diff --git a/.github/workflows/bench-ci.yml b/.github/workflows/bench-ci.yml
new file mode 100644
index 00000000..c36ab2af
--- /dev/null
+++ b/.github/workflows/bench-ci.yml
@@ -0,0 +1,26 @@
+# merget-harness live benchmark CI: the recipe the benchmark VM discovered for
+# this corpus, run on every pull request, on main and on sema's batch branches.
+# The corpus's own workflows are removed on the mirror (docs/SCENARIOS.md "bench").
+name: bench-ci
+on:
+  push:
+    branches:
+      - "main"
+      - "sema/queue/**"
+      - "replay/**"
+  pull_request: {}
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    timeout-minutes: 90
+    steps:
+      - name: "bench:checkout"
+        uses: actions/checkout@v4
+      - name: "bench:toolchain"
+        uses: actions/setup-node@v4
+        with:
+          node-version: "22"
+      - name: "bench:setup"
+        run: if [ -f package-lock.json ] || [ -f npm-shrinkwrap.json ]; then npm ci; else npm install --no-audit --no-fund; fi
+      - name: "bench:test"
+        run: npm test
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
deleted file mode 100644
index 7a9797e0..00000000
--- a/.github/workflows/ci.yml
+++ /dev/null
@@ -1,117 +0,0 @@
-name: ci
-
-on:
-  push:
-    branches:
-      - master
-      - develop
-      - '4.x'
-      - '5.x'
-      - '5.0'
-    paths-ignore:
-      - '*.md'
-  pull_request:
-  workflow_dispatch:
-
-permissions:
-  contents: read
-
-# Cancel in progress workflows
-# in the scenario where we already had a run going for that PR/branch/tag but then triggered a new run
-concurrency:
-  group: "${{ github.workflow }} ✨ ${{ github.event.pull_request.head.label || github.head_ref || github.ref }}"
-  cancel-in-progress: true
-
-jobs:
-  lint:
-    name: Lint
-    runs-on: ubuntu-latest
-    steps:
-      - uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1
-        with:
-          persist-credentials: false
-      - name: Setup Node.js
-        uses: actions/setup-node@395ad3262231945c25e8478fd5baf05154b1d79f # v6.1.0
-        with:
-          node-version: 'lts/*'
-
-      - name: Install dependencies
-        run: npm install --ignore-scripts --include=dev
-
-      - name: Run lint
-        run: npm run lint
-
-  test:
-    strategy:
-      fail-fast: false
-      matrix:
-        os: [ubuntu-latest, windows-latest]
-        node-version: [18, 19, 20, 21, 22, 23, 24, 25]
-        # Node.js release schedule: https://nodejs.org/en/about/releases/
-
-    name: Node.js ${{ matrix.node-version }} - ${{matrix.os}}
-
-    runs-on: ${{ matrix.os }}
-    steps:
-      - uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1
-        with:
-          persist-credentials: false
-
-      - name: Setup Node.js ${{ matrix.node-version }}
-        uses: actions/setup-node@395ad3262231945c25e8478fd5baf05154b1d79f # v6.1.0
-        with:
-          node-version: ${{ matrix.node-version }}
-
-      - name: Configure npm loglevel
-        run: |
-          npm config set loglevel error
-        shell: bash
-
-      - name: Install dependencies
-        run: npm install
-
-      - name: Output Node and NPM versions
-        run: |
-          echo "Node.js version: $(node -v)"
-          echo "NPM version: $(npm -v)"
-
-      - name: Run tests
-        shell: bash
-        run: npm run test-ci
-
-      - name: Upload code coverage
-        uses: actions/upload-artifact@b7c566a772e6b6bfb58ed0dc250532a479d7789f # v6.0.0
-        with:
-          name: coverage-node-${{ matrix.node-version }}-${{ matrix.os }}
-          path: ./coverage/lcov.info
-          retention-days: 1
-
-  coverage:
-    needs: test
-    runs-on: ubuntu-latest
-    permissions:
-      contents: read
-      checks: write
-    steps:
-      - uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1
-        with:
-          persist-credentials: false
-
-      - name: Install lcov
-        shell: bash
-        run: sudo apt-get -y install lcov
-
-      - name: Collect coverage reports
-        uses: actions/download-artifact@37930b1c2abaa49bbe596cd826c3c89aef350131 # v7.0.0
-        with:
-          path: ./coverage
-          pattern: coverage-node-*
-
-      - name: Merge coverage reports
-        shell: bash
-        run: find ./coverage -name lcov.info -exec printf '-a %q\n' {} \; | xargs lcov -o ./lcov.info
-
-      - name: Upload coverage report
-        uses: coverallsapp/github-action@5cbfd81b66ca5d10c19b062c04de0199c215fb6e # v2.3.7
-        with:
-          file: ./lcov.info
diff --git a/.github/workflows/codeql.yml b/.github/workflows/codeql.yml
deleted file mode 100644
index b080ea25..00000000
--- a/.github/workflows/codeql.yml
+++ /dev/null
@@ -1,74 +0,0 @@
-# For most projects, this workflow file will not need changing; you simply need
-# to commit it to your repository.
