diff --git a/.jules/bolt.md b/.jules/bolt.md
new file mode 100644
index 00000000..872bd24b
--- /dev/null
+++ b/.jules/bolt.md
@@ -0,0 +1,3 @@
+## 2026-06-30 - Java HexFormat Optimization
+**Learning:** In Java 21, `java.util.HexFormat.of().formatHex(bytes)` is highly preferred over iterating and using `String.format(\"%02x\", b)` for converting a byte array to a hex string.
+**Action:** Replaced String.format in a loop with HexFormat in DefaultDocumentConversionService to enhance performance during content hash generation.
diff --git a/CHANGELOG.md b/CHANGELOG.md
new file mode 100644
index 00000000..9ce75e9a
--- /dev/null
+++ b/CHANGELOG.md
@@ -0,0 +1 @@
+# Changelog\n\n## [Unreleased]\n- ⚡ `DefaultDocumentConversionService` 내부 해시 생성 시 `String.format`을 `HexFormat`으로 교체하여 성능 개선
diff --git a/src/main/java/com/clearfolio/viewer/service/DefaultDocumentConversionService.java b/src/main/java/com/clearfolio/viewer/service/DefaultDocumentConversionService.java
index 9c595937..a3a68298 100644
--- a/src/main/java/com/clearfolio/viewer/service/DefaultDocumentConversionService.java
+++ b/src/main/java/com/clearfolio/viewer/service/DefaultDocumentConversionService.java
@@ -121,12 +121,8 @@ 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();
+            // 성능 향상을 위해 String.format 대신 HexFormat 사용
+            return java.util.HexFormat.of().formatHex(raw);
         } catch (NoSuchAlgorithmException ex) {
             throw new IllegalStateException("SHA-256 digest unavailable", ex);
         } catch (IOException ex) {