diff --git a/src/main/java/com/openisle/controller/GlobalExceptionHandler.java b/src/main/java/com/openisle/controller/GlobalExceptionHandler.java
index e093748e9..bd7638dba 100644
--- a/src/main/java/com/openisle/controller/GlobalExceptionHandler.java
+++ b/src/main/java/com/openisle/controller/GlobalExceptionHandler.java
@@ -4,6 +4,7 @@ import org.springframework.http.ResponseEntity;
 import org.springframework.web.bind.annotation.ExceptionHandler;
 import org.springframework.web.bind.annotation.RestControllerAdvice;
 import com.openisle.exception.FieldException;
+import com.openisle.exception.NotFoundException;
 
 import java.util.Map;
 
@@ -16,6 +17,11 @@ public class GlobalExceptionHandler {
                 .body(Map.of("error", ex.getMessage(), "field", ex.getField()));
     }
 
+    @ExceptionHandler(NotFoundException.class)
+    public ResponseEntity<?> handleNotFoundException(NotFoundException ex) {
+        return ResponseEntity.status(404).body(Map.of("error", ex.getMessage()));
+    }
+
     @ExceptionHandler(Exception.class)
     public ResponseEntity<?> handleException(Exception ex) {
         return ResponseEntity.badRequest().body(Map.of("error", ex.getMessage()));
diff --git a/src/main/java/com/openisle/exception/NotFoundException.java b/src/main/java/com/openisle/exception/NotFoundException.java
new file mode 100644
index 000000000..875c41bfb
--- /dev/null
+++ b/src/main/java/com/openisle/exception/NotFoundException.java
@@ -0,0 +1,10 @@
+package com.openisle.exception;
+
+/**
+ * Exception representing a missing resource such as a post or user.
+ */
+public class NotFoundException extends RuntimeException {
+    public NotFoundException(String message) {
+        super(message);
+    }
+}
diff --git a/src/main/java/com/openisle/service/AiUsageService.java b/src/main/java/com/openisle/service/AiUsageService.java
index 70c2d6e1b..851cfa309 100644
--- a/src/main/java/com/openisle/service/AiUsageService.java
+++ b/src/main/java/com/openisle/service/AiUsageService.java
@@ -29,7 +29,7 @@ public class AiUsageService {
 
     public int incrementAndGetCount(String username) {
         User user = userRepository.findByUsername(username)
-                .orElseThrow(() -> new IllegalArgumentException("User not found"));
+                .orElseThrow(() -> new com.openisle.exception.NotFoundException("User not found"));
         LocalDate today = LocalDate.now();
         AiFormatUsage usage = usageRepository.findByUserAndUseDate(user, today)
                 .orElseGet(() -> {
@@ -46,7 +46,7 @@ public class AiUsageService {
 
     public int getCount(String username) {
         User user = userRepository.findByUsername(username)
-                .orElseThrow(() -> new IllegalArgumentException("User not found"));
+                .orElseThrow(() -> new com.openisle.exception.NotFoundException("User not found"));
         return usageRepository.findByUserAndUseDate(user, LocalDate.now())
                 .map(AiFormatUsage::getCount)
                 .orElse(0);
diff --git a/src/main/java/com/openisle/service/CommentService.java b/src/main/java/com/openisle/service/CommentService.java
index 262bb0367..77780a630 100644
--- a/src/main/java/com/openisle/service/CommentService.java
+++ b/src/main/java/com/openisle/service/CommentService.java
@@ -34,9 +34,9 @@ public class CommentService {
 
     public Comment addComment(String username, Long postId, String content) {
         User author = userRepository.findByUsername(username)
-                .orElseThrow(() -> new IllegalArgumentException("User not found"));
+                .orElseThrow(() -> new com.openisle.exception.NotFoundException("User not found"));
         Post post = postRepository.findById(postId)
-                .orElseThrow(() -> new IllegalArgumentException("Post not found"));
+                .orElseThrow(() -> new com.openisle.exception.NotFoundException("Post not found"));
         Comment comment = new Comment();
         comment.setAuthor(author);
         comment.setPost(post);
@@ -60,7 +60,7 @@ public class CommentService {
 
