diff --git a/lib/fetch/headers.js b/lib/fetch/headers.js
index 883d7637cbf..a905baef499 100644
--- a/lib/fetch/headers.js
+++ b/lib/fetch/headers.js
@@ -114,7 +114,7 @@ function appendHeader (headers, name, value) {
   //    forbidden response-header name, return.
 
   // 7. Append (name, value) to headers’s header list.
-  return headers[kHeadersList].append(name, value)
+  return headers[kHeadersList].append(name, value, false)
 
   // 8. If headers’s guard is "request-no-cors", then remove
   //    privileged no-CORS request headers from headers
@@ -138,20 +138,14 @@ class HeadersList {
   /**
    * @see https://fetch.spec.whatwg.org/#header-list-contains
    * @param {string} name
+   * @param {boolean} isLowerCase
    */
-  contains (name) {
+  contains (name, isLowerCase) {
     // A header list list contains a header name name if list
     // contains a header whose name is a byte-case-insensitive
     // match for name.
 
-    return this[kHeadersMap].has(name.toLowerCase())
-  }
-
-  /**
-   * @param {string} name
-   */
-  lowerCaseContains (name) {
-    return this[kHeadersMap].has(name)
+    return this[kHeadersMap].has(isLowerCase ? name : name.toLowerCase())
   }
 
   clear () {
@@ -164,13 +158,14 @@ class HeadersList {
    * @see https://fetch.spec.whatwg.org/#concept-header-list-append
    * @param {string} name
    * @param {string} value
+   * @param {boolean} isLowerCase
    */
-  append (name, value) {
+  append (name, value, isLowerCase) {
     this[kHeadersSortedMap] = null
 
     // 1. If list contains name, then set name to the first such
     //    header’s name.
-    const lowercaseName = name.toLowerCase()
+    const lowercaseName = isLowerCase ? name : name.toLowerCase()
     const exists = this[kHeadersMap].get(lowercaseName)
 
     // 2. Append (name, value) to list.
@@ -189,41 +184,15 @@ class HeadersList {
     }
   }
 
-  /**
-   * @param {string} name
-   * @param {string} value
-   */
-  lowerCaseAppend (name, value) {
-    this[kHeadersSortedMap] = null
-
-    // 1. If list contains name, then set name to the first such
-    //    header’s name.
-    const exists = this[kHeadersMap].get(name)
-
-    // 2. Append (name, value) to list.
-    if (exists) {
-      const delimiter = name === 'cookie' ? '; ' : ', '
-      this[kHeadersMap].set(name, {
-        name: exists.name,
-        value: `${exists.value}${delimiter}${value}`
-      })
-    } else {
-      this[kHeadersMap].set(name, { name, value })
-    }
-
-    if (name === 'set-cookie') {
-      (this.cookies ??= []).push(value)
-    }
-  }
-
   /**
    * @see https://fetch.spec.whatwg.org/#concept-header-list-set
    * @param {string} name
    * @param {string} value
+   * @param {boolean} isLowerCase
    */
-  set (name, value) {
+  set (name, value, isLowerCase) {
     this[kHeadersSortedMap] = null
-    const lowercaseName = name.toLowerCase()
+    const lowercaseName = isLowerCase ? name : name.toLowerCase()
 
     if (lowercaseName === 'set-cookie') {
       this.cookies = [value]
@@ -236,35 +205,14 @@ class HeadersList {
     this[kHeadersMap].set(lowercaseName, { name, value })
   }
 
