diff --git a/jetty-core/jetty-http/src/main/java/org/eclipse/jetty/http/HttpFields.java b/jetty-core/jetty-http/src/main/java/org/eclipse/jetty/http/HttpFields.java
index 4de6e6b10eb7..93d73d3825f5 100644
--- a/jetty-core/jetty-http/src/main/java/org/eclipse/jetty/http/HttpFields.java
+++ b/jetty-core/jetty-http/src/main/java/org/eclipse/jetty/http/HttpFields.java
@@ -617,7 +617,7 @@ default Enumeration<String> getFieldNames()
     default Set<String> getFieldNamesCollection()
     {
         Set<HttpHeader> seenByHeader = EnumSet.noneOf(HttpHeader.class);
-        Set<String> seenByName = null;
+        Set<String> buildByName = null;
         List<String> list = new ArrayList<>(size());
 
         for (HttpField f : this)
@@ -625,9 +625,9 @@ default Set<String> getFieldNamesCollection()
             HttpHeader header = f.getHeader();
             if (header == null)
             {
-                if (seenByName == null)
-                    seenByName = new TreeSet<>(String::compareToIgnoreCase);
-                if (seenByName.add(f.getName()))
+                if (buildByName == null)
+                    buildByName = new TreeSet<>(String::compareToIgnoreCase);
+                if (buildByName.add(f.getName()))
                     list.add(f.getName());
             }
             else if (seenByHeader.add(header))
@@ -636,6 +636,8 @@ else if (seenByHeader.add(header))
             }
         }
 
+        Set<String> seenByName = buildByName;
+
         // use the list to retain a rough ordering
         return new AbstractSet<>()
         {
@@ -650,6 +652,14 @@ public int size()
             {
                 return list.size();
             }
+
+            @Override
+            public boolean contains(Object o)
+            {
+                if (o instanceof String s)
+                    return seenByName != null && seenByName.contains(s) || seenByHeader.contains(HttpHeader.CACHE.get(s));
+                return false;
+            }
         };
     }
 
diff --git a/jetty-core/jetty-http/src/main/java/org/eclipse/jetty/http/ImmutableHttpFields.java b/jetty-core/jetty-http/src/main/java/org/eclipse/jetty/http/ImmutableHttpFields.java
index f00e9b85fec7..d8b5063f758b 100644
--- a/jetty-core/jetty-http/src/main/java/org/eclipse/jetty/http/ImmutableHttpFields.java
+++ b/jetty-core/jetty-http/src/main/java/org/eclipse/jetty/http/ImmutableHttpFields.java
@@ -244,15 +244,15 @@ public void set(HttpField field)
     /**
      * An immutable {@link HttpFields} instance, optimized for random field access.
      */
-    private static class RandomAccess implements HttpFields
+    public static class RandomAccess implements HttpFields
     {
         private final HttpFields _httpFields;
         private final EnumMap<HttpHeader, HttpField> _enumMap = new EnumMap<>(HttpHeader.class);
         private final Map<String, HttpField> _stringMap;
 
-        RandomAccess(org.eclipse.jetty.http.ImmutableHttpFields httpFields)
+        RandomAccess(HttpFields httpFields)
         {
-            _httpFields = Objects.requireNonNull(httpFields);
+            _httpFields = Objects.requireNonNull(httpFields.asImmutable());
             Map<String, HttpField> stringMap = null;
             for (HttpField field : httpFields)
             {
@@ -354,7 +354,16 @@ public ListIterator<HttpField> listIterator(int index)
         @Override
         public Set<String> getFieldNamesCollection()
         {
-            LinkedHashSet<String> set = new LinkedHashSet<>();
+            LinkedHashSet<String> set = new LinkedHashSet<>()
+            {
+                @Override
+                public boolean contains(Object o)
+                {
+                    if (o instanceof String s)
+                        return _stringMap.containsKey(s) || _enumMap.containsKey(HttpHeader.CACHE.get(s));
+                    return false;
+                }
+            };
             TreeSet<String> seenByName = null;
             for (HttpField field : _httpFields)
             {
diff --git a/jetty-core/jetty-http/src/test/java/org/eclipse/jetty/http/HttpFieldsTest.java b/jetty-core/jetty-http/src/test/java/org/eclipse/jetty/http/HttpFieldsTest.java
index 938ca15d8676..3913e63903bb 100644
--- a/jetty-core/jetty-http/src/test/java/org/eclipse/jetty/http/HttpFieldsTest.java
+++ b/jetty-core/jetty-http/src/test/java/org/eclipse/jetty/http/HttpFieldsTest.java
@@ -25,6 +25,7 @@
 import java.util.Locale;
 import java.util.Map;
 import java.util.NoSuchElementException;
+import java.util.Set;
 import java.util.function.Consumer;
 import java.util.stream.Collectors;
 import java.util.stream.Stream;
@@ -354,16 +355,27 @@ public void testCaseInsensitive()
         assertThat(header.get("EXPECT"), is("100"));
         assertThat(header.get("eXpEcT"), is("100"));
         assertThat(header.get(HttpHeader.EXPECT), is("100"));
+        assertTrue(header.contains("expect"));
+        assertTrue(header.contains("Expect"));
+        assertTrue(header.contains("EXPECT"));
+        assertTrue(header.contains("eXpEcT"));
 