     public Comment addReply(String username, Long parentId, String content) {
         User author = userRepository.findByUsername(username)
-                .orElseThrow(() -> new IllegalArgumentException("User not found"));
+                .orElseThrow(() -> new com.openisle.exception.NotFoundException("User not found"));
         Comment parent = commentRepository.findById(parentId)
                 .orElseThrow(() -> new IllegalArgumentException("Comment not found"));
         Comment comment = new Comment();
@@ -92,7 +92,7 @@ public class CommentService {
 
     public List<Comment> getCommentsForPost(Long postId) {
         Post post = postRepository.findById(postId)
-                .orElseThrow(() -> new IllegalArgumentException("Post not found"));
+                .orElseThrow(() -> new com.openisle.exception.NotFoundException("Post not found"));
         return commentRepository.findByPostAndParentIsNullOrderByCreatedAtAsc(post);
     }
 
@@ -104,14 +104,14 @@ public class CommentService {
 
     public List<Comment> getRecentCommentsByUser(String username, int limit) {
         User user = userRepository.findByUsername(username)
-                .orElseThrow(() -> new IllegalArgumentException("User not found"));
+                .orElseThrow(() -> new com.openisle.exception.NotFoundException("User not found"));
         Pageable pageable = PageRequest.of(0, limit);
         return commentRepository.findByAuthorOrderByCreatedAtDesc(user, pageable);
     }
 
     public java.util.List<User> getParticipants(Long postId, int limit) {
         Post post = postRepository.findById(postId)
-                .orElseThrow(() -> new IllegalArgumentException("Post not found"));
+                .orElseThrow(() -> new com.openisle.exception.NotFoundException("Post not found"));
         java.util.LinkedHashSet<User> set = new java.util.LinkedHashSet<>();
         set.add(post.getAuthor());
         set.addAll(commentRepository.findDistinctAuthorsByPost(post));
@@ -126,7 +126,7 @@ public class CommentService {
     @org.springframework.transaction.annotation.Transactional
     public void deleteComment(String username, Long id) {
         User user = userRepository.findByUsername(username)
-                .orElseThrow(() -> new IllegalArgumentException("User not found"));
+                .orElseThrow(() -> new com.openisle.exception.NotFoundException("User not found"));
         Comment comment = commentRepository.findById(id)
                 .orElseThrow(() -> new IllegalArgumentException("Comment not found"));
         if (!user.getId().equals(comment.getAuthor().getId()) && user.getRole() != Role.ADMIN) {
diff --git a/src/main/java/com/openisle/service/DraftService.java b/src/main/java/com/openisle/service/DraftService.java
index 794828d50..86efd85c2 100644
--- a/src/main/java/com/openisle/service/DraftService.java
+++ b/src/main/java/com/openisle/service/DraftService.java
@@ -28,7 +28,7 @@ public class DraftService {
     @Transactional
     public Draft saveDraft(String username, Long categoryId, String title, String content, List<Long> tagIds) {
         User user = userRepository.findByUsername(username)
-                .orElseThrow(() -> new IllegalArgumentException("User not found"));
+                .orElseThrow(() -> new com.openisle.exception.NotFoundException("User not found"));
         Draft draft = draftRepository.findByAuthor(user).orElse(new Draft());
         draft.setAuthor(user);
         draft.setTitle(title);
diff --git a/src/main/java/com/openisle/service/NotificationService.java b/src/main/java/com/openisle/service/NotificationService.java
index 3d0c9af59..1859ea2f0 100644
--- a/src/main/java/com/openisle/service/NotificationService.java
+++ b/src/main/java/com/openisle/service/NotificationService.java
@@ -34,7 +34,7 @@ public class NotificationService {
 
     public List<Notification> listNotifications(String username, Boolean read) {
         User user = userRepository.findByUsername(username)
-                .orElseThrow(() -> new IllegalArgumentException("User not found"));
+                .orElseThrow(() -> new com.openisle.exception.NotFoundException("User not found"));
         if (read == null) {
             return notificationRepository.findByUserOrderByCreatedAtDesc(user);
         }
@@ -43,7 +43,7 @@ public class NotificationService {
 