-  /**
-   * @param {string} name
-   * @param {string} value
-   */
-  lowerCaseSet (name, value) {
-    if (name === 'set-cookie') {
-      this.cookies = [value]
-    }
-
-    // 1. If list contains name, then set the value of
-    //    the first such header to value and remove the
-    //    others.
-    // 2. Otherwise, append header (name, value) to list.
-    this[kHeadersMap].set(name, { name, value })
-  }
-
   /**
    * @see https://fetch.spec.whatwg.org/#concept-header-list-delete
    * @param {string} name
+   * @param {boolean} isLowerCase
    */
-  delete (name) {
-    return this.lowerCaseDelete(name.toLowerCase())
-  }
-
-  /**
-   * @param {string} name
-   */
-  lowerCaseDelete (name) {
+  delete (name, isLowerCase) {
     this[kHeadersSortedMap] = null
+    if (!isLowerCase) name = name.toLowerCase()
 
     if (name === 'set-cookie') {
       this.cookies = null
@@ -276,24 +224,15 @@ class HeadersList {
   /**
    * @see https://fetch.spec.whatwg.org/#concept-header-list-get
    * @param {string} name
+   * @param {boolean} isLowerCase
    * @returns {string | null}
    */
-  get (name) {
-    const value = this[kHeadersMap].get(name.toLowerCase())
-
+  get (name, isLowerCase) {
     // 1. If list does not contain name, then return null.
     // 2. Return the values of all headers in list whose name
     //    is a byte-case-insensitive match for name,
     //    separated from each other by 0x2C 0x20, in order.
-    return value === undefined ? null : value.value
-  }
-
-  /**
-   * @param {string} name
-   * @returns {string | null}
-   */
-  lowerCaseGet (name) {
-    return this[kHeadersMap].get(name)?.value ?? null
+    return this[kHeadersMap].get(isLowerCase ? name : name.toLowerCase())?.value ?? null
   }
 
   * [Symbol.iterator] () {
@@ -383,14 +322,14 @@ class Headers {
 
     // 6. If this’s header list does not contain name, then
     //    return.
-    if (!this[kHeadersList].contains(name)) {
+    if (!this[kHeadersList].contains(name, false)) {
       return
     }
 
     // 7. Delete name from this’s header list.
     // 8. If this’s guard is "request-no-cors", then remove
     //    privileged no-CORS request headers from this.
-    this[kHeadersList].delete(name)
+    this[kHeadersList].delete(name, false)
   }
 
   // https://fetch.spec.whatwg.org/#dom-headers-get
@@ -412,7 +351,7 @@ class Headers {
 
     // 2. Return the result of getting name from this’s header
     //    list.
-    return this[kHeadersList].get(name)
+    return this[kHeadersList].get(name, false)
   }
 
   // https://fetch.spec.whatwg.org/#dom-headers-has
@@ -434,7 +373,7 @@ class Headers {
 
     // 2. Return true if this’s header list contains name;
     //    otherwise false.
-    return this[kHeadersList].contains(name)
+    return this[kHeadersList].contains(name, false)
   }
 
   // https://fetch.spec.whatwg.org/#dom-headers-set
@@ -483,7 +422,7 @@ class Headers {
     // 7. Set (name, value) in this’s header list.
     // 8. If this’s guard is "request-no-cors", then remove
     //    privileged no-CORS request headers from this
-    this[kHeadersList].set(name, value)
+    this[kHeadersList].set(name, value, false)
   }
 
   // https://fetch.spec.whatwg.org/#dom-headers-getsetcookie
diff --git a/lib/fetch/index.js b/lib/fetch/index.js
index 0e5fc35a13d..4a25542c310 100644
--- a/lib/fetch/index.js
+++ b/lib/fetch/index.js
@@ -475,7 +475,7 @@ function fetching ({
   }
 
   // 12. If request’s header list does not contain `Accept`, then:
-  if (!request.headersList.lowerCaseContains('accept')) {
+  if (!request.headersList.contains('accept', true)) {
     // 1. Let value be `*/*`.
     const value = '*/*'
 
@@ -492,14 +492,14 @@ function fetching ({
     // TODO
 
     // 3. Append `Accept`/value to request’s header list.
-    request.headersList.lowerCaseAppend('accept', value)
+    request.headersList.append('accept', value, true)
   }
 
   // 13. If request’s header list does not contain `Accept-Language`, then
   // user agents should append `Accept-Language`/an appropriate value to
   // request’s header list.
-  if (!request.headersList.lowerCaseContains('accept-language')) {
-    request.headersList.lowerCaseAppend('accept-language', '*')
+  if (!request.headersList.contains('accept-language', true)) {
+    request.headersList.append('accept-language', '*', true)
   }
 
