diff --git a/.jules/bolt.md b/.jules/bolt.md
new file mode 100644
index 00000000..108b134f
--- /dev/null
+++ b/.jules/bolt.md
@@ -0,0 +1,3 @@
+## 2026-07-02 - String.format in loops
+**Learning:** `String.format` is slow for hex conversion inside loops.
+**Action:** Replace it with `HexFormat.of().formatHex(bytes)` which is significantly faster and doesn't do intermediate string creations.
diff --git a/src/main/java/com/clearfolio/viewer/artifact/ArtifactLinkService.java b/src/main/java/com/clearfolio/viewer/artifact/ArtifactLinkService.java
index 454b0448..33e5fe4e 100644
--- a/src/main/java/com/clearfolio/viewer/artifact/ArtifactLinkService.java
+++ b/src/main/java/com/clearfolio/viewer/artifact/ArtifactLinkService.java
@@ -38,6 +38,8 @@
 @Service
 public class ArtifactLinkService {
 
+    private static final java.util.HexFormat HEX_FORMAT = java.util.HexFormat.of();
+
     /**
      * Query parameter used by PDF.js-compatible artifact URLs.
      */
@@ -423,11 +425,7 @@ private static String sha256Hex(byte[] bytes) {
         try {
             MessageDigest digest = MessageDigest.getInstance("SHA-256");
             byte[] raw = digest.digest(bytes);
-            StringBuilder hex = new StringBuilder(raw.length * 2);
-            for (byte b : raw) {
-                hex.append(String.format("%02x", b));
-            }
-            return hex.toString();
+            return HEX_FORMAT.formatHex(raw);
         } catch (GeneralSecurityException ex) {
             throw new IllegalStateException("SHA-256 digest unavailable", ex);
         }
diff --git a/src/main/java/com/clearfolio/viewer/service/DefaultDocumentConversionService.java b/src/main/java/com/clearfolio/viewer/service/DefaultDocumentConversionService.java
index b8787f35..7ef568b5 100644
--- a/src/main/java/com/clearfolio/viewer/service/DefaultDocumentConversionService.java
+++ b/src/main/java/com/clearfolio/viewer/service/DefaultDocumentConversionService.java
@@ -26,6 +26,8 @@
 @Service
 public class DefaultDocumentConversionService implements DocumentConversionService {
 
+    private static final java.util.HexFormat HEX_FORMAT = java.util.HexFormat.of();
+
     private final ConversionJobRepository repository;
     private final ConversionJobStateStore stateStore;
     private final DocumentValidationService validationService;
@@ -160,12 +162,7 @@ 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 HEX_FORMAT.formatHex(raw);
         } catch (NoSuchAlgorithmException ex) {
             throw new IllegalStateException("SHA-256 digest unavailable", ex);
         } catch (IOException ex) {