     public void markRead(String username, List<Long> ids) {
         User user = userRepository.findByUsername(username)
-                .orElseThrow(() -> new IllegalArgumentException("User not found"));
+                .orElseThrow(() -> new com.openisle.exception.NotFoundException("User not found"));
         List<Notification> notifs = notificationRepository.findAllById(ids);
         for (Notification n : notifs) {
             if (n.getUser().getId().equals(user.getId())) {
@@ -55,7 +55,7 @@ public class NotificationService {
 
     public long countUnread(String username) {
         User user = userRepository.findByUsername(username)
-                .orElseThrow(() -> new IllegalArgumentException("User not found"));
+                .orElseThrow(() -> new com.openisle.exception.NotFoundException("User not found"));
         return notificationRepository.countByUserAndRead(user, false);
     }
 }
diff --git a/src/main/java/com/openisle/service/PostReadService.java b/src/main/java/com/openisle/service/PostReadService.java
index 029bbde0f..35fcddf69 100644
--- a/src/main/java/com/openisle/service/PostReadService.java
+++ b/src/main/java/com/openisle/service/PostReadService.java
@@ -21,9 +21,9 @@ public class PostReadService {
     public void recordRead(String username, Long postId) {
         if (username == null) return;
         User user = userRepository.findByUsername(username)
-                .orElseThrow(() -> new IllegalArgumentException("User not found"));
+                .orElseThrow(() -> new com.openisle.exception.NotFoundException("User not found"));
         Post post = postRepository.findById(postId)
-                .orElseThrow(() -> new IllegalArgumentException("Post not found"));
+                .orElseThrow(() -> new com.openisle.exception.NotFoundException("Post not found"));
         postReadRepository.findByUserAndPost(user, post).ifPresentOrElse(pr -> {
             pr.setLastReadAt(LocalDateTime.now());
             postReadRepository.save(pr);
@@ -38,7 +38,7 @@ public class PostReadService {
 
     public long countReads(String username) {
         User user = userRepository.findByUsername(username)
-                .orElseThrow(() -> new IllegalArgumentException("User not found"));
+                .orElseThrow(() -> new com.openisle.exception.NotFoundException("User not found"));
         return postReadRepository.countByUser(user);
     }
 
diff --git a/src/main/java/com/openisle/service/PostService.java b/src/main/java/com/openisle/service/PostService.java
index 1a0522fb5..adb2bb8ea 100644
--- a/src/main/java/com/openisle/service/PostService.java
+++ b/src/main/java/com/openisle/service/PostService.java
@@ -90,7 +90,7 @@ public class PostService {
             throw new IllegalArgumentException("At most two tags allowed");
         }
         User author = userRepository.findByUsername(username)
-                .orElseThrow(() -> new IllegalArgumentException("User not found"));
+                .orElseThrow(() -> new com.openisle.exception.NotFoundException("User not found"));
         Category category = categoryRepository.findById(categoryId)
                 .orElseThrow(() -> new IllegalArgumentException("Category not found"));
         java.util.List<com.openisle.model.Tag> tags = tagRepository.findAllById(tagIds);
@@ -132,15 +132,15 @@ public class PostService {
 
     public Post viewPost(Long id, String viewer) {
         Post post = postRepository.findById(id)
-                .orElseThrow(() -> new IllegalArgumentException("Post not found"));
+                .orElseThrow(() -> new com.openisle.exception.NotFoundException("Post not found"));
         if (post.getStatus() != PostStatus.PUBLISHED) {
             if (viewer == null) {
-                throw new IllegalArgumentException("Post not found");
+                throw new com.openisle.exception.NotFoundException("Post not found");
             }
             User viewerUser = userRepository.findByUsername(viewer)
-                    .orElseThrow(() -> new IllegalArgumentException("User not found"));
+                    .orElseThrow(() -> new com.openisle.exception.NotFoundException("User not found"));
             if (!viewerUser.getRole().equals(com.openisle.model.Role.ADMIN) && !viewerUser.getId().equals(post.getAuthor().getId())) {
-                throw new IllegalArgumentException("Post not found");
+                throw new com.openisle.exception.NotFoundException("Post not found");
             }
         }
         post.setViews(post.getViews() + 1);
@@ -243,7 +243,7 @@ public class PostService {
 
