diff --git a/src/test/java/com/taskflow/model/TaskModelTest.java b/src/test/java/com/taskflow/model/TaskModelTest.java
new file mode 100644
index 0000000..b66ecc6
--- /dev/null
+++ b/src/test/java/com/taskflow/model/TaskModelTest.java
@@ -0,0 +1,102 @@
+package com.taskflow.model;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+/**
+ * Unit tests for the {@link Task} model.
+ *
+ * <p>Bug pins for {@link Task#toString()}:
+ * <ul>
+ *   <li>If {@code assignee_id} is null, {@code assignee_id.toString()} throws NPE.</li>
+ *   <li>If {@code due_date} is null, {@code due_date.trim()} throws NPE.</li>
+ * </ul>
+ */
+class TaskModelTest {
+
+    // --- constructor tests ---
+
+    @Test
+    void shortConstructor_setsFieldsCorrectly() {
+        Task task = new Task("Fix login", 3, "bug", "Login fails on empty password", 42L);
+        assertEquals("Fix login", task.getTitle());
+        assertEquals(3, task.getPriority());
+        assertEquals("bug", task.type);
+        assertEquals("Fix login", task.getTitle());
+        assertEquals(42L, task.assignee_id);
+        assertEquals(0, task.getStatus()); // default status is 0 (todo)
+        assertNotNull(task.createdDate);
+    }
+
+    @Test
+    void defaultConstructor_allFieldsDefaultToNull() {
+        Task task = new Task();
+        assertNull(task.getId());
+        assertNull(task.getTitle());
+        assertEquals(0, task.getStatus());
+        assertEquals(0, task.getPriority());
+    }
+
+    // --- toString() NPE bug-pin tests ---
+
+    /**
+     * Bug pin: Task.toString() calls assignee_id.toString() without null check.
+     * Constructing a Task via no-arg constructor leaves assignee_id=null → NPE.
+     */
+    @Test
+    void toString_nullAssigneeId_throwsNPE() {
+        Task task = new Task();
+        task.setTitle("Test task");
+        task.due_date = "2026-01-01";
+        // BUG: assignee_id.toString() throws NullPointerException
+        assertThrows(NullPointerException.class, task::toString);
+    }
+
+    /**
+     * Bug pin: Task.toString() calls due_date.trim() without null check.
+     */
+    @Test
+    void toString_nullDueDate_throwsNPE() {
+        Task task = new Task();
+        task.setTitle("Test task");
+        task.assignee_id = 1L;
+        // due_date is null → BUG: due_date.trim() throws NullPointerException
+        assertThrows(NullPointerException.class, task::toString);
+    }
+
+    /**
+     * Bug pin: Task constructed via no-arg constructor has null assignee_id and due_date;
+     * calling toString() throws NPE even for a task that is otherwise valid.
+     */
+    @Test
+    void toString_defaultConstructorFields_throwsNPE() {
+        Task task = new Task();
+        task.setTitle("Minimal task");
+        assertThrows(NullPointerException.class, task::toString);
+    }
+
+    /**
+     * Positive case: toString() succeeds when both assignee_id and due_date are set.
+     */
+    @Test
+    void toString_allRequiredFieldsSet_noException() {
+        Task task = new Task();
+        task.setTitle("Deploy");
+        task.setStatus(1);
+        task.assignee_id = 7L;
+        task.due_date = "2026-12-31";
+        String result = assertDoesNotThrow(task::toString);
+        assertTrue(result.contains("Deploy"));
+        assertTrue(result.contains("7"));
+    }
+
+    // --- attachmentUrls initial state ---
+
+    @Test
+    void attachmentUrls_initializedAsEmptyList() {
+        Task task = new Task();
+        assertNotNull(task.attachmentUrls);
+        assertTrue(task.attachmentUrls.isEmpty());
+    }
+}
diff --git a/src/test/java/com/taskflow/model/UserModelTest.java b/src/test/java/com/taskflow/model/UserModelTest.java
new file mode 100644
index 0000000..f42560f
--- /dev/null
+++ b/src/test/java/com/taskflow/model/UserModelTest.java
@@ -0,0 +1,121 @@
+package com.taskflow.model;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+/**
+ * Unit tests for {@link User#getDisplayName()}.
