diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index 9cebbb20bef..a23d5edec71 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -48,6 +48,6 @@ jobs:
 
       - name: Coverage Report
         if: inputs.runs-on == 'ubuntu-latest' && inputs.node-version == 20
-        uses: codecov/codecov-action@54bcd8715eee62d40e33596ef5e8f0f48dbbccab # v4.1.0
+        uses: codecov/codecov-action@c16abc29c95fcf9174b58eb7e1abf4c866893bc8 # v4.1.1
         with:
           token: ${{ secrets.CODECOV_TOKEN }}
diff --git a/docs/docs/api/DiagnosticsChannel.md b/docs/docs/api/DiagnosticsChannel.md
index 099c072f6c6..cb0421d84d4 100644
--- a/docs/docs/api/DiagnosticsChannel.md
+++ b/docs/docs/api/DiagnosticsChannel.md
@@ -20,8 +20,6 @@ diagnosticsChannel.channel('undici:request:create').subscribe(({ request }) => {
   console.log('method', request.method)
   console.log('path', request.path)
   console.log('headers') // array of strings, e.g: ['foo', 'bar']
-  request.addHeader('hello', 'world')
-  console.log('headers', request.headers) // e.g. ['foo', 'bar', 'hello', 'world']
 })
 ```
 
diff --git a/lib/core/request.js b/lib/core/request.js
index 37839d3c949..6d9ebc8bc8f 100644
--- a/lib/core/request.js
+++ b/lib/core/request.js
@@ -91,6 +91,8 @@ class Request {
 
     this.abort = null
 
+    this.publicInterface = null
+
     if (body == null) {
       this.body = null
     } else if (isStream(body)) {
@@ -187,10 +189,32 @@ class Request {
     this[kHandler] = handler
 
     if (channels.create.hasSubscribers) {
-      channels.create.publish({ request: this })
+      channels.create.publish({ request: this.getPublicInterface() })
     }
   }
 
+  getPublicInterface () {
+    const self = this
+    this.publicInterface ??= {
+      get origin () {
+        return self.origin
+      },
+      get method () {
+        return self.method
+      },
+      get path () {
+        return self.path
+      },
+      get headers () {
+        return self.headers
+      },
+      get completed () {
+        return self.completed
+      }
+    }
+    return this.publicInterface
+  }
+
   onBodySent (chunk) {
     if (this[kHandler].onBodySent) {
       try {
@@ -203,7 +227,7 @@ class Request {
 
   onRequestSent () {
     if (channels.bodySent.hasSubscribers) {
-      channels.bodySent.publish({ request: this })
+      channels.bodySent.publish({ request: this.getPublicInterface() })
     }
 
     if (this[kHandler].onRequestSent) {
@@ -236,7 +260,7 @@ class Request {
     assert(!this.completed)
 
     if (channels.headers.hasSubscribers) {
-      channels.headers.publish({ request: this, response: { statusCode, headers, statusText } })
+      channels.headers.publish({ request: this.getPublicInterface(), response: { statusCode, headers, statusText } })
     }
 
     try {
@@ -272,7 +296,7 @@ class Request {
 
     this.completed = true
     if (channels.trailers.hasSubscribers) {
-      channels.trailers.publish({ request: this, trailers })
+      channels.trailers.publish({ request: this.getPublicInterface(), trailers })
     }
 
     try {
@@ -287,7 +311,7 @@ class Request {
     this.onFinally()
 
     if (channels.error.hasSubscribers) {
-      channels.error.publish({ request: this, error })
+      channels.error.publish({ request: this.getPublicInterface(), error })
     }
 
     if (this.aborted) {
@@ -309,11 +333,6 @@ class Request {
       this.endHandler = null
     }
   }
-
-  addHeader (key, value) {
-    processHeader(this, key, value)
