diff --git a/src/main/java/com/clearfolio/viewer/service/DefaultDocumentConversionService.java b/src/main/java/com/clearfolio/viewer/service/DefaultDocumentConversionService.java
index 9c59593..6d9295c 100644
--- a/src/main/java/com/clearfolio/viewer/service/DefaultDocumentConversionService.java
+++ b/src/main/java/com/clearfolio/viewer/service/DefaultDocumentConversionService.java
@@ -1,12 +1,12 @@
 package com.clearfolio.viewer.service;
 
-import java.util.Optional;
-import java.util.UUID;
-
 import java.io.IOException;
 import java.io.InputStream;
 import java.security.MessageDigest;
 import java.security.NoSuchAlgorithmException;
+import java.util.HexFormat;
+import java.util.Optional;
+import java.util.UUID;
 
 import org.springframework.stereotype.Service;
 import org.springframework.web.multipart.MultipartFile;
@@ -62,6 +62,8 @@ public UUID submit(MultipartFile file, PolicyOverrideRequest overrideRequest) {
         PolicyOverrideRequest effectiveOverride = overrideRequest == null
                 ? PolicyOverrideRequest.none()
                 : overrideRequest;
+        // Security: File type, size limits, and basic content validation are enforced here.
+        // Processing sandboxing is handled by the worker environment.
         validationService.validateOrThrow(file, effectiveOverride);
 
         String contentHash = contentHash(file);
@@ -120,13 +122,7 @@ private String contentHash(MultipartFile file) {
                 digest.update(buffer, 0, read);
             }
 
-            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 HexFormat.of().formatHex(digest.digest());
         } catch (NoSuchAlgorithmException ex) {
             throw new IllegalStateException("SHA-256 digest unavailable", ex);
         } catch (IOException ex) {
diff --git a/src/test/java/com/clearfolio/viewer/service/DefaultDocumentConversionServiceTest.java b/src/test/java/com/clearfolio/viewer/service/DefaultDocumentConversionServiceTest.java
index 7d07d6b..8b8280f 100644
--- a/src/test/java/com/clearfolio/viewer/service/DefaultDocumentConversionServiceTest.java
+++ b/src/test/java/com/clearfolio/viewer/service/DefaultDocumentConversionServiceTest.java
@@ -143,6 +143,33 @@ void returnsSameJobIdForDuplicatePayloads() {
         assertEquals(1, worker.enqueuedCount());
     }
 
+    @Test
+    void submitStoresKnownSha256HexContentHash() {
+        InMemoryConversionJobRepository repository = new InMemoryConversionJobRepository();
+        RecordingConversionWorker worker = new RecordingConversionWorker();
+        DocumentConversionService service = new DefaultDocumentConversionService(
+                repository,
+                new DefaultDocumentValidationService(new ConversionProperties()),
+                worker,
+                new ConversionProperties()
+        );
+
+        MockMultipartFile file = new MockMultipartFile(
+                "file",
+                "contract.docx",
+                "application/octet-stream",
+                "hello".getBytes(StandardCharsets.UTF_8)
+        );
+
+        service.submit(file);
+
+        String expected = "2cf24dba5fb0a30e26e83b2ac5b9e29e1b161e5c1fa7425e73043362938b9824";
+        ConversionJob job = repository.findByContentHash(expected).orElseThrow();
+        assertEquals(64, job.getContentHash().length());
+        assertTrue(job.getContentHash().matches("[0-9a-f]{64}"));
+        assertEquals(expected, job.getContentHash());
+    }
+
     @Test
     void returnsDifferentJobIdsForDifferentPayloads() {
         ConversionJobRepository repository = new InMemoryConversionJobRepository();