-#
-# You may wish to alter this file to override the set of languages analyzed,
-# or to provide custom queries or build logic.
-#
-# ******** NOTE ********
-# We have attempted to detect the languages in your repository. Please check
-# the `language` matrix defined below to confirm you have the correct set of
-# supported CodeQL languages.
-#
-name: "CodeQL"
-
-on:
-  push:
-    branches: ["master"]
-  pull_request:
-    # The branches below must be a subset of the branches above
-    branches: ["master"]
-  schedule:
-    - cron: "0 0 * * 1"
-  workflow_dispatch:
-
-permissions:
-  contents: read
-
-jobs:
-  analyze:
-    name: Analyze
-    runs-on: ubuntu-latest
-    permissions:
-      actions: read
-      contents: read
-      security-events: write
-    strategy:
-      fail-fast: false
-      matrix:
-        language: [javascript, actions]
-
-    steps:
-      - name: Checkout repository
-        uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1
-        with:
-          persist-credentials: false
-
-      # Initializes the CodeQL tools for scanning.
-      - name: Initialize CodeQL
-        uses: github/codeql-action/init@5d4e8d1aca955e8d8589aabd499c5cae939e33c7 # v3.29.5
-        with:
-          languages: ${{ matrix.language }}
-          config: |
-            paths-ignore:
-              - test
-          # If you wish to specify custom queries, you can do so here or in a config file.
-          # By default, queries listed here will override any specified in a config file.
-          # Prefix the list here with "+" to use these queries and those in the config file.
-
-      # Autobuild attempts to build any compiled languages  (C/C++, C#, or Java).
-      # If this step fails, then you should remove it and run the build manually (see below)
-      # - name: Autobuild
-      #   uses: github/codeql-action/autobuild@3ab4101902695724f9365a384f86c1074d94e18c # v3.24.7
-
-      # ℹ️ Command-line programs to run using the OS shell.
-      # 📚 See https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#jobsjob_idstepsrun
-
-      #   If the Autobuild fails above, remove it and uncomment the following three lines.
-      #   modify them (or add more) to build your code if your project, please refer to the EXAMPLE below for guidance.