-    return this
-  }
 }
 
 function processHeader (request, key, val) {
diff --git a/lib/core/util.js b/lib/core/util.js
index a62396e23e0..e7b5d9c1edd 100644
--- a/lib/core/util.js
+++ b/lib/core/util.js
@@ -246,9 +246,6 @@ function bufferToLowerCasedHeaderName (value) {
  * @returns {Record<string, string | string[]>}
  */
 function parseHeaders (headers, obj) {
-  // For H2 support
-  if (!Array.isArray(headers)) return headers
-
   if (obj === undefined) obj = {}
   for (let i = 0; i < headers.length; i += 2) {
     const key = headerNameToString(headers[i])
diff --git a/lib/dispatcher/client-h1.js b/lib/dispatcher/client-h1.js
index 62a3e29ef24..da70f7d8313 100644
--- a/lib/dispatcher/client-h1.js
+++ b/lib/dispatcher/client-h1.js
@@ -993,7 +993,7 @@ function writeH1 (client, request) {
   }
 
   if (channels.sendHeaders.hasSubscribers) {
-    channels.sendHeaders.publish({ request, headers: header, socket })
+    channels.sendHeaders.publish({ request: request.getPublicInterface(), headers: header, socket })
   }
 
   /* istanbul ignore else: assertion */
diff --git a/lib/dispatcher/client-h2.js b/lib/dispatcher/client-h2.js
index d593eae4fca..0a53b75e501 100644
--- a/lib/dispatcher/client-h2.js
+++ b/lib/dispatcher/client-h2.js
@@ -54,6 +54,20 @@ const {
   }
 } = http2
 
+function parseH2Headers (headers) {
+  // set-cookie is always an array. Duplicates are added to the array.
+  // For duplicate cookie headers, the values are joined together with '; '.
+  headers = Object.entries(headers).flat(2)
+
+  const result = []
+
+  for (const header of headers) {
+    result.push(Buffer.from(header))
+  }
+
+  return result
+}
+
 async function connectH2 (client, socket) {
   client[kSocket] = socket
 
@@ -391,7 +405,7 @@ function writeH2 (client, request) {
     const { [HTTP2_HEADER_STATUS]: statusCode, ...realHeaders } = headers
     request.onResponseStarted()
 
-    if (request.onHeaders(Number(statusCode), realHeaders, stream.resume.bind(stream), '') === false) {
+    if (request.onHeaders(Number(statusCode), parseH2Headers(realHeaders), stream.resume.bind(stream), '') === false) {
       stream.pause()
     }
 
diff --git a/lib/web/fetch/index.js b/lib/web/fetch/index.js
index d8c20c59bf7..58d61585c06 100644
--- a/lib/web/fetch/index.js
+++ b/lib/web/fetch/index.js
@@ -2141,29 +2141,6 @@ async function httpNetworkFetch (
               codings = contentEncoding.toLowerCase().split(',').map((x) => x.trim())
             }
             location = headersList.get('location', true)
-          } else {
-            const keys = Object.keys(rawHeaders)
-            for (let i = 0; i < keys.length; ++i) {
-              // The header names are already in lowercase.
-              const key = keys[i]
-              const value = rawHeaders[key]
-              if (key === 'set-cookie') {
-                for (let j = 0; j < value.length; ++j) {
-                  headersList.append(key, value[j], true)
-                }
-              } else {
-                headersList.append(key, value, true)
-              }
-            }
-            // For H2, The header names are already in lowercase,
-            // so we can avoid the `HeadersList#get` call here.
-            const contentEncoding = rawHeaders['content-encoding']
-            if (contentEncoding) {
-              // https://www.rfc-editor.org/rfc/rfc7231#section-3.1.2.1
-              // "All content-coding values are case-insensitive..."
