jobs:
             git checkout -b "${RELEASE_BRANCH}"
           fi
 
-          node scripts/release-metadata.js sync-package-versions "${NEXT_VERSION}"
+          node scripts/release-metadata.js sync-package-versions "${RELEASE_BASE}" "${NEXT_VERSION}"
 
           git add package.json packages/*/package.json
 
@@ -136,7 +141,7 @@ jobs:
               --json number,title,author \
               --jq '.[] | "- [#\(.number)](https://github.com/${{ github.repository }}/pull/\(.number)) \(.title) (@\(.author.login))"' \
               2>/dev/null || echo "")
-            if [ -n "$PR_LINES" ]; then
+            if [ -n "$PR_LINES" ] && ! grep -q "^### v${NEXT_VERSION}\\b" CHANGELOG.md; then
               TODAY=$(date +%Y-%m-%d)
               {
                 head -1 CHANGELOG.md
@@ -150,30 +155,16 @@ jobs:
                 tail -n +2 CHANGELOG.md
               } > CHANGELOG.tmp && mv CHANGELOG.tmp CHANGELOG.md
               git add CHANGELOG.md
+            elif grep -q "^### v${NEXT_VERSION}\\b" CHANGELOG.md; then
+              echo "CHANGELOG.md already has a v${NEXT_VERSION} section; keeping the curated release notes."
             fi
           fi
 
-          COMMITTED=false
           if git diff --cached --quiet; then
-            echo "No version changes; branch is already at v${NEXT_VERSION}"
+            echo "No manifest changes; creating an explicit release commit for v${NEXT_VERSION}"
+            git commit --allow-empty -m "${TITLE}"
           else
             git commit -m "${TITLE}"
-            COMMITTED=true
-          fi
-
-          # If no new commit was created the release branch has no commits
-          # ahead of master, so pushing it and trying to open a PR would fail
-          # with "no commits between head and base". Instead, just report
-          # whether an existing release PR is open and exit cleanly.
-          if [ "$COMMITTED" = "false" ]; then
-            EXISTING=$(gh pr list --head "${RELEASE_BRANCH}" --base "${RELEASE_BASE}" \
-              --json number --jq '.[0].number' 2>/dev/null || echo "")
-            if [ -n "${EXISTING}" ]; then
-              echo "✅ No new changes; release PR #${EXISTING} already exists"
-            else
-              echo "✅ No version bump needed and no existing release PR; nothing to do"
-            fi
-            exit 0
           fi
 
           # --force-with-lease refuses to overwrite if the remote has advanced
@@ -264,9 +255,9 @@ jobs:
           PREVIOUS_VERSION=$(node -p "require('./packages/less/package.json').version")
           VERSION=$(node .release-scripts/release-metadata.js parse-title "$RELEASE_BASE" "$RELEASE_TITLE")
           if [ "$RELEASE_BASE" = "alpha" ]; then
-            NPM_VERSION=$(npm view less dist-tags.alpha)
+            NPM_VERSION=$(npm view less dist-tags.alpha 2>/dev/null || echo "")
           else
-            NPM_VERSION=$(npm view less version)
+            NPM_VERSION=$(npm view less version 2>/dev/null || echo "")
           fi
           node .release-scripts/release-metadata.js validate-title-sync "$RELEASE_BASE" "$VERSION" "$PREVIOUS_VERSION" "$NPM_VERSION"
           TITLE=$(node .release-scripts/release-metadata.js title "$RELEASE_BASE" "$VERSION")

.github/workflows/publish.yml

@@ -81,8 +81,8 @@ jobs:
           node scripts/release-metadata.js validate "$RELEASE_BASE" "$TITLE_VERSION" "$NPM_VERSION"
           echo "✅ Release title, package.json, and npm version checks agree on v${TITLE_VERSION}"
 
-      - name: Run node tests (ESM + CJS)
-        run: pnpm run test:node
+      - name: Run Less v5 alpha support contract
+        run: pnpm run test:alpha
 
       - name: Build
         run: |
@@ -202,7 +202,15 @@ jobs:
             PRERELEASE="--prerelease"
             BODY="## Alpha Release
 
-          This is an alpha release from the alpha branch.
+          This Less 5 alpha is a Jess-powered compiler rewrite for early testing, not a drop-in Less 4.x replacement yet.
+
+          Highlights:
+          - CSS nesting is preserved by default.
+          - Less-style ampersand joining for BEM-style modifiers and similar selector patterns is still supported.
+          - Nested and extended selector output may use modern selectors such as :is(); selector compatibility remains part of the alpha feedback surface.
+          - Browser compilation is not included in this alpha; a new browser build mechanism will be introduced in a future alpha.
+
+          See CHANGELOG.md for the current supported surface and work-in-progress areas.
 
