diff --git a/src/string_bytes.cc b/src/string_bytes.cc
index 1d4ee3a81803b2..37ea21e5966128 100644
--- a/src/string_bytes.cc
+++ b/src/string_bytes.cc
@@ -537,6 +537,24 @@ Maybe<size_t> StringBytes::Size(Isolate* isolate,
     }                                                                          \
   } while (0)
 
+// Converts known-valid UTF-8 (buflen >= 32) to a V8 string via the fast
+// UTF-16 path. Callers must ensure buflen is range-checked.
+static MaybeLocal<Value> EncodeValidNonAsciiUtf8(Isolate* isolate,
+                                                 const char* buf,
+                                                 size_t buflen) {
+  size_t u16size = simdutf::utf16_length_from_utf8(buf, buflen);
+  if (u16size > static_cast<size_t>(v8::String::kMaxLength)) {
+    isolate->ThrowException(ERR_STRING_TOO_LONG(isolate));
+    return MaybeLocal<Value>();
+  }
+  return EncodeTwoByteString(
+      isolate, u16size, [buf, buflen, u16size](uint16_t* dst) {
+        size_t written = simdutf::convert_valid_utf8_to_utf16(
+            buf, buflen, reinterpret_cast<char16_t*>(dst));
+        CHECK_EQ(written, u16size);
+      });
+}
+
 MaybeLocal<Value> StringBytes::Encode(Isolate* isolate,
                                       const char* buf,
                                       size_t buflen,
@@ -586,17 +604,7 @@ MaybeLocal<Value> StringBytes::Encode(Isolate* isolate,
         // We know that we are non-ASCII (and are unlikely Latin1), use 2-byte
         // In the most likely case of valid UTF-8, we can use this fast impl
         // For very short input, it is slower, so we limit min size
-        size_t u16size = simdutf::utf16_length_from_utf8(buf, buflen);
-        if (u16size > static_cast<size_t>(v8::String::kMaxLength)) {
-          isolate->ThrowException(ERR_STRING_TOO_LONG(isolate));
-          return MaybeLocal<Value>();
-        }
-        return EncodeTwoByteString(
-            isolate, u16size, [buf, buflen, u16size](uint16_t* dst) {
-              size_t written = simdutf::convert_valid_utf8_to_utf16(
-                  buf, buflen, reinterpret_cast<char16_t*>(dst));
-              CHECK_EQ(written, u16size);
-            });
+        return EncodeValidNonAsciiUtf8(isolate, buf, buflen);
       }
 
       val =
@@ -671,40 +679,6 @@ MaybeLocal<Value> StringBytes::Encode(Isolate* isolate,
   }
 }
 
-MaybeLocal<Value> StringBytes::EncodeValidUtf8(Isolate* isolate,
-                                               const char* buf,
-                                               size_t buflen) {
-  CHECK_BUFLEN_IN_RANGE(buflen);
-  if (!buflen) return String::Empty(isolate);
-  buflen = keep_buflen_in_range(buflen);
-
-  // ASCII fast path
-  if (!simdutf::validate_ascii_with_errors(buf, buflen).error) {
-    return ExternOneByteString::NewFromCopy(isolate, buf, buflen);
-  }
-
-  if (buflen >= 32) {
-    size_t u16size = simdutf::utf16_length_from_utf8(buf, buflen);
-    if (u16size > static_cast<size_t>(v8::String::kMaxLength)) {
-      isolate->ThrowException(ERR_STRING_TOO_LONG(isolate));
-      return MaybeLocal<Value>();
-    }
-    return EncodeTwoByteString(
-        isolate, u16size, [buf, buflen, u16size](uint16_t* dst) {
-          size_t written = simdutf::convert_valid_utf8_to_utf16(
-              buf, buflen, reinterpret_cast<char16_t*>(dst));
-          CHECK_EQ(written, u16size);
-        });
-  }
-
-  Local<String> str;
-  if (!String::NewFromUtf8(isolate, buf, v8::NewStringType::kNormal, buflen)
-           .ToLocal(&str)) {
-    isolate->ThrowException(node::ERR_STRING_TOO_LONG(isolate));
-  }
-  return str;
-}
-
 MaybeLocal<Value> StringBytes::Encode(Isolate* isolate,
                                       const uint16_t* buf,
                                       size_t buflen) {
@@ -733,4 +707,19 @@ MaybeLocal<Value> StringBytes::Encode(Isolate* isolate,
   return Encode(isolate, buf, len, encoding);
 }
 
+MaybeLocal<Value> StringBytes::EncodeValidUtf8(Isolate* isolate,
+                                               const char* buf,
+                                               size_t buflen) {
+  buflen = keep_buflen_in_range(buflen);
+  if (!simdutf::validate_ascii_with_errors(buf, buflen).error) {
+    return ExternOneByteString::NewFromCopy(isolate, buf, buflen);
+  }
+  if (buflen >= 32) return EncodeValidNonAsciiUtf8(isolate, buf, buflen);
+  Local<String> str;
+  if (!String::NewFromUtf8(isolate, buf, v8::NewStringType::kNormal, buflen)
+           .ToLocal(&str))
+    isolate->ThrowException(node::ERR_STRING_TOO_LONG(isolate));
+  return str;
+}
+
 }  // namespace node
diff --git a/src/string_bytes.h b/src/string_bytes.h
index 71aa9ff1f90a7c..a9935b68db9647 100644
--- a/src/string_bytes.h
+++ b/src/string_bytes.h
@@ -83,7 +83,8 @@ class StringBytes {
                                           size_t buflen,
                                           enum encoding encoding);
 
-  // Like Encode(..., UTF8) but does not re-validate. Input must be valid UTF-8.
+  // Like Encode(..., UTF8) but skips UTF-8 validation. Caller must guarantee
+  // that buf contains valid UTF-8.
   static v8::MaybeLocal<v8::Value> EncodeValidUtf8(v8::Isolate* isolate,
                                                    const char* buf,
                                                    size_t buflen);