         assertThat(header.get("random"), is("value"));
         assertThat(header.get("Random"), is("value"));
         assertThat(header.get("RANDOM"), is("value"));
         assertThat(header.get("rAnDoM"), is("value"));
         assertThat(header.get("RaNdOm"), is("value"));
+        assertTrue(header.contains("random"));
+        assertTrue(header.contains("Random"));
+        assertTrue(header.contains("RANDOM"));
+        assertTrue(header.contains("rAnDoM"));
+        assertTrue(header.contains("RaNdOm"));
 
         assertThat(header.get("Accept-Charset"), is("UTF-8"));
         assertThat(header.get("accept-charset"), is("UTF-8"));
         assertThat(header.get(HttpHeader.ACCEPT_CHARSET), is("UTF-8"));
+        assertTrue(header.contains("Accept-Charset"));
+        assertTrue(header.contains("accept-charset"));
 
         assertThat(header.getValuesList("Accept-Charset"), contains("UTF-8", "UTF-16"));
         assertThat(header.getValuesList("accept-charset"), contains("UTF-8", "UTF-16"));
@@ -373,9 +385,19 @@ public void testCaseInsensitive()
         assertThat(header.get("Foo-Bar"), is("one"));
         assertThat(header.getValuesList("foo-bar"), contains("one", "two"));
         assertThat(header.getValuesList("Foo-Bar"), contains("one", "two"));
+        assertTrue(header.contains("foo-bar"));
+        assertTrue(header.contains("Foo-Bar"));
 
         // We know the order of the set is deterministic
-        assertThat(header.getFieldNamesCollection(), contains("expect", "RaNdOm", "Accept-Charset", "foo-bar"));
+        Set<String> names = header.getFieldNamesCollection();
+        assertThat(names, contains("expect", "RaNdOm", "Accept-Charset", "foo-bar"));
+        assertTrue(names.contains("expect"));
+        assertTrue(names.contains("Expect"));
+        assertTrue(names.contains("random"));
+        assertTrue(names.contains("accept-charset"));
+        assertTrue(names.contains("Accept-Charset"));
+        assertTrue(names.contains("foo-bar"));
+        assertTrue(names.contains("Foo-Bar"));
     }
 
     @ParameterizedTest
@@ -1492,16 +1514,27 @@ public void testRandomAccess()
         assertThat(header.get("EXPECT"), is("100"));
         assertThat(header.get("eXpEcT"), is("100"));
         assertThat(header.get(HttpHeader.EXPECT), is("100"));
+        assertTrue(header.contains("expect"));
+        assertTrue(header.contains("Expect"));
+        assertTrue(header.contains("EXPECT"));
+        assertTrue(header.contains("eXpEcT"));
 
         assertThat(header.get("random"), is("value"));
         assertThat(header.get("Random"), is("value"));
         assertThat(header.get("RANDOM"), is("value"));
         assertThat(header.get("rAnDoM"), is("value"));
         assertThat(header.get("RaNdOm"), is("value"));
+        assertTrue(header.contains("random"));
+        assertTrue(header.contains("Random"));
+        assertTrue(header.contains("RANDOM"));
+        assertTrue(header.contains("rAnDoM"));
+        assertTrue(header.contains("RaNdOm"));
 
         assertThat(header.get("Accept-Charset"), is("UTF-8"));
         assertThat(header.get("accept-charset"), is("UTF-8"));
         assertThat(header.get(HttpHeader.ACCEPT_CHARSET), is("UTF-8"));
+        assertTrue(header.contains("Accept-Charset"));
+        assertTrue(header.contains("accept-charset"));
 
         assertThat(header.getValuesList("Accept-Charset"), contains("UTF-8", "UTF-16"));
         assertThat(header.getValuesList("accept-charset"), contains("UTF-8", "UTF-16"));
@@ -1511,8 +1544,18 @@ public void testRandomAccess()
         assertThat(header.get("Foo-Bar"), is("one"));
         assertThat(header.getValuesList("foo-bar"), contains("one", "two"));
         assertThat(header.getValuesList("Foo-Bar"), contains("one", "two"));
+        assertTrue(header.contains("foo-bar"));
+        assertTrue(header.contains("Foo-Bar"));
 
         // We know the order of the set is deterministic
-        assertThat(header.getFieldNamesCollection(), contains("Expect", "RaNdOm", "Accept-Charset", "foo-bar"));
+        Set<String> names = header.getFieldNamesCollection();
+        assertThat(names, contains("Expect", "RaNdOm", "Accept-Charset", "foo-bar"));
+        assertTrue(names.contains("expect"));
+        assertTrue(names.contains("Expect"));
+        assertTrue(names.contains("random"));
+        assertTrue(names.contains("accept-charset"));
+        assertTrue(names.contains("Accept-Charset"));
+        assertTrue(names.contains("foo-bar"));
+        assertTrue(names.contains("Foo-Bar"));
     }
 }