   // 14. If request’s priority is null, then use request’s initiator and
@@ -718,7 +718,7 @@ async function mainFetch (fetchParams, recursive = false) {
     response.type === 'opaque' &&
     internalResponse.status === 206 &&
     internalResponse.rangeRequested &&
-    !request.headers.lowerCaseContains('range')
+    !request.headers.contains('range', true)
   ) {
     response = internalResponse = makeNetworkError()
   }
@@ -840,7 +840,7 @@ function schemeFetch (fetchParams) {
 
       // 8. If request’s header list does not contain `Range`:
       // 9. Otherwise:
-      if (!request.headersList.lowerCaseContains('range')) {
+      if (!request.headersList.contains('range', true)) {
         // 1. Let bodyWithType be the result of safely extracting blob.
         // Note: in the FileAPI a blob "object" is a Blob *or* a MediaSource.
         // In node, this can only ever be a Blob. Therefore we can safely
@@ -854,14 +854,14 @@ function schemeFetch (fetchParams) {
         response.body = bodyWithType[0]
 
         // 4. Set response’s header list to « (`Content-Length`, serializedFullLength), (`Content-Type`, type) ».
-        response.headersList.lowerCaseSet('content-length', serializedFullLength)
-        response.headersList.lowerCaseSet('content-type', type)
+        response.headersList.set('content-length', serializedFullLength, true)
+        response.headersList.set('content-type', type, true)
       } else {
         // 1. Set response’s range-requested flag.
         response.rangeRequested = true
 
         // 2. Let rangeHeader be the result of getting `Range` from request’s header list.
-        const rangeHeader = request.headersList.lowerCaseGet('range')
+        const rangeHeader = request.headersList.get('range', true)
 
         // 3. Let rangeValue be the result of parsing a single range header value given rangeHeader and true.
         const rangeValue = simpleRangeHeaderValue(rangeHeader, true)
@@ -921,9 +921,9 @@ function schemeFetch (fetchParams) {
 
         // 15. Set response’s header list to « (`Content-Length`, serializedSlicedLength),
         //     (`Content-Type`, type), (`Content-Range`, contentRange) ».
-        response.headersList.lowerCaseSet('content-length', serializedSlicedLength)
-        response.headersList.lowerCaseSet('content-type', type)
-        response.headersList.lowerCaseSet('content-range', contentRange)
+        response.headersList.set('content-length', serializedSlicedLength, true)
+        response.headersList.set('content-type', type, true)
+        response.headersList.set('content-range', contentRange, true)
       }
 
       // 10. Return response.
@@ -1040,7 +1040,7 @@ function fetchFinale (fetchParams, response) {
         responseStatus = response.status
 
         // 2. Let mimeType be the result of extracting a MIME type from response’s header list.
-        const mimeType = parseMIMEType(response.headersList.lowerCaseGet('content-type')) // TODO: fix
+        const mimeType = parseMIMEType(response.headersList.get('content-type', true)) // TODO: fix
 
         // 3. If mimeType is not failure, then set bodyInfo’s content type to the result of minimizing a supported MIME type given mimeType.
         if (mimeType !== 'failure') {
@@ -1336,11 +1336,11 @@ function httpRedirectFetch (fetchParams, response) {
   //     delete headerName from request’s header list.
   if (!sameOrigin(requestCurrentURL(request), locationURL)) {
     // https://fetch.spec.whatwg.org/#cors-non-wildcard-request-header-name
-    request.headersList.lowerCaseDelete('authorization')
+    request.headersList.delete('authorization', true)
 
     // "Cookie" and "Host" are forbidden request-headers, which undici doesn't implement.
-    request.headersList.lowerCaseDelete('cookie')
-    request.headersList.lowerCaseDelete('host')
+    request.headersList.delete('cookie', true)
+    request.headersList.delete('host', true)
   }
 