           ## Installation
 
@@ -229,14 +237,26 @@ jobs:
           \`\`\`"
           fi
 
+          ASSETS=()
+          if [ "$IS_ALPHA" != "true" ]; then
+            for asset in packages/less/dist/less.js packages/less/dist/less.min.js; do
+              if [ -f "$asset" ]; then
+                ASSETS+=("$asset")
+              fi
+            done
+          else
+            echo "Alpha release has no browser build assets; browser compilation will return through a future build mechanism."
+          fi
+
           if gh release view "$TAG" &>/dev/null; then
-            echo "Release $TAG already exists, uploading assets to existing release"
-            gh release upload "$TAG" packages/less/dist/less.js packages/less/dist/less.min.js --clobber
+            echo "Release $TAG already exists"
+            if [ "${#ASSETS[@]}" -gt 0 ]; then
+              gh release upload "$TAG" "${ASSETS[@]}" --clobber
+            fi
           else
             gh release create "$TAG" \
               --title "$TITLE" \
               $PRERELEASE \
               --notes "$BODY" \
-              packages/less/dist/less.js \
-              packages/less/dist/less.min.js
+              "${ASSETS[@]}"
           fi

.gitignore

@@ -17,3 +17,9 @@ npm-debug.log
 .nyc_output
 coverage
 *.lcov
+
+# Build output
+dist
+
+# Claude Code
+.claude/

.husky/post-merge

@@ -3,3 +3,7 @@
 
 # Post-merge hook to preserve alpha versions when merging master into alpha
 node scripts/post-merge-version-fix.js
+
+
+
+

.husky/pre-commit

@@ -1 +1,9 @@
+branch="$(git branch --show-current)"
+
+if [ "$branch" = "alpha" ]; then
+  echo "Skipping pre-commit verification on alpha branch"
+  exit 0
+fi
+
+cd packages/less && npm run typecheck && cd ../..
 pnpm test