-
-      # - run: |
-      #   echo "Run, Build Application using script"
-      #   ./location_of_script_within_repo/buildscript.sh
-
-      - name: Perform CodeQL Analysis
-        uses: github/codeql-action/analyze@5d4e8d1aca955e8d8589aabd499c5cae939e33c7 # v3.29.5
diff --git a/.github/workflows/legacy.yml b/.github/workflows/legacy.yml
deleted file mode 100644
index e31f55e3..00000000
--- a/.github/workflows/legacy.yml
+++ /dev/null
@@ -1,101 +0,0 @@
-name: legacy
-
-on:
-  push:
-    branches:
-      - master
-      - develop
-      - '4.x'
-      - '5.x'
-      - '5.0'
-    paths-ignore:
-      - '*.md'
-  pull_request:
-    paths-ignore:
-      - '*.md'
-  workflow_dispatch:
-  
-permissions:
-  contents: read
-
-# Cancel in progress workflows
-# in the scenario where we already had a run going for that PR/branch/tag but then triggered a new run
-concurrency:
-  group: "${{ github.workflow }} ✨ ${{ github.event.pull_request.head.label || github.head_ref || github.ref }}"
-  cancel-in-progress: true
-
-jobs:
-  test:
-    strategy:
-      fail-fast: false
-      matrix:
-        os: [ubuntu-latest, windows-latest]
-        node-version: [16, 17]
-        # Node.js release schedule: https://nodejs.org/en/about/releases/
-
-    name: Node.js ${{ matrix.node-version }} - ${{matrix.os}}
-
-    runs-on: ${{ matrix.os }}
-    steps:
-      - uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1
-        with:
-          persist-credentials: false
-
-      - name: Setup Node.js ${{ matrix.node-version }}
-        uses: actions/setup-node@395ad3262231945c25e8478fd5baf05154b1d79f # v6.1.0
-        with:
-          node-version: ${{ matrix.node-version }}
-
-      - name: Configure npm loglevel
-        run: |
-          npm config set loglevel error
-        shell: bash
-
-      - name: Install dependencies
-        run: npm install
-
-      - name: Output Node and NPM versions
-        run: |
-          echo "Node.js version: $(node -v)"
-          echo "NPM version: $(npm -v)"
-
-      - name: Run tests
-        shell: bash
-        run: npm run test-ci
-
-      - name: Upload code coverage
-        uses: actions/upload-artifact@b7c566a772e6b6bfb58ed0dc250532a479d7789f # v6.0.0
-        with:
-          name: coverage-node-${{ matrix.node-version }}-${{ matrix.os }}
-          path: ./coverage/lcov.info
-          retention-days: 1
-
-  coverage:
-    needs: test
-    runs-on: ubuntu-latest
-    permissions:
-      contents: read
-      checks: write
-    steps:
-      - uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1
-        with:
-          persist-credentials: false
-
-      - name: Install lcov
-        shell: bash
-        run: sudo apt-get -y install lcov
-
-      - name: Collect coverage reports
-        uses: actions/download-artifact@37930b1c2abaa49bbe596cd826c3c89aef350131 # v7.0.0
-        with:
-          path: ./coverage
-          pattern: coverage-node-*
-
-      - name: Merge coverage reports
-        shell: bash
-        run: find ./coverage -name lcov.info -exec printf '-a %q\n' {} \; | xargs lcov -o ./lcov.info
-
-      - name: Upload coverage report
-        uses: coverallsapp/github-action@5cbfd81b66ca5d10c19b062c04de0199c215fb6e # v2.3.7
-        with:
-          file: ./lcov.info
diff --git a/.github/workflows/scorecard.yml b/.github/workflows/scorecard.yml
deleted file mode 100644
index 28b34e01..00000000
--- a/.github/workflows/scorecard.yml
+++ /dev/null
@@ -1,72 +0,0 @@
-# This workflow uses actions that are not certified by GitHub. They are provided
-# by a third-party and are governed by separate terms of service, privacy
-# policy, and support documentation.
-
-name: Scorecard supply-chain security
-on:
-  # For Branch-Protection check. Only the default branch is supported. See
-  # https://github.com/ossf/scorecard/blob/main/docs/checks.md#branch-protection
-  branch_protection_rule:
-  # To guarantee Maintained check is occasionally updated. See
-  # https://github.com/ossf/scorecard/blob/main/docs/checks.md#maintained
-  schedule:
-    - cron: '16 21 * * 1'
-  push:
-    branches: [ "master" ]
-
-# Declare default permissions as read only.
-permissions: read-all
-
-jobs:
-  analysis:
-    name: Scorecard analysis
-    runs-on: ubuntu-latest
-    permissions:
-      # Needed to upload the results to code-scanning dashboard.
-      security-events: write
-      # Needed to publish results and get a badge (see publish_results below).
-      id-token: write
-      # Uncomment the permissions below if installing in a private repository.
-      # contents: read
-      # actions: read
-
-    steps:
-      - name: "Checkout code"
-        uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1
-        with:
-          persist-credentials: false
-
-      - name: "Run analysis"
-        uses: ossf/scorecard-action@4eaacf0543bb3f2c246792bd56e8cdeffafb205a # v2.4.3
-        with:
-          results_file: results.sarif
-          results_format: sarif
-          # (Optional) "write" PAT token. Uncomment the `repo_token` line below if:
-          # - you want to enable the Branch-Protection check on a *public* repository, or
-          # - you are installing Scorecard on a *private* repository
-          # To create the PAT, follow the steps in https://github.com/ossf/scorecard-action#authentication-with-pat.