-              codings = contentEncoding.toLowerCase().split(',').map((x) => x.trim()).reverse()
-            }
-            location = rawHeaders.location
           }
 
           this.body = new Readable({ read: resume })
diff --git a/package.json b/package.json
index f7d7bfd5b9a..a3dc801c4dc 100644
--- a/package.json
+++ b/package.json
@@ -96,7 +96,7 @@
     "@sinonjs/fake-timers": "^11.1.0",
     "@types/node": "^18.0.3",
     "abort-controller": "^3.0.0",
-    "borp": "^0.9.1",
+    "borp": "^0.10.0",
     "c8": "^9.1.0",
     "cross-env": "^7.0.3",
     "dns-packet": "^5.4.0",
@@ -112,7 +112,7 @@
     "proxy": "^2.1.1",
     "snazzy": "^9.0.0",
     "standard": "^17.0.0",
-    "tsd": "^0.30.1",
+    "tsd": "^0.31.0",
     "typescript": "^5.0.2",
     "ws": "^8.11.0"
   },
diff --git a/test/fetch/http2.js b/test/fetch/http2.js
index 0793a21556c..1cef1d00ce6 100644
--- a/test/fetch/http2.js
+++ b/test/fetch/http2.js
@@ -34,7 +34,7 @@ test('[Fetch] Issue#2311', async (t) => {
     res.end(body)
   })
 
-  const { strictEqual } = tspl(t, { plan: 1 })
+  const { strictEqual } = tspl(t, { plan: 2 })
 
   server.listen()
   await once(server, 'listening')
@@ -65,6 +65,7 @@ test('[Fetch] Issue#2311', async (t) => {
   t.after(closeClientAndServerAsPromise(client, server))
 
   strictEqual(responseBody, expectedBody)
+  strictEqual(response.headers.get('x-custom-h2'), 'foo')
 })
 
 test('[Fetch] Simple GET with h2', async (t) => {
diff --git a/test/http2.js b/test/http2.js
index 3319ffddb1e..849a0cc43ba 100644
--- a/test/http2.js
+++ b/test/http2.js
@@ -851,8 +851,10 @@ test('Should handle h2 request with body (string or buffer) - dispatch', async t
         },
         onHeaders (statusCode, headers) {
           t.strictEqual(statusCode, 200)
-          t.strictEqual(headers['content-type'], 'text/plain; charset=utf-8')
-          t.strictEqual(headers['x-custom-h2'], 'foo')
+          t.strictEqual(headers[0].toString('utf-8'), 'content-type')
+          t.strictEqual(headers[1].toString('utf-8'), 'text/plain; charset=utf-8')
+          t.strictEqual(headers[2].toString('utf-8'), 'x-custom-h2')
+          t.strictEqual(headers[3].toString('utf-8'), 'foo')
         },
         onData (chunk) {
           response.push(chunk)
diff --git a/test/node-test/diagnostics-channel/get.js b/test/node-test/diagnostics-channel/get.js
index 397dfa3bc5f..261c136f740 100644
--- a/test/node-test/diagnostics-channel/get.js
+++ b/test/node-test/diagnostics-channel/get.js
@@ -7,7 +7,7 @@ const { Client } = require('../../..')
 const { createServer } = require('node:http')
 
 test('Diagnostics channel - get', (t) => {
-  const assert = tspl(t, { plan: 32 })
+  const assert = tspl(t, { plan: 31 })
   const server = createServer((req, res) => {
     res.setHeader('Content-Type', 'text/plain')
     res.setHeader('trailer', 'foo')
@@ -33,8 +33,6 @@ test('Diagnostics channel - get', (t) => {
     assert.equal(request.method, 'GET')
     assert.equal(request.path, '/')
     assert.deepStrictEqual(request.headers, ['bar', 'bar'])
-    request.addHeader('hello', 'world')
-    assert.deepStrictEqual(request.headers, ['bar', 'bar', 'hello', 'world'])
   })
 
   let _connector
@@ -77,8 +75,7 @@ test('Diagnostics channel - get', (t) => {
       'GET / HTTP/1.1',
       `host: localhost:${server.address().port}`,
       'connection: keep-alive',
-      'bar: bar',
-      'hello: world'
+      'bar: bar'
     ]
 
     assert.deepStrictEqual(headers, expectedHeaders.join('\r\n') + '\r\n')
diff --git a/test/node-test/diagnostics-channel/post-stream.js b/test/node-test/diagnostics-channel/post-stream.js
index 881873a7c1c..9cc5540290b 100644
--- a/test/node-test/diagnostics-channel/post-stream.js
+++ b/test/node-test/diagnostics-channel/post-stream.js
@@ -8,7 +8,7 @@ const { Client } = require('../../..')