CHANGELOG.md

@@ -1,5 +1,90 @@
 ## Change Log
 
+### v5.0.0-alpha.1 (unreleased)
+
+This is the first public alpha for Less 5, a major rewrite of the Less compiler
+on top of [Jess](https://github.com/jesscss/jess). Jess is a new CSS-family
+compiler engine that Less now uses for parsing, evaluation, rendering, imports,
+and the `lessc` command.
+
+This alpha is meant for early testing of the new compiler path. It is not yet a
+drop-in replacement for Less 4.x.
+
+#### Highlights
+
+- Introduces the Jess-powered Less compiler and CLI through the existing
+  `less` package.
+- Keeps `lessc` owned by Less. The separate `jess` package has its own CLI and
+  no longer installs a competing `lessc` binary.
+- Preserves authored CSS nesting by default. Use `collapseNesting: true` or
+  `lessc --collapse-nesting` to emit flattened selector output.
+- Still supports Less-style ampersand joining for BEM-style modifiers and
+  similar selector patterns, such as `.block { &__item { ... } }`.
+- Preserves cascade order when nested rules are collapsed, so declarations
+  after a nested child remain after that child in the generated CSS.
+- May use modern selector output such as `:is()` in generated nested and
+  extended selectors. Selector compatibility remains part of the alpha feedback
+  surface.
+- Adds an alpha readiness test path for the supported Less 5 surface, including
+  package loading, `lessc`, file imports, malformed-input diagnostics, and clean
+  npm consumer installation.
+
+#### Current alpha support
+
+Less 5 alpha.1 currently focuses on core compile behavior: `less.render()`,
+`less.renderFile()`, `lessc`, variables, arithmetic, mixin calls, sibling file
+imports, and nested-rule output.
+
+Less 5 alpha.1 requires Node.js `^20.19.0 || >=22.12.0`, matching its Jess
+runtime dependencies.
+
+The broader Less 4.x compatibility surface is still in progress. Known
+work-in-progress areas include legacy plugin execution, file-manager and
+pre/post-processor hooks, source maps, URL rewriting options, compressed-output
+parity, browser compilation, and the remaining long-tail Less 4 fixture corpus.
+Less 5 alpha.1 does not include browser compilation support; a new browser
+build mechanism will be introduced in a future alpha.
+Unsupported syntax should fail with filename, line, column, and source context
+rather than raw parser offsets.
+
+### v4.6.0 (2026-03-09)
+
+#### Bug Fixes
+
+- [#4414](https://github.com/less/less.js/pull/4414) Fix pre-existing bugs in tree nodes: selector `this` binding, atrule parenting, mixin-call error propagation, container/media functionRegistry guard (@matthew-dean)
+- [#4408](https://github.com/less/less.js/pull/4408) Fix [#4358](https://github.com/less/less.js/issues/4358) Resolve parent selectors in comma-separated pseudo-selector lists (@matthew-dean)
+- [#4407](https://github.com/less/less.js/pull/4407) Fix [#4331](https://github.com/less/less.js/issues/4331) Exclude CSS at-rule keywords from declarationCall parsing (@matthew-dean)
+- [#4389](https://github.com/less/less.js/pull/4389) Fix [#4354](https://github.com/less/less.js/issues/4354) Unknown at-rule expression commas (@puckowski)
+- [#4404](https://github.com/less/less.js/pull/4404) Fix no-prototype-builtins issues in Ruleset and ToCSSVisitor (@matthew-dean)
+- [#4236](https://github.com/less/less.js/pull/4236) Fix import subpath module bug (@nicolo-ribaudo)
+- [#4327](https://github.com/less/less.js/pull/4327) Remove duplicate length check from expression.genCSS() (@nicolo-ribaudo)
+- [#3791](https://github.com/less/less.js/pull/3791) Handle the lack of optional dependencies (@nicolo-ribaudo)
+
+#### Features & Improvements
+
+- [#4413](https://github.com/less/less.js/pull/4413) Add JSDoc type annotations for all tree node files (@matthew-dean)
+- [#4412](https://github.com/less/less.js/pull/4412) Convert prototype-based tree nodes to ES6 classes (@matthew-dean)
+- [#4411](https://github.com/less/less.js/pull/4411) Migrate to native ESM with no build step (@matthew-dean)
+- [#4410](https://github.com/less/less.js/pull/4410) Optimize hot paths and fix benchmark infrastructure (@matthew-dean)
+- [#4409](https://github.com/less/less.js/pull/4409) Code quality cleanup for container queries and related code (@matthew-dean)
+
+#### Deprecation Warnings
+
+- [#4402](https://github.com/less/less.js/pull/4402) Add deprecation warnings for features removed in Less 5.x, container query variable name fix, deprecation notice fix (@matthew-dean, @puckowski)
+
+#### Chores
+
+- [#4406](https://github.com/less/less.js/pull/4406) Add test for number with underscore parsing (@matthew-dean)
+- [#4386](https://github.com/less/less.js/pull/4386) Update README.md copyright (@matthew-dean)
+- [#3782](https://github.com/less/less.js/pull/3782) Remove phantom stuff (@nicolo-ribaudo)
+- [#3702](https://github.com/less/less.js/pull/3702) Replace deprecated String.prototype.substr() (@nicolo-ribaudo)
+- [#4265](https://github.com/less/less.js/pull/4265) Remove redundant return from parsers.blockRuleset() (@nicolo-ribaudo)
+- [#4271](https://github.com/less/less.js/pull/4271) Remove unused parsers.entities.propertyCurly() (@nicolo-ribaudo)
+
+### v4.5.1 (2025-12-28)
+
+_Automated patch release — no user-facing changes._
+
 ### v4.4.2 (2025-08-27)
 
 - [#4357](https://github.com/less/less.js/pull/4357) Migrate Less test data to use valid CSS (@matthew-dean)