     public List<Post> getRecentPostsByUser(String username, int limit) {
         User user = userRepository.findByUsername(username)
-                .orElseThrow(() -> new IllegalArgumentException("User not found"));
+                .orElseThrow(() -> new com.openisle.exception.NotFoundException("User not found"));
         Pageable pageable = PageRequest.of(0, limit);
         return postRepository.findByAuthorAndStatusOrderByCreatedAtDesc(user, PostStatus.PUBLISHED, pageable);
     }
@@ -311,7 +311,7 @@ public class PostService {
 
     public Post approvePost(Long id) {
         Post post = postRepository.findById(id)
-                .orElseThrow(() -> new IllegalArgumentException("Post not found"));
+                .orElseThrow(() -> new com.openisle.exception.NotFoundException("Post not found"));
         // publish all pending tags along with the post
         for (com.openisle.model.Tag tag : post.getTags()) {
             if (!tag.isApproved()) {
@@ -327,7 +327,7 @@ public class PostService {
 
     public Post rejectPost(Long id) {
         Post post = postRepository.findById(id)
-                .orElseThrow(() -> new IllegalArgumentException("Post not found"));
+                .orElseThrow(() -> new com.openisle.exception.NotFoundException("Post not found"));
         // remove user created tags that are only linked to this post
         java.util.Set<com.openisle.model.Tag> tags = new java.util.HashSet<>(post.getTags());
         for (com.openisle.model.Tag tag : tags) {
@@ -348,9 +348,9 @@ public class PostService {
     @org.springframework.transaction.annotation.Transactional
     public void deletePost(Long id, String username) {
         Post post = postRepository.findById(id)
-                .orElseThrow(() -> new IllegalArgumentException("Post not found"));
+                .orElseThrow(() -> new com.openisle.exception.NotFoundException("Post not found"));
         User user = userRepository.findByUsername(username)
-                .orElseThrow(() -> new IllegalArgumentException("User not found"));
+                .orElseThrow(() -> new com.openisle.exception.NotFoundException("User not found"));
         if (!user.getId().equals(post.getAuthor().getId()) && user.getRole() != Role.ADMIN) {
             throw new IllegalArgumentException("Unauthorized");
         }
diff --git a/src/main/java/com/openisle/service/ReactionService.java b/src/main/java/com/openisle/service/ReactionService.java
index 104db91c3..95ba9af99 100644
--- a/src/main/java/com/openisle/service/ReactionService.java
+++ b/src/main/java/com/openisle/service/ReactionService.java
@@ -25,9 +25,9 @@ public class ReactionService {
 
     public Reaction reactToPost(String username, Long postId, ReactionType type) {
         User user = userRepository.findByUsername(username)
-                .orElseThrow(() -> new IllegalArgumentException("User not found"));
+                .orElseThrow(() -> new com.openisle.exception.NotFoundException("User not found"));
         Post post = postRepository.findById(postId)
-                .orElseThrow(() -> new IllegalArgumentException("Post not found"));
+                .orElseThrow(() -> new com.openisle.exception.NotFoundException("Post not found"));
         java.util.Optional<Reaction> existing =
                 reactionRepository.findByUserAndPostAndType(user, post, type);
         if (existing.isPresent()) {
@@ -47,7 +47,7 @@ public class ReactionService {
 
     public Reaction reactToComment(String username, Long commentId, ReactionType type) {
         User user = userRepository.findByUsername(username)
-                .orElseThrow(() -> new IllegalArgumentException("User not found"));
+                .orElseThrow(() -> new com.openisle.exception.NotFoundException("User not found"));
         Comment comment = commentRepository.findById(commentId)
                 .orElseThrow(() -> new IllegalArgumentException("Comment not found"));
         java.util.Optional<Reaction> existing =
@@ -70,7 +70,7 @@ public class ReactionService {
 
