fix(res.send): add Content-Length header only if Transfer-Encoding is not present (#4893)

YuryShkoda · bjohansebas · web-flow · commit 18e5985b8a9d · 2026-06-15T22:06:31.000-05:00
* fix(respond): add Content-Length header only if Transfer-Encoding is not present

* fix(response): used get('Transfer-Encoding')

* test(res.send): covered Transfer-Encoding and Content-Length headers logic

* chore(response): removed comment

* chore(res.redirect): revert changes to focus on res.send

* test(res.send): added tests for all transfer encodings

* docs: update History.md to include bug fix for HTTP header conflict in res.send()

---------

Co-authored-by: Sebastian Beltran &lt;bjohansebas@gmail.com&gt;
diff --git 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
@@ -162,9 +162,10 @@ 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
@@ -592,4 +592,29 @@ 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);
+      })
+    });
+  })
 })