-          # repo_token: ${{ secrets.SCORECARD_TOKEN }}
-
-          # Public repositories:
-          #   - Publish results to OpenSSF REST API for easy access by consumers
-          #   - Allows the repository to include the Scorecard badge.
-          #   - See https://github.com/ossf/scorecard-action#publishing-results.
-          # For private repositories:
-          #   - `publish_results` will always be set to `false`, regardless
-          #     of the value entered here.
-          publish_results: true
-
-      # Upload the results as artifacts (optional). Commenting out will disable uploads of run results in SARIF
-      # format to the repository Actions tab.
-      - name: "Upload artifact"
-        uses: actions/upload-artifact@b7c566a772e6b6bfb58ed0dc250532a479d7789f # v6.0.0
-        with:
-          name: SARIF file
-          path: results.sarif
-          retention-days: 5
-
-      # Upload the results to GitHub's code scanning dashboard.
-      - name: "Upload to code-scanning"
-        uses: github/codeql-action/upload-sarif@5d4e8d1aca955e8d8589aabd499c5cae939e33c7 # v3.29.5
-        with:
-          sarif_file: results.sarif
diff --git a/History.md b/History.md
index 689c08a9..d2b49551 100644
--- a/History.md
+++ b/History.md
@@ -1,5 +1,12 @@
 # Unreleased Changes
 
+## 🐞 Bug fixes
+
+- Fixed HTTP header conflict between Content-Length and Transfer-Encoding in res.send - by [@YuryShkoda](https://github.com/YuryShkoda) in [#4893](https://github.com/expressjs/express/pull/4893)
+
+
+    Fixed the behavior of `res.send()` to prevent conflicts between `Content-Length` and `Transfer-Encoding` HTTP headers in responses. The `Content-Length` header in `res.send()` is now only added when a `Transfer-Encoding` header is not present, complying with the HTTP specification that states both headers should not coexist in the same response
+
 ## 🚀 Improvements
 
 - Improve HTML structure in `res.redirect()` responses when HTML format is accepted by adding `<!DOCTYPE html>`, `<title>`, and `<body>` tags for better browser compatibility - by [@Bernice55231](https://github.com/Bernice55231) in [#5167](https://github.com/expressjs/express/pull/5167)
diff --git a/lib/response.js b/lib/response.js
index 731afb78..02c3dfb2 100644
--- a/lib/response.js
+++ b/lib/response.js
@@ -168,9 +168,10 @@ res.send = function send(body) {
   var etagFn = app.get('etag fn')
   var generateETag = !this.get('ETag') && typeof etagFn === 'function'
 
-  // populate Content-Length
-  var len
-  if (chunk !== undefined) {
+  // Because Content-Length and Transfer-Encoding can't be present in the response headers together,
+  // Content-Length should be added only if there is no Transfer-Encoding header
+  var len;
+  if (chunk !== undefined && !this.get('Transfer-Encoding')) {
     if (Buffer.isBuffer(chunk)) {
       // get length of Buffer
       len = chunk.length
diff --git a/test/res.send.js b/test/res.send.js
index 8547b776..213ab381 100644
--- a/test/res.send.js
+++ b/test/res.send.js
@@ -566,4 +566,29 @@ describe('res', function(){
       })
     })
   })
+
+  describe('when Transfer-Encoding header is present', function(){
+    var transferEncodings = [
+      'chunked',
+      'compress',
+      'deflate',
+      'gzip'
+    ];
+
+    transferEncodings.forEach(function(encoding){
+      it('should not add Content-Length header if Transfer-Encoding header is equal to ' + encoding, function(done){
+        var app = express();
+
+        app.use(function(_, res){
+          res.status(200).set('Transfer-Encoding', encoding).send('');
+        });
+
+        request(app)
+          .get('/')
+          .expect(utils.shouldNotHaveHeader('Content-Length'))
+          .expect(utils.shouldHaveHeader('Transfer-Encoding'))
+          .expect(200, '', done);
+      })
+    });
+  })
 })