   // 14. If request’s body is non-null, then set request’s body to the first return
@@ -1456,7 +1456,7 @@ async function httpNetworkOrCacheFetch (
   //    `Content-Length`/contentLengthHeaderValue to httpRequest’s header
   //    list.
   if (contentLengthHeaderValue != null) {
-    httpRequest.headersList.lowerCaseAppend('content-length', contentLengthHeaderValue)
+    httpRequest.headersList.append('content-length', contentLengthHeaderValue, true)
   }
 
   //    9. If contentLengthHeaderValue is non-null, then append (`Content-Length`,
@@ -1472,7 +1472,7 @@ async function httpNetworkOrCacheFetch (
   //    `Referer`/httpRequest’s referrer, serialized and isomorphic encoded,
   //     to httpRequest’s header list.
   if (httpRequest.referrer instanceof URL) {
-    httpRequest.headersList.lowerCaseAppend('referer', isomorphicEncode(httpRequest.referrer.href))
+    httpRequest.headersList.append('referer', isomorphicEncode(httpRequest.referrer.href), true)
   }
 
   //    12. Append a request `Origin` header for httpRequest.
@@ -1484,8 +1484,8 @@ async function httpNetworkOrCacheFetch (
   //    14. If httpRequest’s header list does not contain `User-Agent`, then
   //    user agents should append `User-Agent`/default `User-Agent` value to
   //    httpRequest’s header list.
-  if (!httpRequest.headersList.lowerCaseContains('user-agent')) {
-    httpRequest.headersList.lowerCaseAppend('user-agent', typeof esbuildDetection === 'undefined' ? 'undici' : 'node')
+  if (!httpRequest.headersList.contains('user-agent', true)) {
+    httpRequest.headersList.append('user-agent', typeof esbuildDetection === 'undefined' ? 'undici' : 'node', true)
   }
 
   //    15. If httpRequest’s cache mode is "default" and httpRequest’s header
@@ -1494,11 +1494,11 @@ async function httpNetworkOrCacheFetch (
   //    httpRequest’s cache mode to "no-store".
   if (
     httpRequest.cache === 'default' &&
-    (httpRequest.headersList.lowerCaseContains('if-modified-since') ||
-      httpRequest.headersList.lowerCaseContains('if-none-match') ||
-      httpRequest.headersList.lowerCaseContains('if-unmodified-since') ||
-      httpRequest.headersList.lowerCaseContains('if-match') ||
-      httpRequest.headersList.lowerCaseContains('if-range'))
+    (httpRequest.headersList.contains('if-modified-since', true) ||
+      httpRequest.headersList.contains('if-none-match', true) ||
+      httpRequest.headersList.contains('if-unmodified-since', true) ||
+      httpRequest.headersList.contains('if-match', true) ||
+      httpRequest.headersList.contains('if-range', true))
   ) {
     httpRequest.cache = 'no-store'
   }
@@ -1510,44 +1510,44 @@ async function httpNetworkOrCacheFetch (
   if (
     httpRequest.cache === 'no-cache' &&
     !httpRequest.preventNoCacheCacheControlHeaderModification &&
-    !httpRequest.headersList.lowerCaseContains('cache-control')
+    !httpRequest.headersList.contains('cache-control', true)
   ) {
-    httpRequest.headersList.lowerCaseAppend('cache-control', 'max-age=0')
+    httpRequest.headersList.append('cache-control', 'max-age=0', true)
   }
 
   //    17. If httpRequest’s cache mode is "no-store" or "reload", then:
   if (httpRequest.cache === 'no-store' || httpRequest.cache === 'reload') {
     // 1. If httpRequest’s header list does not contain `Pragma`, then append
     // `Pragma`/`no-cache` to httpRequest’s header list.
-    if (!httpRequest.headersList.lowerCaseContains('pragma')) {
-      httpRequest.headersList.lowerCaseAppend('pragma', 'no-cache')
+    if (!httpRequest.headersList.contains('pragma', true)) {
+      httpRequest.headersList.append('pragma', 'no-cache', true)
     }
 
