diff --git a/src/lib.rs b/src/lib.rs
index 5941618..e66a276 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -20,11 +20,16 @@ lazy_static! {
 }
 
 #[pg_extern(strict)]
-fn cache_json_schema(schema_id: &str, schema: JsonB) -> JsonB {
+fn cache_json_schema(schema_id: &str, schema: JsonB, strict: bool) -> JsonB {
   let mut cache = SCHEMA_CACHE.write().unwrap();
-  let schema_value: Value = schema.0;
+  let mut schema_value: Value = schema.0;
   let schema_path = format!("urn:{}", schema_id);
 
+  // Apply strict validation to all objects in the schema if requested
+  if strict {
+    apply_strict_validation(&mut schema_value);
+  }
+
   let mut compiler = Compiler::new();
   compiler.enable_format_assertions();
 
@@ -75,6 +80,32 @@ fn cache_json_schema(schema_id: &str, schema: JsonB) -> JsonB {
   }
 }
 
+// Helper function to recursively apply strict validation to all objects in a schema
+fn apply_strict_validation(schema: &mut Value) {
+  match schema {
+      Value::Object(map) => {
+          // If this is an object type schema, add additionalProperties: false
+          if let Some(Value::String(t)) = map.get("type") {
+              if t == "object" && !map.contains_key("additionalProperties") {
+                  map.insert("additionalProperties".to_string(), Value::Bool(false));
+              }
+          }
+          
+          // Recurse into all properties
+          for (_, value) in map.iter_mut() {
+              apply_strict_validation(value);
+          }
+      }
+      Value::Array(arr) => {
+          // Recurse into array items
+          for item in arr.iter_mut() {
+              apply_strict_validation(item);
+          }
+      }
+      _ => {}
+  }
+}
+
 #[pg_extern(strict, parallel_safe)]
 fn validate_json_schema(schema_id: &str, instance: JsonB) -> JsonB {
   let cache = SCHEMA_CACHE.read().unwrap();
diff --git a/src/tests.rs b/src/tests.rs
index 706caa8..b259c08 100644
--- a/src/tests.rs
+++ b/src/tests.rs
@@ -143,7 +143,7 @@ fn test_cache_and_validate_json_schema() {
   let invalid_instance_type = json!({ "name": "Bob", "age": -5 });
   let invalid_instance_missing = json!({ "name": "Charlie" });
 
-  let cache_result = cache_json_schema(schema_id, jsonb(schema.clone()));
+  let cache_result = cache_json_schema(schema_id, jsonb(schema.clone()), false);
   assert_success_with_json!(cache_result, "Cache operation should succeed.");
 
   let valid_result = validate_json_schema(schema_id, jsonb(valid_instance));
@@ -190,7 +190,7 @@ fn test_cache_invalid_json_schema() {
     "type": ["invalid_type_value"] 
   });
 
-  let cache_result = cache_json_schema(schema_id, jsonb(invalid_schema));
+  let cache_result = cache_json_schema(schema_id, jsonb(invalid_schema), false);
 
   // Expect 2 leaf errors because the meta-schema validation fails at the type value
   // and within the type array itself.
@@ -233,7 +233,7 @@ fn test_validate_json_schema_detailed_validation_errors() {
     },
     "required": ["address"]
   });
-  let _ = cache_json_schema(schema_id, jsonb(schema));
+  let _ = cache_json_schema(schema_id, jsonb(schema), false);
 
   let invalid_instance = json!({
     "address": {
@@ -272,7 +272,7 @@ fn test_validate_json_schema_oneof_validation_errors() {
     ]
   });
 
-  let _ = cache_json_schema(schema_id, jsonb(schema));
+  let _ = cache_json_schema(schema_id, jsonb(schema), false);
 
