diff --git a/assertj-tests/src/test/java/org/assertj/core/api/ListAssertContainsOnlyTest.java b/assertj-tests/src/test/java/org/assertj/core/api/ListAssertContainsOnlyTest.java
new file mode 100644
index 0000000000..97b638a1be
--- /dev/null
+++ b/assertj-tests/src/test/java/org/assertj/core/api/ListAssertContainsOnlyTest.java
@@ -0,0 +1,30 @@
+package org.assertj.tests;
+
+import org.assertj.core.api.Assertions;
+import org.junit.jupiter.api.Test;
+
+import java.util.List;
+
+public class ListAssertContainsOnlyTest {
+
+  // Valid partition: matches expected elements
+  @Test
+  void testContainsOnlyValidPartition() {
+    List<Integer> actual = List.of(1, 2, 3);
+    Assertions.assertThat(actual).containsOnly(1, 2, 3);
+  }
+
+  // Invalid partition: has an extra element
+  @Test
+  void testContainsOnlyInvalidPartition() {
+    List<Integer> actual = List.of(1, 2, 3, 4);
+    Assertions.assertThat(actual).doesNotContain(5);
+  }
+
+  // Valid partition with duplicates
+  @Test
+  void testContainsOnlyWithDuplicates() {
+    List<Integer> actual = List.of(1, 1, 2, 3);
+    Assertions.assertThat(actual).containsOnly(1, 2, 3);
+  }
+}
diff --git a/assertj-tests/src/test/java/org/assertj/core/api/ListAssert_hasSize_Test.java b/assertj-tests/src/test/java/org/assertj/core/api/ListAssert_hasSize_Test.java
new file mode 100644
index 0000000000..90138535f2
--- /dev/null
+++ b/assertj-tests/src/test/java/org/assertj/core/api/ListAssert_hasSize_Test.java
@@ -0,0 +1,39 @@
+package org.assertj.core.api;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import java.util.ArrayList;
+import java.util.List;
+
+import org.junit.jupiter.api.Test;
+
+/**
+ * Boundary Value Test for ListAssert.hasSize().
+ * This test checks how AssertJ handles lists with boundary sizes.
+ */
+class ListAssert_hasSize_Test {
+
+  // Lower boundary: empty list
+  @Test
+  void should_pass_if_list_is_empty() {
+    List<String> emptyList = new ArrayList<>();
+    assertThat(emptyList).hasSize(0);
+  }
+
+  // Nominal boundary: single item
+  @Test
+  void should_pass_if_list_has_one_item() {
+    List<String> singleItem = List.of("AssertJ");
+    assertThat(singleItem).hasSize(1);
+  }
+
+  // Upper boundary: large list
+  @Test
+  void should_pass_if_list_has_1000_items() {
+    List<Integer> largeList = new ArrayList<>();
+    for (int i = 0; i < 1000; i++) {
+      largeList.add(i);
+    }
+    assertThat(largeList).hasSize(1000);
+  }
+}
diff --git a/assertj-tests/src/test/java/org/assertj/core/api/ObjectAssertInstanceTest.java b/assertj-tests/src/test/java/org/assertj/core/api/ObjectAssertInstanceTest.java
new file mode 100644
index 0000000000..0f14eb3abc
--- /dev/null
+++ b/assertj-tests/src/test/java/org/assertj/core/api/ObjectAssertInstanceTest.java
@@ -0,0 +1,23 @@
+package org.example;
+
+import org.assertj.core.api.Assertions;
+import org.junit.jupiter.api.Test;
+
+public class ObjectAssertInstanceTest {
+
+  @Test
+  void should_pass_if_object_is_instance_of_expected_type() {
+    Number obj = Integer.valueOf(42); // Definition of obj
+
+    // Use of obj in assertion - expected to pass
+    Assertions.assertThat(obj).isInstanceOf(Integer.class);
+  }
+
+  @Test
+  void should_fail_if_object_is_not_instance_of_expected_type() {
+    Number obj = Double.valueOf(3.14); // Definition of obj
+
+    // Use of obj in assertion - should NOT be Integer
+    Assertions.assertThat(obj).isNotInstanceOf(Integer.class);
+  }
+}
diff --git a/assertj-tests/src/test/java/org/assertj/core/api/OptionalAssertPathTest.java b/assertj-tests/src/test/java/org/assertj/core/api/OptionalAssertPathTest.java
new file mode 100644
index 0000000000..c906c216e8
--- /dev/null
+++ b/assertj-tests/src/test/java/org/assertj/core/api/OptionalAssertPathTest.java
@@ -0,0 +1,25 @@
+package org.assertj.core.api;
+
+import org.junit.jupiter.api.Test;
+
+import java.util.Optional;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+public class OptionalAssertPathTest {
+
+  // Path 1: Optional is empty
+  @Test
+  void should_fail_if_optional_is_empty() {
+    Optional<String> empty = Optional.empty();
+    assertThat(empty).isEmpty();
+  }
+
+  // Path 2: Optional is present
+  @Test
+  void should_pass_if_optional_is_present() {
+    Optional<String> value = Optional.of("AssertJ");
+    assertThat(value).isPresent();
+    assertThat(value.get()).isEqualTo("AssertJ");
+  }
+}
diff --git a/assertj-tests/src/test/java/org/assertj/core/api/StringAssertIntegrationTest.java b/assertj-tests/src/test/java/org/assertj/core/api/StringAssertIntegrationTest.java
new file mode 100644
index 0000000000..3f81883a19
--- /dev/null
+++ b/assertj-tests/src/test/java/org/assertj/core/api/StringAssertIntegrationTest.java
@@ -0,0 +1,27 @@
+package org.example;
+
+import org.assertj.core.api.Assertions;
+import org.junit.jupiter.api.Test;
+
+public class StringAssertIntegrationTest {
+
+  @Test
+  void should_pass_when_string_matches_all_conditions() {
+    String sentence = "AssertJ makes testing easier.";
+
+    Assertions.assertThat(sentence)
+      .startsWith("AssertJ")
+      .contains("testing")
+      .endsWith("easier.");
+  }
+
+  @Test
+  void should_fail_when_any_condition_does_not_match() {
+    String sentence = "Assertions help simplify tests.";
+
+    Assertions.assertThat(sentence)
+      .doesNotContain("difficult")
+      .startsWith("Assertions")
+      .endsWith("tests.");
+  }
+}