     // 2. If httpRequest’s header list does not contain `Cache-Control`,
     // then append `Cache-Control`/`no-cache` to httpRequest’s header list.
-    if (!httpRequest.headersList.lowerCaseContains('cache-control')) {
-      httpRequest.headersList.lowerCaseAppend('cache-control', 'no-cache')
+    if (!httpRequest.headersList.contains('cache-control', true)) {
+      httpRequest.headersList.append('cache-control', 'no-cache', true)
     }
   }
 
   //    18. If httpRequest’s header list contains `Range`, then append
   //    `Accept-Encoding`/`identity` to httpRequest’s header list.
-  if (httpRequest.headersList.lowerCaseContains('range')) {
-    httpRequest.headersList.lowerCaseAppend('accept-encoding', 'identity')
+  if (httpRequest.headersList.contains('range', true)) {
+    httpRequest.headersList.append('accept-encoding', 'identity', true)
   }
 
   //    19. Modify httpRequest’s header list per HTTP. Do not append a given
   //    header if httpRequest’s header list contains that header’s name.
   //    TODO: https://github.com/whatwg/fetch/issues/1285#issuecomment-896560129
-  if (!httpRequest.headersList.lowerCaseContains('accept-encoding')) {
+  if (!httpRequest.headersList.contains('accept-encoding', true)) {
     if (urlHasHttpsScheme(requestCurrentURL(httpRequest))) {
-      httpRequest.headersList.lowerCaseAppend('accept-encoding', 'br, gzip, deflate')
+      httpRequest.headersList.append('accept-encoding', 'br, gzip, deflate', true)
     } else {
-      httpRequest.headersList.lowerCaseAppend('accept-encoding', 'gzip, deflate')
+      httpRequest.headersList.append('accept-encoding', 'gzip, deflate', true)
     }
   }
 
-  httpRequest.headersList.lowerCaseDelete('host')
+  httpRequest.headersList.delete('host', true)
 
   //    20. If includeCredentials is true, then:
   if (includeCredentials) {
@@ -1630,7 +1630,7 @@ async function httpNetworkOrCacheFetch (
 
   // 12. If httpRequest’s header list contains `Range`, then set response’s
   // range-requested flag.
-  if (httpRequest.headersList.lowerCaseContains('range')) {
+  if (httpRequest.headersList.contains('range', true)) {
     response.rangeRequested = true
   }
 
@@ -2121,15 +2121,15 @@ async function httpNetworkFetch (
           // We distinguish between them and iterate accordingly
           if (Array.isArray(rawHeaders)) {
             for (let i = 0; i < rawHeaders.length; i += 2) {
-              headersList.lowerCaseAppend(bufferToLowerCasedHeaderName(rawHeaders[i]), rawHeaders[i + 1].toString('latin1'))
+              headersList.append(bufferToLowerCasedHeaderName(rawHeaders[i]), rawHeaders[i + 1].toString('latin1'), true)
             }
-            const contentEncoding = headersList.lowerCaseGet('content-encoding')
+            const contentEncoding = headersList.get('content-encoding', true)
             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())
             }
-            location = headersList.lowerCaseGet('location')
+            location = headersList.get('location', true)
           } else {
             const keys = Object.keys(rawHeaders)
             for (let i = 0; i < keys.length; ++i) {
@@ -2243,7 +2243,7 @@ async function httpNetworkFetch (
           const headersList = new HeadersList()
 
           for (let i = 0; i < rawHeaders.length; i += 2) {
-            headersList.lowerCaseAppend(bufferToLowerCasedHeaderName(rawHeaders[i]), rawHeaders[i + 1].toString('latin1'))
+            headersList.append(bufferToLowerCasedHeaderName(rawHeaders[i]), rawHeaders[i + 1].toString('latin1'), true)
           }
 
           resolve({
diff --git a/lib/fetch/response.js b/lib/fetch/response.js
index 54ef3595416..f8894b692eb 100644
--- a/lib/fetch/response.js
+++ b/lib/fetch/response.js
@@ -126,7 +126,7 @@ class Response {
     const value = isomorphicEncode(URLSerializer(parsedURL))
 