   // --- Test case 1: Fails string maxLength (in branch 0) AND missing number_prop (in branch 1) ---
   let invalid_string_instance = json!({ "string_prop": "toolongstring" });
@@ -340,7 +340,7 @@ fn test_clear_json_schemas() {
   
   let schema_id = "schema_to_clear";
   let schema = json!({ "type": "string" });
-  let cache_result = cache_json_schema(schema_id, jsonb(schema.clone()));
+  let cache_result = cache_json_schema(schema_id, jsonb(schema.clone()), false);
   assert_success_with_json!(cache_result);
 
   let show_result1 = show_json_schemas(); 
@@ -366,8 +366,8 @@ fn test_show_json_schemas() {
   let schema_id2 = "schema2";
   let schema = json!({ "type": "boolean" });
 
-  let _ = cache_json_schema(schema_id1, jsonb(schema.clone()));
-  let _ = cache_json_schema(schema_id2, jsonb(schema.clone()));
+  let _ = cache_json_schema(schema_id1, jsonb(schema.clone()), false);
+  let _ = cache_json_schema(schema_id2, jsonb(schema.clone()), false);
 
   let result = show_json_schemas();
   let schemas = result.0["response"].as_array().unwrap();
@@ -375,3 +375,167 @@ fn test_show_json_schemas() {
   assert!(schemas.contains(&json!(schema_id1)));
   assert!(schemas.contains(&json!(schema_id2)));
 }
+
+#[pg_test]
+fn test_auto_strict_validation() {
+  clear_json_schemas();
+  let schema_id = "strict_test";
+  let schema_id_non_strict = "non_strict_test";
+  
+  // Schema without explicit additionalProperties: false
+  let schema = json!({
+    "type": "object",
+    "properties": {
+      "name": { "type": "string" },
+      "profile": {
+        "type": "object",
+        "properties": {
+          "age": { "type": "number" },
+          "preferences": {
+            "type": "object",
+            "properties": {
+              "theme": { "type": "string" }
+            }
+          }
+        }
+      },
+      "tags": {
+        "type": "array",
+        "items": {
+          "type": "object",
+          "properties": {
+            "id": { "type": "string" },
+            "value": { "type": "string" }
+          }
+        }
+      }
+    }
+  });
+  
+  // Cache the same schema twice - once with strict=true, once with strict=false
+  let cache_result_strict = cache_json_schema(schema_id, jsonb(schema.clone()), true);
+  assert_success_with_json!(cache_result_strict, "Schema caching with strict=true should succeed");
+  
+  let cache_result_non_strict = cache_json_schema(schema_id_non_strict, jsonb(schema.clone()), false);
+  assert_success_with_json!(cache_result_non_strict, "Schema caching with strict=false should succeed");
+  
+  // Test 1: Valid instance with no extra properties (should pass for both)
+  let valid_instance = json!({
+    "name": "John",
+    "profile": {
+      "age": 30,
+      "preferences": {
+        "theme": "dark"
+      }
+    },
+    "tags": [
+      {"id": "1", "value": "rust"},
+      {"id": "2", "value": "postgres"}
+    ]
+  });
+  
+  let valid_result_strict = validate_json_schema(schema_id, jsonb(valid_instance.clone()));
+  assert_success_with_json!(valid_result_strict, "Valid instance should pass with strict schema");
+  
+  let valid_result_non_strict = validate_json_schema(schema_id_non_strict, jsonb(valid_instance));
+  assert_success_with_json!(valid_result_non_strict, "Valid instance should pass with non-strict schema");
+  
+  // Test 2: Root level extra property
+  let invalid_root_extra = json!({
+    "name": "John",
+    "extraField": "should fail"  // Extra property at root
+  });
+  
+  // Should fail with strict schema
+  let result_root_strict = validate_json_schema(schema_id, jsonb(invalid_root_extra.clone()));
+  assert_failure_with_json!(result_root_strict, 1, "Object contains properties that are not allowed");
+  let errors_root = result_root_strict.0["errors"].as_array().unwrap();
+  assert_eq!(errors_root[0]["code"], "ADDITIONAL_PROPERTIES_NOT_ALLOWED");
+  assert_eq!(errors_root[0]["details"]["path"], "");
+  
+  // Should pass with non-strict schema
