diff --git a/src/main/java/net/javahippie/fitpub/service/AchievementService.java b/src/main/java/net/javahippie/fitpub/service/AchievementService.java
index 702a581..0c21c5e 100644
--- a/src/main/java/net/javahippie/fitpub/service/AchievementService.java
+++ b/src/main/java/net/javahippie/fitpub/service/AchievementService.java
@@ -41,10 +41,8 @@ public class AchievementService {
      */
     @Transactional
     public List<Achievement> checkAndAwardAchievements(Activity activity) {
-        List<Achievement> newAchievements = new ArrayList<>();
-
         if (activity.getUserId() == null || activity.getStartedAt() == null || activity.getEndedAt() == null) {
-            return newAchievements;
+            return List.of();
         }
 
         UUID userId = activity.getUserId();
@@ -59,6 +57,15 @@ public class AchievementService {
             existing.add(a.getAchievementType());
         }
 
+        return checkAndAwardAchievements(activity, progress, existing);
+    }
+
+    private List<Achievement> checkAndAwardAchievements(Activity activity,
+                                                        ActivityProgress progress,
+                                                        Set<Achievement.AchievementType> existing) {
+        List<Achievement> newAchievements = new ArrayList<>();
+        UUID userId = activity.getUserId();
+
         // Check first activity achievements
         newAchievements.addAll(checkFirstActivityAchievements(userId, activity, progress, existing));
 
@@ -83,6 +90,10 @@ public class AchievementService {
         // Check speed achievements
         newAchievements.addAll(checkSpeedAchievements(userId, activity, progress, existing));
 
+        for (Achievement achievement : newAchievements) {
+            existing.add(achievement.getAchievementType());
+        }
+
         return newAchievements;
     }
 
@@ -103,9 +114,15 @@ public class AchievementService {
 
         achievementRepository.deleteByUserId(userId);
 
+        Set<Achievement.AchievementType> existing = EnumSet.noneOf(Achievement.AchievementType.class);
         List<Achievement> rebuiltAchievements = new ArrayList<>();
-        for (Activity activity : activityHistory) {
-            rebuiltAchievements.addAll(checkAndAwardAchievements(activity));
+        for (int i = 0; i < activityHistory.size(); i++) {
+            Activity activity = activityHistory.get(i);
+            rebuiltAchievements.addAll(checkAndAwardAchievements(
+                    activity,
+                    ActivityProgress.fromHistory(activityHistory, i),
+                    existing
+            ));
         }
 
         return rebuiltAchievements;
@@ -560,6 +577,10 @@ public class AchievementService {
                 throw new IllegalStateException("Current activity missing from chronological history: " + currentActivity.getId());
             }
 
+            return fromHistory(activityHistory, currentIndex);
+        }
+
+        private static ActivityProgress fromHistory(List<Activity> activityHistory, int currentIndex) {
             return new ActivityProgress(
                     List.copyOf(activityHistory.subList(0, currentIndex)),
                     List.copyOf(activityHistory.subList(0, currentIndex + 1))
diff --git a/src/test/java/net/javahippie/fitpub/service/AchievementServiceTest.java b/src/test/java/net/javahippie/fitpub/service/AchievementServiceTest.java
index a38e05b..f6f5665 100644
--- a/src/test/java/net/javahippie/fitpub/service/AchievementServiceTest.java
+++ b/src/test/java/net/javahippie/fitpub/service/AchievementServiceTest.java
@@ -414,6 +414,27 @@ class AchievementServiceTest {
         inOrder.verify(achievementRepository, atLeastOnce()).save(any(Achievement.class));
     }
 
+    @Test
+    @DisplayName("Should rebuild achievements from a stable history snapshot")
+    void testRebuildAchievementsForUser_UsesStableHistorySnapshot() {
+        Activity previous = createActivity(Activity.ActivityType.RUN, 9000L, BigDecimal.ZERO);
+        previous.setStartedAt(LocalDateTime.of(2025, 11, 30, 10, 0));
+        previous.setEndedAt(LocalDateTime.of(2025, 11, 30, 11, 0));
+        Activity activity = createActivity(Activity.ActivityType.RUN, 2000L, BigDecimal.ZERO);
+        activity.setStartedAt(LocalDateTime.of(2025, 12, 1, 7, 15));
+        activity.setEndedAt(LocalDateTime.of(2025, 12, 1, 8, 5));
+
+        when(activityRepository.findByUserIdOrderByStartedAtAsc(userId))
+                .thenReturn(List.of(previous, activity))
+                .thenReturn(List.of(previous));
+        when(achievementRepository.save(any(Achievement.class))).thenAnswer(invocation -> invocation.getArgument(0));
+
+        List<Achievement> rebuilt = achievementService.rebuildAchievementsForUser(userId);
+
+        assertTrue(rebuilt.stream().anyMatch(a -> a.getAchievementType() == Achievement.AchievementType.DISTANCE_10K));
+        verify(activityRepository, times(1)).findByUserIdOrderByStartedAtAsc(userId);
+    }
+
     @Test
     @DisplayName("Should get user achievements")
     void testGetUserAchievements() {