     // 7. Append `Location`/value to responseObject’s response’s header list.
-    responseObject[kState].headersList.lowerCaseAppend('location', value)
+    responseObject[kState].headersList.append('location', value, true)
 
     // 8. Return responseObject.
     return responseObject
@@ -496,8 +496,8 @@ function initializeResponse (response, init, body) {
 
     // 3. If body's type is non-null and response's header list does not contain
     //    `Content-Type`, then append (`Content-Type`, body's type) to response's header list.
-    if (body.type != null && !response[kState].headersList.lowerCaseContains('content-type')) {
-      response[kState].headersList.lowerCaseAppend('content-type', body.type)
+    if (body.type != null && !response[kState].headersList.contains('content-type', true)) {
+      response[kState].headersList.append('content-type', body.type, true)
     }
   }
 }
diff --git a/lib/fetch/util.js b/lib/fetch/util.js
index 13c4b468cc6..2693d05bfcf 100644
--- a/lib/fetch/util.js
+++ b/lib/fetch/util.js
@@ -35,7 +35,7 @@ function responseLocationURL (response, requestFragment) {
 
   // 2. Let location be the result of extracting header list values given
   // `Location` and response’s header list.
-  let location = response.headersList.lowerCaseGet('location')
+  let location = response.headersList.get('location', true)
 
   // 3. If location is a header value, then set location to the result of
   //    parsing location with response’s URL.
@@ -153,7 +153,7 @@ function setRequestReferrerPolicyOnRedirect (request, actualResponse) {
   // 2. Let policy be the empty string.
   // 3. For each token in policy-tokens, if token is a referrer policy and token is not the empty string, then set policy to token.
   // 4. Return policy.
-  const policyHeader = (headersList.lowerCaseGet('referrer-policy') ?? '').split(',')
+  const policyHeader = (headersList.get('referrer-policy', true) ?? '').split(',')
 
   // Note: As the referrer-policy can contain multiple policies
   // separated by comma, we need to loop through all of them
@@ -212,7 +212,7 @@ function appendFetchMetadata (httpRequest) {
   header = httpRequest.mode
 
   //  4. Set a structured field value `Sec-Fetch-Mode`/header in r’s header list.
-  httpRequest.headersList.lowerCaseSet('sec-fetch-mode', header)
+  httpRequest.headersList.set('sec-fetch-mode', header, true)
 
   //  https://w3c.github.io/webappsec-fetch-metadata/#sec-fetch-site-header
   //  TODO
@@ -229,7 +229,7 @@ function appendRequestOriginHeader (request) {
   // 2. If request’s response tainting is "cors" or request’s mode is "websocket", then append (`Origin`, serializedOrigin) to request’s header list.
   if (request.responseTainting === 'cors' || request.mode === 'websocket') {
     if (serializedOrigin) {
-      request.headersList.lowerCaseAppend('origin', serializedOrigin)
+      request.headersList.append('origin', serializedOrigin, true)
     }
 
   // 3. Otherwise, if request’s method is neither `GET` nor `HEAD`, then:
@@ -260,7 +260,7 @@ function appendRequestOriginHeader (request) {
 
     if (serializedOrigin) {
       // 2. Append (`Origin`, serializedOrigin) to request’s header list.
-      request.headersList.lowerCaseAppend('origin', serializedOrigin)
+      request.headersList.append('origin', serializedOrigin, true)
     }
   }
 }
diff --git a/test/fetch/headers.js b/test/fetch/headers.js
index 48461103d78..fcf6f5f4891 100644
--- a/test/fetch/headers.js
+++ b/test/fetch/headers.js
@@ -741,3 +741,12 @@ tap.test('Headers.prototype.getSetCookie', (t) => {
 
   t.end()
 })
+
+tap.test('When the value is updated, update the cache', (t) => {
+  t.plan(2)
+  const expected = [['a', 'a'], ['b', 'b'], ['c', 'c']]
+  const headers = new Headers(expected)
+  t.same([...headers], expected)
+  headers.append('d', 'd')
+  t.same([...headers], [...expected, ['d', 'd']])
+})