+  let result_root_non_strict = validate_json_schema(schema_id_non_strict, jsonb(invalid_root_extra));
+  assert_success_with_json!(result_root_non_strict, "Extra property should be allowed with non-strict schema");
+  
+  // Test 3: Nested object extra property
+  let invalid_nested_extra = json!({
+    "name": "John",
+    "profile": {
+      "age": 30,
+      "extraNested": "should fail"  // Extra property in nested object
+    }
+  });
+  
+  // Should fail with strict schema
+  let result_nested_strict = validate_json_schema(schema_id, jsonb(invalid_nested_extra.clone()));
+  assert_failure_with_json!(result_nested_strict, 1, "Object contains properties that are not allowed");
+  let errors_nested = result_nested_strict.0["errors"].as_array().unwrap();
+  assert_eq!(errors_nested[0]["code"], "ADDITIONAL_PROPERTIES_NOT_ALLOWED");
+  assert_eq!(errors_nested[0]["details"]["path"], "/profile");
+  
+  // Should pass with non-strict schema
+  let result_nested_non_strict = validate_json_schema(schema_id_non_strict, jsonb(invalid_nested_extra));
+  assert_success_with_json!(result_nested_non_strict, "Extra nested property should be allowed with non-strict schema");
+  
+  // Test 4: Deeply nested object extra property
+  let invalid_deep_extra = json!({
+    "name": "John",
+    "profile": {
+      "age": 30,
+      "preferences": {
+        "theme": "dark",
+        "extraDeep": "should fail"  // Extra property in deeply nested object
+      }
+    }
+  });
+  
+  // Should fail with strict schema
+  let result_deep_strict = validate_json_schema(schema_id, jsonb(invalid_deep_extra.clone()));
+  assert_failure_with_json!(result_deep_strict, 1, "Object contains properties that are not allowed");
+  let errors_deep = result_deep_strict.0["errors"].as_array().unwrap();
+  assert_eq!(errors_deep[0]["code"], "ADDITIONAL_PROPERTIES_NOT_ALLOWED");
+  assert_eq!(errors_deep[0]["details"]["path"], "/profile/preferences");
+  
+  // Should pass with non-strict schema
+  let result_deep_non_strict = validate_json_schema(schema_id_non_strict, jsonb(invalid_deep_extra));
+  assert_success_with_json!(result_deep_non_strict, "Extra deep property should be allowed with non-strict schema");
+  
+  // Test 5: Array item extra property
+  let invalid_array_item_extra = json!({
+    "name": "John",
+    "tags": [
+      {"id": "1", "value": "rust", "extraInArray": "should fail"}  // Extra property in array item
+    ]
+  });
+  
+  // Should fail with strict schema
+  let result_array_strict = validate_json_schema(schema_id, jsonb(invalid_array_item_extra.clone()));
+  assert_failure_with_json!(result_array_strict, 1, "Object contains properties that are not allowed");
+  let errors_array = result_array_strict.0["errors"].as_array().unwrap();
+  assert_eq!(errors_array[0]["code"], "ADDITIONAL_PROPERTIES_NOT_ALLOWED");
+  assert_eq!(errors_array[0]["details"]["path"], "/tags/0");
+  
+  // Should pass with non-strict schema
+  let result_array_non_strict = validate_json_schema(schema_id_non_strict, jsonb(invalid_array_item_extra));
+  assert_success_with_json!(result_array_non_strict, "Extra array item property should be allowed with non-strict schema");
+  
+  // Test 6: Schema with explicit additionalProperties: true should allow extras even with strict=true
+  let schema_id_permissive = "permissive_test";
+  let permissive_schema = json!({
+    "type": "object",
+    "properties": {
+      "name": { "type": "string" }
+    },
+    "additionalProperties": true  // Explicitly allow additional properties
+  });
+  
+  let _ = cache_json_schema(schema_id_permissive, jsonb(permissive_schema), true); // Note: strict=true
+  
+  let instance_with_extra = json!({
+    "name": "John",
+    "extraAllowed": "should pass"
+  });
+  
+  let result_permissive = validate_json_schema(schema_id_permissive, jsonb(instance_with_extra));
+  assert_success_with_json!(result_permissive, "Instance with extra property should pass when additionalProperties is explicitly true, even with strict=true");
+}