Try exposing IsCharacterHeadNode to Primitive.

src/main/java/org/truffleruby/core/encoding/CharacterNodes.java

@@ -0,0 +1,60 @@
+/*
+ * Copyright (c) 2020, 2023 Oracle and/or its affiliates. All rights reserved. This
+ * code is released under a tri EPL/GPL/LGPL license. You can use it,
+ * redistribute it and/or modify it under the terms of the:
+ *
+ * Eclipse Public License version 2.0, or
+ * GNU General Public License version 2, or
+ * GNU Lesser General Public License version 2.1.
+ */
+package org.truffleruby.core.encoding;
+
+import com.oracle.truffle.api.CompilerDirectives.TruffleBoundary;
+import com.oracle.truffle.api.dsl.Cached;
+import com.oracle.truffle.api.dsl.Specialization;
+import com.oracle.truffle.api.strings.AbstractTruffleString;
+import com.oracle.truffle.api.strings.TruffleString;
+import org.truffleruby.annotations.CoreModule;
+import org.truffleruby.annotations.Primitive;
+import org.truffleruby.builtins.PrimitiveArrayArgumentsNode;
+
+@CoreModule("Truffle::Encoding")
+public abstract class CharacterNodes {
+
+    /** Whether the position at byteOffset is the start of a character and not in the middle of a character */
+    @Primitive(name = "is_character_head", lowerFixnum = 2)
+    public abstract static class IsCharacterHeadNode extends PrimitiveArrayArgumentsNode {
+
+        public abstract boolean execute(RubyEncoding enc, AbstractTruffleString string, int byteOffset);
+
+        @Specialization(guards = "enc.isSingleByte")
+        protected boolean singleByte(RubyEncoding enc, AbstractTruffleString string, int byteOffset) {
+            // return offset directly (org.jcodings.SingleByteEncoding#leftAdjustCharHead)
+            return true;
+        }
+
+        @Specialization(guards = { "!enc.isSingleByte", "enc.jcoding.isUTF8()" })
+        protected boolean utf8(RubyEncoding enc, AbstractTruffleString string, int byteOffset,
+                @Cached TruffleString.ReadByteNode readByteNode) {
+            // based on org.jcodings.specific.BaseUTF8Encoding#leftAdjustCharHead
+            return utf8IsLead(readByteNode.execute(string, byteOffset, enc.tencoding));
+
+        }
+
+        @TruffleBoundary
+        @Specialization(guards = { "!enc.isSingleByte", "!enc.jcoding.isUTF8()" })
+        protected boolean other(RubyEncoding enc, AbstractTruffleString string, int byteOffset,
+                @Cached TruffleString.GetInternalByteArrayNode getInternalByteArrayNode) {
+            var byteArray = getInternalByteArrayNode.execute(string, enc.tencoding);
+            int addedOffsets = byteArray.getOffset() + byteOffset;
+            return enc.jcoding.leftAdjustCharHead(byteArray.getArray(), byteArray.getOffset(), addedOffsets,
+                    byteArray.getEnd()) == addedOffsets;
+        }
+
+        /** Copied from org.jcodings.specific.BaseUTF8Encoding */
+        private static boolean utf8IsLead(int c) {
+            return ((c & 0xc0) & 0xff) != 0x80;
+        }
+
+    }
+}

src/main/java/org/truffleruby/core/string/StringNodes.java

@@ -115,7 +115,7 @@
 import org.truffleruby.core.cast.ToStrNode;
 import org.truffleruby.core.cast.ToStrNodeGen;
 import org.truffleruby.core.encoding.EncodingNodes.NegotiateCompatibleStringEncodingNode;
-import org.truffleruby.core.encoding.IsCharacterHeadNode;
+import org.truffleruby.core.encoding.CharacterNodes.IsCharacterHeadNode;
 import org.truffleruby.core.encoding.EncodingNodes.CheckEncodingNode;
 import org.truffleruby.core.encoding.EncodingNodes.GetActualEncodingNode;
 import org.truffleruby.core.encoding.Encodings;