     public java.util.List<Reaction> getReactionsForPost(Long postId) {
         Post post = postRepository.findById(postId)
-                .orElseThrow(() -> new IllegalArgumentException("Post not found"));
+                .orElseThrow(() -> new com.openisle.exception.NotFoundException("Post not found"));
         return reactionRepository.findByPost(post);
     }
 
diff --git a/src/main/java/com/openisle/service/TagService.java b/src/main/java/com/openisle/service/TagService.java
index 203762dd2..0f7c51abc 100644
--- a/src/main/java/com/openisle/service/TagService.java
+++ b/src/main/java/com/openisle/service/TagService.java
@@ -28,7 +28,7 @@ public class TagService {
         tag.setApproved(approved);
         if (creatorUsername != null) {
             User creator = userRepository.findByUsername(creatorUsername)
-                    .orElseThrow(() -> new IllegalArgumentException("User not found"));
+                    .orElseThrow(() -> new com.openisle.exception.NotFoundException("User not found"));
             tag.setCreator(creator);
         }
         return tagRepository.save(tag);
@@ -94,14 +94,14 @@ public class TagService {
 
     public List<Tag> getRecentTagsByUser(String username, int limit) {
         User user = userRepository.findByUsername(username)
-                .orElseThrow(() -> new IllegalArgumentException("User not found"));
+                .orElseThrow(() -> new com.openisle.exception.NotFoundException("User not found"));
         Pageable pageable = PageRequest.of(0, limit);
         return tagRepository.findByCreatorOrderByCreatedAtDesc(user, pageable);
     }
 
     public List<Tag> getTagsByUser(String username) {
         User user = userRepository.findByUsername(username)
-                .orElseThrow(() -> new IllegalArgumentException("User not found"));
+                .orElseThrow(() -> new com.openisle.exception.NotFoundException("User not found"));
         return tagRepository.findByCreator(user);
     }
 }
diff --git a/src/main/java/com/openisle/service/UserService.java b/src/main/java/com/openisle/service/UserService.java
index 1d97097ba..bcf241e64 100644
--- a/src/main/java/com/openisle/service/UserService.java
+++ b/src/main/java/com/openisle/service/UserService.java
@@ -104,14 +104,14 @@ public class UserService {
 
     public User updateAvatar(String username, String avatarUrl) {
         User user = userRepository.findByUsername(username)
-                .orElseThrow(() -> new IllegalArgumentException("User not found"));
+                .orElseThrow(() -> new com.openisle.exception.NotFoundException("User not found"));
         user.setAvatar(avatarUrl);
         return userRepository.save(user);
     }
 
     public User updateProfile(String currentUsername, String newUsername, String introduction) {
         User user = userRepository.findByUsername(currentUsername)
-                .orElseThrow(() -> new IllegalArgumentException("User not found"));
+                .orElseThrow(() -> new com.openisle.exception.NotFoundException("User not found"));
         if (newUsername != null && !newUsername.equals(currentUsername)) {
             usernameValidator.validate(newUsername);
             userRepository.findByUsername(newUsername).ifPresent(u -> {
diff --git a/src/main/java/com/openisle/service/UserVisitService.java b/src/main/java/com/openisle/service/UserVisitService.java
index 305c860c3..7ae912851 100644
--- a/src/main/java/com/openisle/service/UserVisitService.java
+++ b/src/main/java/com/openisle/service/UserVisitService.java
@@ -17,7 +17,7 @@ public class UserVisitService {
 
     public void recordVisit(String username) {
         User user = userRepository.findByUsername(username)
-                .orElseThrow(() -> new IllegalArgumentException("User not found"));
+                .orElseThrow(() -> new com.openisle.exception.NotFoundException("User not found"));
         LocalDate today = LocalDate.now();
         userVisitRepository.findByUserAndVisitDate(user, today).orElseGet(() -> {
             UserVisit visit = new UserVisit();
@@ -29,7 +29,7 @@ public class UserVisitService {
 
     public long countVisits(String username) {
         User user = userRepository.findByUsername(username)
-                .orElseThrow(() -> new IllegalArgumentException("User not found"));
+                .orElseThrow(() -> new com.openisle.exception.NotFoundException("User not found"));
         return userVisitRepository.countByUser(user);
     }
 }