+ *
+ * <p>getDisplayName() has three branches:
+ * <ol>
+ *   <li>Both firstName AND lastName non-null → "firstName lastName"</li>
+ *   <li>Either first/last is null, but full_name non-null → full_name</li>
+ *   <li>full_name also null → username</li>
+ * </ol>
+ *
+ * <p>Bug documented: if firstName is set but lastName is null (or vice versa),
+ * the set name is silently ignored and the method falls through to full_name/username.
+ */
+class UserModelTest {
+
+    // --- happy path ---
+
+    @Test
+    void getDisplayName_bothFirstAndLastName_returnsCombined() {
+        User user = new User("jsmith", "j@example.com", "pass");
+        user.setFirstName("John");
+        user.setLastName("Smith");
+        assertEquals("John Smith", user.getDisplayName());
+    }
+
+    @Test
+    void getDisplayName_noFirstOrLastName_usesFullName() {
+        User user = new User("jsmith", "j@example.com", "pass");
+        user.setFull_name("John Smith");
+        assertEquals("John Smith", user.getDisplayName());
+    }
+
+    @Test
+    void getDisplayName_noNamesAtAll_usesUsername() {
+        User user = new User("jsmith", "j@example.com", "pass");
+        assertEquals("jsmith", user.getDisplayName());
+    }
+
+    // --- bug-pin tests: partial name silently ignored ---
+
+    /**
+     * Bug pin: firstName set but lastName is null.
+     * The caller might expect "John" but the method skips to full_name/username.
+     * Current (buggy) behaviour: falls through to full_name if set, else username.
+     */
+    @Test
+    void getDisplayName_firstNameSetButLastNameNull_fallsBackToFullName() {
+        User user = new User("jsmith", "j@example.com", "pass");
+        user.setFirstName("John");
+        // lastName is null; full_name is set
+        user.setFull_name("John Smith (full)");
+        // BUG: firstName is ignored; full_name is returned instead
+        assertEquals("John Smith (full)", user.getDisplayName());
+    }
+
+    @Test
+    void getDisplayName_firstNameSetLastNameNullNoFullName_fallsBackToUsername() {
+        User user = new User("jsmith", "j@example.com", "pass");
+        user.setFirstName("John");
+        // lastName is null; full_name is also null
+        // BUG: the caller-supplied firstName is silently discarded
+        assertEquals("jsmith", user.getDisplayName());
+    }
+
+    @Test
+    void getDisplayName_lastNameSetButFirstNameNull_fallsBackToUsername() {
+        User user = new User("jsmith", "j@example.com", "pass");
+        user.setLastName("Smith");
+        // firstName is null; full_name is also null
+        // BUG: the caller-supplied lastName is silently discarded
+        assertEquals("jsmith", user.getDisplayName());
+    }
+
+    // --- precedence ---
+
+    /**
+     * firstName+lastName wins over full_name when both are set.
+     */
+    @Test
+    void getDisplayName_firstAndLastNamePrecedeFullName() {
+        User user = new User("jsmith", "j@example.com", "pass");
+        user.setFirstName("John");
+        user.setLastName("Smith");
+        user.setFull_name("J. Smith (legacy)");
+        assertEquals("John Smith", user.getDisplayName());
+    }
+
+    /**
+     * full_name wins over username when first/last are missing.
+     */
+    @Test
+    void getDisplayName_fullNamePrecedesUsername() {
+        User user = new User("jsmith", "j@example.com", "pass");
+        user.setFull_name("John Smith");
+        assertEquals("John Smith", user.getDisplayName());
+    }
+
+    // --- edge cases ---
+
+    @Test
+    void getDisplayName_emptyFirstAndLastName_returnsSpaceSeparated() {
+        // Both are non-null (empty strings) → concatenated as " "
+        User user = new User("jsmith", "j@example.com", "pass");
+        user.setFirstName("");
+        user.setLastName("");
+        assertEquals(" ", user.getDisplayName());
+    }
+
+    @Test
+    void getDisplayName_defaultConstructor_noNPE() {
+        User user = new User();
+        // All name fields are null; username is null too → returns null (no NPE)
+        assertNull(user.getDisplayName());
+    }
+}