diff --git a/src/main/java/ai/rhesis/sdk/synthesizers/SchemaBuilder.java b/src/main/java/ai/rhesis/sdk/synthesizers/SchemaBuilder.java
index 7ca01a1..fb94a05 100644
--- a/src/main/java/ai/rhesis/sdk/synthesizers/SchemaBuilder.java
+++ b/src/main/java/ai/rhesis/sdk/synthesizers/SchemaBuilder.java
@@ -12,6 +12,7 @@ public static Map<String, Object> buildFlatTestsSchema(
     items.put("type", "object");
     items.put("properties", properties);
     items.put("required", requiredFields);
+    items.put("additionalProperties", false);
 
     Map<String, Object> testsProp = new HashMap<>();
     testsProp.put("type", "array");
@@ -22,9 +23,12 @@ public static Map<String, Object> buildFlatTestsSchema(
 
     Map<String, Object> jsonSchema = new HashMap<>();
     jsonSchema.put("name", "FlatTests");
-    jsonSchema.put(
-        "schema",
-        Map.of("type", "object", "properties", schemaProps, "required", List.of("tests")));
+    Map<String, Object> schema = new HashMap<>();
+    schema.put("type", "object");
+    schema.put("properties", schemaProps);
+    schema.put("required", List.of("tests"));
+    schema.put("additionalProperties", false);
+    jsonSchema.put("schema", schema);
     jsonSchema.put("strict", true);
 
     Map<String, Object> rootSchema = new HashMap<>();
diff --git a/src/test/java/ai/rhesis/sdk/unit/synthesizers/SchemaBuilderTest.java b/src/test/java/ai/rhesis/sdk/unit/synthesizers/SchemaBuilderTest.java
new file mode 100644
index 0000000..e56b3d3
--- /dev/null
+++ b/src/test/java/ai/rhesis/sdk/unit/synthesizers/SchemaBuilderTest.java
@@ -0,0 +1,42 @@
+package ai.rhesis.sdk.unit.synthesizers;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+import ai.rhesis.sdk.synthesizers.SchemaBuilder;
+import java.util.Map;
+import org.junit.jupiter.api.Test;
+
+class SchemaBuilderTest {
+
+  @Test
+  @SuppressWarnings("unchecked")
+  void singleTurnSchemaHasAdditionalPropertiesFalseAtAllLevels() {
+    Map<String, Object> root = SchemaBuilder.buildSingleTurnSchema();
+    Map<String, Object> jsonSchema = (Map<String, Object>) root.get("json_schema");
+    Map<String, Object> schema = (Map<String, Object>) jsonSchema.get("schema");
+
+    assertEquals(false, schema.get("additionalProperties"), "root schema object");
+
+    Map<String, Object> props = (Map<String, Object>) schema.get("properties");
+    Map<String, Object> tests = (Map<String, Object>) props.get("tests");
+    Map<String, Object> items = (Map<String, Object>) tests.get("items");
+
+    assertEquals(false, items.get("additionalProperties"), "test item object");
+  }
+
+  @Test
+  @SuppressWarnings("unchecked")
+  void multiTurnSchemaHasAdditionalPropertiesFalseAtAllLevels() {
+    Map<String, Object> root = SchemaBuilder.buildMultiTurnSchema();
+    Map<String, Object> jsonSchema = (Map<String, Object>) root.get("json_schema");
+    Map<String, Object> schema = (Map<String, Object>) jsonSchema.get("schema");
+
+    assertEquals(false, schema.get("additionalProperties"), "root schema object");
+
+    Map<String, Object> props = (Map<String, Object>) schema.get("properties");
+    Map<String, Object> tests = (Map<String, Object>) props.get("tests");
+    Map<String, Object> items = (Map<String, Object>) tests.get("items");
+
+    assertEquals(false, items.get("additionalProperties"), "test item object");
+  }
+}