diff --git a/lib/web/fetch/data-url.js b/lib/web/fetch/data-url.js
index 2996feca92a..5d2b310aa4a 100644
--- a/lib/web/fetch/data-url.js
+++ b/lib/web/fetch/data-url.js
@@ -136,8 +136,8 @@ function URLSerializer (url, excludeFragment = false) {
   return serialized
 }
 
-// https://infra.spec.whatwg.org/#collect-a-sequence-of-code-points
 /**
+ * @see https://infra.spec.whatwg.org/#collect-a-sequence-of-code-points
  * @param {(char: string) => boolean} condition
  * @param {string} input
  * @param {{ position: number }} position
diff --git a/lib/web/fetch/headers.js b/lib/web/fetch/headers.js
index 44b1f52bb3f..e3bc2eb26ca 100644
--- a/lib/web/fetch/headers.js
+++ b/lib/web/fetch/headers.js
@@ -4,10 +4,13 @@
 
 const { kConstruct } = require('../../core/symbols')
 const { kEnumerableProperty } = require('../../core/util')
+const { forbiddenMethods } = require('./constants')
 const {
+  gettingDecodingSplitting,
   iteratorMixin,
   isValidHeaderName,
-  isValidHeaderValue
+  isValidHeaderValue,
+  isomorphicEncode
 } = require('./util')
 const { webidl } = require('./webidl')
 const assert = require('node:assert')
@@ -79,6 +82,75 @@ function fill (headers, object) {
   }
 }
 
+/**
+ * @see https://fetch.spec.whatwg.org/#forbidden-request-header
+ * @param {string} name
+ * @param {string} value
+ * @returns {boolean} Returns true if the header is forbidden.
+ */
+function isForbiddenRequestHeader (name, value) {
+  // 1. If name is a byte-case-insensitive match of one of:
+  if (
+    [
+      'accept-charset',
+      'accept-encoding',
+      'access-control-request-headers',
+      'access-control-request-method',
+      'connection',
+      'content-length',
+      'cookie',
+      'cookie2',
+      'date',
+      'dnt',
+      'expect',
+      'host',
+      'keep-alive',
+      'origin',
+      'referer',
+      'set-cookie',
+      'te',
+      'trailer',
+      'transfer-encoding',
+      'upgrade',
+      'via'
+    ].includes(name.toLowerCase())
+  ) {
+    // then return true.
+    return true
+  }
+
+  // 2. If name when byte-lowercased starts with `proxy-` or `sec-`,
+  // then return true.
+  if (
+    name.toLowerCase().startsWith('proxy-') ||
+    name.toLowerCase().startsWith('sec-')
+  ) {
+    return true
+  }
+
+  // 3. If name is a byte-case-insensitive match for one of
+  if (
+    ['x-http-method', 'x-http-method-override', 'x-method-override'].includes(
+      name.toLowerCase()
+    )
+  ) {
+    // 1. Let parsedValues be the result of getting, decoding, and
+    // splitting value.
+    const parsedValues = gettingDecodingSplitting(value)
+
+    // 2. For each method of parsedValues:
+    for (const method of parsedValues) {
+      // if the isomorphic encoding of method is a forbidden method,
+      // then return true.
+      if (forbiddenMethods.includes(isomorphicEncode(method).toUpperCase())) {
+        return true
+      }
+    }
+  }
+
+  return false
+}
+
 /**
  * @see https://fetch.spec.whatwg.org/#concept-headers-append
  * @param {Headers} headers
@@ -89,8 +161,9 @@ function appendHeader (headers, name, value) {
   // 1. Normalize value.
   value = headerValueNormalize(value)
 
-  // 2. If name is not a header name or value is not a
-  //    header value, then throw a TypeError.
+  // 2. If validating (name, value) for headers returns false, then return.
+  //    1. If name is not a header name or value is not a header value,
+  //    then throw a TypeError.
   if (!isValidHeaderName(name)) {
     throw webidl.errors.invalidArgument({
       prefix: 'Headers.append',
@@ -105,18 +178,23 @@ function appendHeader (headers, name, value) {
     })
   }
 
-  // 3. If headers’s guard is "immutable", then throw a TypeError.
-  // 4. Otherwise, if headers’s guard is "request" and name is a
-  //    forbidden header name, return.
-  // 5. Otherwise, if headers’s guard is "request-no-cors":
-  //    TODO
-  // Note: undici does not implement forbidden header names
+  //    2. If headers’s guard is "immutable", then throw a TypeError.
   if (getHeadersGuard(headers) === 'immutable') {
     throw new TypeError('immutable')
   }
 
-  // 6. Otherwise, if headers’s guard is "response" and name is a
-  //    forbidden response-header name, return.
+  //    3. If headers’s guard is "request" and (name, value) is a forbidden
+  //    request-header, then return.
+  if (
+    getHeadersGuard(headers) === 'request' &&
+    isForbiddenRequestHeader(name, value)
+  ) {
+    return
+  }
+
+  //    4. If headers’s guard is "response" and name is a forbidden
+  //    response-header name, then return.
+  //    TODO
 
   // 7. Append (name, value) to headers’s header list.
   return getHeadersList(headers).append(name, value, false)
diff --git a/lib/web/fetch/util.js b/lib/web/fetch/util.js
index daa2483c943..c47b99b711c 100644
--- a/lib/web/fetch/util.js
+++ b/lib/web/fetch/util.js
@@ -1448,6 +1448,7 @@ function extractMimeType (headers) {
 /**
  * @see https://fetch.spec.whatwg.org/#header-value-get-decode-and-split
  * @param {string|null} value
+ * @returns {string[]} Returns the parsed values.
  */
 function gettingDecodingSplitting (value) {
   // 1. Let input be the result of isomorphic decoding value.
@@ -1622,6 +1623,7 @@ module.exports = {
   parseMetadata,
   createInflate,
   extractMimeType,
+  gettingDecodingSplitting,
   getDecodeSplit,
   utf8DecodeBytes,
   environmentSettingsObject
diff --git a/test/wpt/status/fetch.status.json b/test/wpt/status/fetch.status.json
index 081b693493c..187db016378 100644
--- a/test/wpt/status/fetch.status.json
+++ b/test/wpt/status/fetch.status.json
@@ -52,10 +52,6 @@
           "origin-when-cross-origin policy on a same-origin URL"
         ]
       },
-      "request-forbidden-headers.any.js": {
-        "note": "undici doesn't filter headers",
-        "skip": true
-      },
       "request-headers.any.js": {
         "fail": [
           "Fetch with Chicken",