 const { createServer } = require('node:http')
 
 test('Diagnostics channel - post stream', (t) => {
-  const assert = tspl(t, { plan: 33 })
+  const assert = tspl(t, { plan: 31 })
   const server = createServer((req, res) => {
     req.resume()
     res.setHeader('Content-Type', 'text/plain')
@@ -34,9 +34,6 @@ test('Diagnostics channel - post stream', (t) => {
     assert.equal(request.method, 'POST')
     assert.equal(request.path, '/')
     assert.deepStrictEqual(request.headers, ['bar', 'bar'])
-    request.addHeader('hello', 'world')
-    assert.deepStrictEqual(request.headers, ['bar', 'bar', 'hello', 'world'])
-    assert.deepStrictEqual(request.body, body)
   })
 
   let _connector
@@ -79,8 +76,7 @@ test('Diagnostics channel - post stream', (t) => {
       'POST / HTTP/1.1',
       `host: localhost:${server.address().port}`,
       'connection: keep-alive',
-      'bar: bar',
-      'hello: world'
+      'bar: bar'
     ]
 
     assert.equal(headers, expectedHeaders.join('\r\n') + '\r\n')
diff --git a/test/node-test/diagnostics-channel/post.js b/test/node-test/diagnostics-channel/post.js
index 1408ffbf023..06c56b673ac 100644
--- a/test/node-test/diagnostics-channel/post.js
+++ b/test/node-test/diagnostics-channel/post.js
@@ -7,7 +7,7 @@ const { Client } = require('../../../')
 const { createServer } = require('node:http')
 
 test('Diagnostics channel - post', (t) => {
-  const assert = tspl(t, { plan: 33 })
+  const assert = tspl(t, { plan: 31 })
   const server = createServer((req, res) => {
     req.resume()
     res.setHeader('Content-Type', 'text/plain')
@@ -32,9 +32,6 @@ test('Diagnostics channel - post', (t) => {
     assert.equal(request.method, 'POST')
     assert.equal(request.path, '/')
     assert.deepStrictEqual(request.headers, ['bar', 'bar'])
-    request.addHeader('hello', 'world')
-    assert.deepStrictEqual(request.headers, ['bar', 'bar', 'hello', 'world'])
-    assert.deepStrictEqual(request.body, Buffer.from('hello world'))
   })
 
   let _connector
@@ -77,8 +74,7 @@ test('Diagnostics channel - post', (t) => {
       'POST / HTTP/1.1',
       `host: localhost:${server.address().port}`,
       'connection: keep-alive',
-      'bar: bar',
-      'hello: world'
+      'bar: bar'
     ]
 
     assert.deepStrictEqual(headers, expectedHeaders.join('\r\n') + '\r\n')
diff --git a/types/diagnostics-channel.d.ts b/types/diagnostics-channel.d.ts
index 85d44823978..a037d1e0b2c 100644
--- a/types/diagnostics-channel.d.ts
+++ b/types/diagnostics-channel.d.ts
@@ -9,8 +9,7 @@ declare namespace DiagnosticsChannel {
     completed: boolean;
     method?: Dispatcher.HttpMethod;
     path: string;
-    headers: string;
-    addHeader(key: string, value: string): Request;
+    headers: any;
   }
   interface Response {
     statusCode: number;