diff --git a/.jules/bolt.md b/.jules/bolt.md
new file mode 100644
index 00000000..c8d052dd
--- /dev/null
+++ b/.jules/bolt.md
@@ -0,0 +1,3 @@
+## 2026-06-29 - Use HexFormat for Byte Array to Hex String Conversion
+**Learning:** In Java 21, using `String.format("%02x", b)` inside a loop to convert a byte array to a hex string is inefficient and causes unnecessary object allocations and CPU overhead.
+**Action:** Always prefer using `java.util.HexFormat.of().formatHex(bytes)` for converting byte arrays to hex strings in this Java 21 codebase to improve performance.
diff --git a/src/main/java/com/clearfolio/viewer/service/DefaultDocumentConversionService.java b/src/main/java/com/clearfolio/viewer/service/DefaultDocumentConversionService.java
index 9c595937..4a55e7d4 100644
--- a/src/main/java/com/clearfolio/viewer/service/DefaultDocumentConversionService.java
+++ b/src/main/java/com/clearfolio/viewer/service/DefaultDocumentConversionService.java
@@ -110,7 +110,7 @@ public RetryDeadLetterResult retryDeadLettered(UUID jobId, String operatorId) {
         return RetryDeadLetterResult.ACCEPTED;
     }
 
-    private String contentHash(MultipartFile file) {
+    private String contentHash(final MultipartFile file) {
         try (InputStream stream = file.getInputStream()) {
             MessageDigest digest = MessageDigest.getInstance("SHA-256");
             byte[] buffer = new byte[8192];
@@ -121,12 +121,7 @@ private String contentHash(MultipartFile file) {
             }
 
             byte[] raw = digest.digest();
-            StringBuilder hex = new StringBuilder(raw.length * 2);
-            for (byte b : raw) {
-                hex.append(String.format("%02x", b));
-            }
-
-            return hex.toString();
+            return java.util.HexFormat.of().formatHex(raw);
         } catch (NoSuchAlgorithmException ex) {
             throw new IllegalStateException("SHA-256 digest unavailable", ex);
         } catch (IOException ex) {